1. Introduction
Nutraceuticals are biologically active molecules found in foods that may not be essential for maintaining normal human functions, but may enhance human health and wellbeing by inhibiting certain diseases or improving human performance (Gupta, 2016; Wildman & Kelley, 2007). Numerous different classes of nutraceuticals are found in both natural and processed foods including carotenoids, flavonoids, curcuminoids, phytosterols, and certain fatty acids (Gupta, 2016).
Many of these nutraceuticals have the potential to act as anticancer agents, and may therefore be suitable for incorporation into functional or medical foods as a means of preventing or treating certain types of cancer. Nutraceuticals vary considerably in their chemical structures, physiochemical properties, and biological effects (Bagchi, Preuss, & Sarwoop, 2015; Gupta, 2016).
For example, nutraceuticals vary in their molar mass, structure, polarity, charge and functional groups, which influences their chemical reactivity, physical state, solubility characteristics, and biological fate and functions (McClements, 2015b).
Some nutraceuticals are naturally present in whole foods, such as fruits, vegetables, and cereals, and are therefore often consumed in this form. Conversely, other nutraceuticals are isolated from their natural states and converted into additives that can be incorporated into functional foods, dietary supplements, or pharmaceuticals.
In this article, we will mainly focus on the delivery of nutraceuticals using foods, as it has been proposed that increased consumption of foods rich in nutraceuticals may decrease the risk of certain types of chronic diseases, including cancer. However, if consumers are going to benefit from consuming foods containing nutraceuticals, it is important that they have certain characteristics:
There are a number of factors that currently limit the utilization of many types of
anticancer nutraceuticals in functional foods (Gleeson, Ryan, & Brayden, 2016;
McClements, 2015a; McClements, Li, & Xiao, 2015b). Firstly, many of nutraceuticals
cannot easily be incorporated into foods because they have poor-solubility characteristics,
or they cause undesirable changes in appearance, texture, or flavor of foods. Second,
many nutraceuticals are chemically or biochemically unstable and therefore lose their
bioactivity because they are degraded within food products or the human body. Third,
many nutraceuticals have a low bioavailability and therefore only a small fraction of
them are actually absorbed and utilized by the body. Fourth, for some nutraceuticals the
optimum dose has not been established, and therefore it is unclear how much to deliver in
a bioactive form, e.g., the anticancer efficacy of resveratrol actually decreases as the dose
increases (Cai, Scott, Kholghi, Andreadi, Rufini, Karmokar, et al., 2015)
The purpose of this review article is to highlight how food matrices can be designed
to enhance the biological activity of anticancer nutraceuticals. In particular, we focus on
two different approaches that can be utilized for this purpose (Figure 1): very systems: In this approach, nutraceuticals isolated from their original environment are encapsulated within a delivery system that is specifically designed to enhance their bioavailability (Arora & Jaglan, 2016; Gleeson, Ryan, & Brayden, 2016; McClements, Decker, & Weiss, 2007; Sagalowicz & Leser, 2010). An example of this approach would be a carotenoid ingredient isolated from carrots that is encapsulated within an oil-in-water emulsion that could then be added to foods such as soft drinks, yogurts, dressings, or sauces (Salvia- Trujillo, Qian, Martin-Belloso, & McClements, 2013a).
In this approach, a nutraceutical-rich food is co-ingested with an excipient food, which is again specially designed to improve the bioavailability of the nutraceuticals (McClements & Xiao, 2014). An example of this approach would be an oil-in-water excipient emulsion consumed at the same time as carotenoid-rich carrots (Zhang, Zhang, Zou, Xiao, Zhang, Decker, et al., 2016b).
nutraceuticals should initially be present in functional foods at a sufficiently high level to have a beneficial physiological effect. The nutraceuticals should remain stable within the functional foods during manufacturing, storage, and utilization, otherwise they may lose their beneficial health effects.
nutraceuticals should not have an adverse effect on the color, taste, or shelf- life of a food product. After ingestion, the nutraceuticals should be released from the functional foods and delivered to the appropriate site-of-action within the human body.
Initially, a brief overview of some of the most important anticancer nutraceuticals that have been identified in foods is given. Some of the major factors limiting the bioavailability of nutraceuticals is then discussed, and potential approaches to overcome these limitations are highlighted. The design of delivery and excipient systems to enhance the bioavailability of anticancer nutraceuticals is then described.
2. Anticancer nutraceuticals
Accumulating evidence suggests that many nutraceuticals, such as curcumin, resveratrol, tea polyphenols, sulforaphane, anthocyanins, genistein, quercetin and
lycopene, exhibit anticancer activities against various forms of cancer (Arvanitoyannis & van Houwelingen-Koukaliaroglou, 2005; Ullah & Khan, 2008).
One of the important advantages of utilizing nutraceuticals to prevent and treat cancer is that they generally exhibit little or no adverse effects frequently associated with pharmaceutical agents after long-term administration. Nutraceuticals have been found to exert a wide range of cellular effects. The possible mechanisms of action of anticancer nutraceuticals include induction of cell cycle arrest and apoptosis in cancerous cells, detoxification of highly reactive molecules, activation of the host immune system, and sensitization of malignant cells to cytotoxic agents (Kotecha, Takami, & Espinoza, 2016; Pan & Ho, 2008). In this section, we focus on several important anticancer nutraceuticals that have beenintensively investigated with particular emphasis on the clinical evidence supporting the safety and efficacy of these compounds in cancer prevention and treatment.
Curcumin: Curcumin is a polyphenol found in turmeric (Curcuma longa), a member of the ginger family (Zingiberaceae) (Joe, Vijaykumar, & Lokesh, 2004). A large number of in vitro and in vivo studies have shown that curcumin inhibits the development of various cancers by inducing cell cycle arrest and cellular apoptosis, through pleiotropic modulation on several key cancer targets such as Wnt/-catenin, nuclear factor kappa B (NF- B), cyclooxygenase-2 (COX-2), tumor necrosis factor alpha (TNF- ), STAT-3 and cyclin D1(Perrone, Ardito, Giannatempo, Dioguardi, Troiano, Lo Russo, et al., 2015). Several phase I and phase II clinical trials have been conducted and demonstrated the safety and anticancer effects of curcumin in patients with different malignancies including myeloma, pancreatic and colorectal cancer (Kotecha, Takami, & Espinoza, 2016). Since curcumin is preferentially distributed in the colonic mucosa, in comparison to other tissues, many clinical trials have been focused on its chemo-preventive efficacy on colorectal cancer (Bar-Sela, Epelbaum, & Schaffer, 2010).
In a phase IIa clinical trial, patients taking 4 grams of curcumin daily were found to have a 40% decrease in the number of aberrant crypt foci (ACF) lesions, which are one of the early histologic signs seen in the colon that may lead to cancer (Carroll, 2012). One of the most important factors limiting the bioefficacy of curcumin is its poor bioavailability.
In animal studies, various edible delivery systems have been developed to improve the bioavailability and bioactivities of curcumin, including liposomes, phospholipid complexes, organogel-based nanoemulsions, chitosan-based nanoparticles and self-emulsifying drug delivery systems (Ting, Jiang, Ho, & Huang, 2014). For example, polylactide co-glycolide nanocapsulated curcumin suppressed cell proliferation, induced cancer cell apoptosis and improved pathological structures in a hepatocellular carcinoma model, whereas an identical concentration of free curcumin was found to be ineffective (Ghosh, Choudhury, Ghosh, Mandal, Sarkar, Ghosh, et al., 2012).
In a recent crossover study, liquid micellar formulations of curcumin showed a 185-fold enhancement in bioavailability within 24 hours without increased toxicity in healthy subjects, compared to powdered curcuminoids (Schiborr, Kocher, Behnam, Jandasek, stede, & Frank, 2014).
Resveratrol is a natural phenol produced by many fruits and plants such as grapes, blueberries, raspberries and peanuts (Bhat, Kosmeder, & Pezzuto, 2001; Kundu & Surh, 2008; Smoliga, Baur, & Hausenblas, 2011a). Animal studies have shown protective effects of resveratrol against several types of cancer, such as breast, skin, gastric, colon, prostate and pancreatic cancers, by interfering with multiple stages of carcinogenesis (Shukla & Singh, 2011).
Recently, clinical trials have established the safety and potential anticancer effects of resveratrol as both a single agent and a constituent of foods (Smoliga, Baur, & Hausenblas, 2011b). A phase I pilot study in colorectal cancer patients determined the anticancer effects of freeze-dried grape powder containing a low dose of resveratrol in combination with other bioactive components. The results suggest that dietary intake of the dry grape powder inhibited the Wnt signaling pathway in the colon, which may have contributed to the observed inhibitory effects on colon carcinogenesis (Nguyen, Martinez, Stamos, Moyer, Planutis, Hope, et al., 2009).
The anticancer effect of resveratrol also has been heavily investigated in breast cancer patients. In a randomized, double-blind placebo-controlled trial, twice daily resveratrol supplement (5 or 50 mg) for 12 weeks decreased the methylation of four cancer-related genes in mammary tissue in women with high risk for developing breast cancer (Zhu, Qin, Zhang, Rottinghaus, Chen, Kliethermes, et al., 2012). Research has been conducted to enhance the bioavailability of resveratrol. Using a prostate cancer mice model, liposome encapsulation of resveratrol and curcumin successfully was shown to increase the oral bioavailablity of both nautraceuticals.
Moreover, encapsulation of both nutraeuticals in the same liposome system resulted in a synergistic reduction of prostate cancer incidence (Narayanan, Nargi, Randolph, & Narayanan, 2009). However, as mentioned earlier, the optimum dose for resveratrol has not been established yet, since some studies have shown that higher levels are less effective at inhibiting cancer than low doses (Cai, et al., 2015).
Sulforaphane: Sulforaphane is an isothiocyanate mainly found in cruciferous vegetables, especially abundant in broccoli and broccoli sprouts (Dinkova- Kostova & Kostov, 2012; Houghton, Fassett, & Coombes, 2013). A large number of cell culture and animal studies have shown that sulforaphane is a potent chemo-preventive agent against various type of cancer.
The molecular targets of sulforaphane vary upon cancer type and stage. The major anticancer mechanism by which sulforaphane protects normal cells from carcinogenesis is through Nrf2- mediated induction of phase II antioxidant and detoxifying enzymes. These enzyme systems enhance cell defense against oxidative damage, and facilitate the removal of carcinogens. Sulforaphane also exerts anticancer activities through various mechanisms of action that are involved in regulating cell proliferation, differentiation, apoptosis, and cell cycle progression (Clarke, Dashwood, & Ho, 2008; Juge, Mithen, & Traka, 2007). To date, only a few clinical trials have been conducted on sulforaphane in cancer patients or high-risk populations. In a phase II study, patients who had recurrent prostate cancer were given 200 moles/day of Sulforaphane-rich extracts for a maximum period of 20 weeks.
Although there was no large decline (by ≥50 %) in prostate-specific antigen (PSA), 7 out of 20 patients experienced moderate PSA declines (by <50 %). Moreover, the on- treatment PSA doubling time (PSADT) was significantly lengthened compared to the pre-treatment PSADT (Alumkal, Slottke, Schwartzman, Cherala, Munar, Graff, et al., 2015). In another clinical trial with Helicobacter pylori-infected humans, daily oral intake of sulforaphane-rich broccoli sprouts (70 gram/day; containing 420 moles of sulforaphane precursor) for 2 months reduced the incidence of infection in humans. This trial suggests that sulforaphane may enhance chemo-protection of the gastric mucosa against H. pylori-induced oxidative stress (Yanaka, Fahey, Fukumoto, Nakayama, Inoue, Zhang, et al., 2009).
In a melanoma mouse model, sulforaphane-loaded albumin microspheres demonstrated a significantly stronger suppression on tumor growth as compared to non-encapsulated sulforaphane, and no adverse effects were observed (Do, Pai, Rizvi, & D'Souza, 2010).
polyphenols: Tea polyphenols are the major bioactive components in tea leaves, and include epigallocatechin-3-gallate (EGCG) and epicatechin-3-gallate (ECG) (Cabrera, Artacho, & Gimenez, 2006; Crozier, Jaganath, & Clifford, 2009; Nijveldt, van Nood, van Hoorn, Boelens, van Norren, & van Leeuwen, 2001). EGCG and ECG can act as potent antioxidants to prevent DNA damage by scavenging reactive oxygen species (ROS), which in turn prevents carcinogenic mutagenesis in normal cells. In pre-clinical studies, tea polyphenols have been found to regulate multiple key cell signaling pathways, resulting in the suppression of angiogenesis, the modulation of the immune system, and the activation of phase II detoxifying enzymes (Yang, Li, Yang, Guan, Chen, & Ju, 2013).
Several clinical trials have suggested that teas and tea polyphenols have the potential to prevent multiple types of cancer, including oral leukoplakia, liver, lung and bladder cancer (Kotecha, Takami, & Espinoza, 2016).
In a recent phase II randomized presurgical placebo-controlled trial, bladder cancer patients taking 800 or 1200 mg of polyphenon E (a green tea polyphenol formulation mainly consisting of EGCG) showed a dose-dependent tissue accumulation of EGCG in benign and malignant bladder urothelium, which was associated with reduction of cell proliferation and increase of apoptosis (Gee, Saltzstein, Kim, Kolesar, Huang, Havighurst, et al., 2015).
However, many clinical studies showed overall inconsistent results regarding the potential anticancer effects of tea polyphenols in different populations, suggesting different types of teas and variable tea preparations may significantly affect the chemo-preventive properties of teas (Sun, Yuan, Koh, & Yu, 2006). Moreover, the food matrix and gastrointestinal tract are known to impact the bioavailability of tea polyphenols (Manach, Williamson, Morand, Scalbert, & Remesy, 2005).
flavonoids: Flavonoids are a large group of polyphenolic secondary metabolites of fruits, vegetables, and other plants with a broad-spectrum of bioactivities, including inhibitory effects on a wide range of human cancers (Tapas, Sakarkar, & Kakde, 2008; Weng & Yen, 2012). Epidemiological
evidence suggests a positive correlation between flavonoids-rich diets and low
risk of colon, breast and prostate cancer. Flavonoids can be categorized into
flavones, flavonols, flavanones, flavanols, anthocyanins and isoflavones based on
their structures. Flavonoids can target multiple signaling pathways during
carcinogenesis to inhibit cancer cell proliferation, suppress tumor angiogenesis,
and induce apoptosis in cancer cells (Batra & Sharma, 2013). Quercetin is one of
the most-studied flavones, and is abundant in onions and apples (Chirumbolo,
2013). Many in vitro and in vivo studies have shown the anticancer potential of
quercetin against a variety of human cancers, such as cervical, breast, colon,
prostate, liver and lung cancer (Yang, Song, Wang, Wang, Xu, & Xing, 2015). A
recent phase I study in patients with chronic Hepatitis C, a major causative factor
of liver cancer, reported that quercetin was safe when consumed at levels up to 5
grams per day, and that there was a modest reduction in viral load, suggesting a
potential for liver cancer prevention (Lu, Crespi, Liu, Vu, Ahmadieh, Wu, et al.,
2016). Quercetin normally exhibits a poor oral bioavailability due to its low
absorption. A microencapsulation approach has been shown to enhance the effects
of quercetin in reducing the oxidative damage and attenuating inflammation in a
mouse colitis model, presumably due to increased absorption (Guazelli, Fattori,
Colombo, Georgetti, Vicentini, Casagrande, et al., 2013). Tangeretin is another
well-studied flavonoid mainly found in citrus fruits (Weng & Yen, 2012). Poor
water-solubility limits the oral bioavailability and efficacy of tangeretin. In a
recent study, an emulsion-based delivery system was shown to significantly
enhance the inhibitory activity of tangeretin on colitis-associated colon
carcinogenesis in mice (Ting, Chiou, Pan, Ho, & Huang, 2015).
3. Factors limiting the bioavailability of anticancer nutraceuticals
The design of functional foods to improve the oral bioavailability of anticancer
nutraceuticals relies on an understanding of their fate within the human gastrointestinal
tract (GIT), as well as of the physicochemical and physiological events that typically
restrict their bioavailability. The factors limiting the oral bioavailability (BA) of
anticancer nutraceuticals can be divided into three main categories, as highlighted
schematically in Figure 2 (Arnott & Planey, 2012; Gleeson, Ryan, & Brayden, 2016;
McClements, 2013; McClements, Li, & Xiao, 2015b; Yao, McClements, & Xiao, 2015):
285 BA = B* A* T* (1) 286
The parameters B*, A* and T* represent the fractions of the nutraceuticals that are
bioaccessible, absorbed, and biologically active within the GIT, respectively. These
parameters depend on: (i) the molecular and physicochemical characteristics of the
anticancer nutraceuticals; (ii) the composition and structure of the surrounding food
matrices; and (iii) the complex events occurring within the human GIT. Improved
knowledge of the major factors limiting the BA of specific nutraceuticals will lead to a
more rational design of foods with enhanced beneficial health effects.
3.1. Bioaccessibility
The bioaccessibility (B*) of an anticancer nutraceutical represents the fraction of the total
amount of orally consumed nutraceuticals that is in a form that can be readily absorbed
by the GIT (Figure 2):
B*= mB/mI (2) 300
Here, mB is the mass of nutraceutical that is in a form that can be absorbed, and mI is the
total mass of the nutraceutical that is ingested. In the case of lipophilic nutraceuticals, B*
is usually taken to be the fraction of nutraceuticals solubilized in the mixed micelles in
the small intestinal fluids.
The bioaccessibility may be affected by three main factors in the GIT: liberation;
solubility; and interactions (McClements, 2015c; McClements, Li, & Xiao, 2015b).
3.2. Absorption
After an anticancer nutraceutical is released from any structures containing it, and
then solubilized within the gastrointestinal fluids, it has to be transported through the GIT
contents and then be absorbed by the epithelial cells lining the GIT (Gleeson, Ryan, &
Brayden, 2016). The absorption of nutraceuticals is affected by numerous
physicochemical and physiological factors
3.3. Transformation
The bioavailability and bioactivity of anticancer nutraceuticals is a result of their
precise chemical structures and mole
cular conformation. Changes in the structure or
conformation of these nutraceuticals due to chemical or biochemical reactions within the
GIT fluids may therefore alter their bio-efficacy.
nutraceuticals by using food components that modulate their chemical or
biochemical changes within the GIT.
4. Classification of the bioavailability of anticancer nutraceuticals
Foods can be specifically designed to improve the bioavailability profiles of
anticancer nutraceuticals by modulating their bioaccessibility, absorption, and
transformation in the GIT. Nutraceuticals can be classified according to the major factors
that limit their bioavailability, which means that generic food compositions and structures
can often be developed for a broad range of different nutraceuticals. A brief overview of
a recently developed classification schemes for nutraceuticals is given here (McClements,
Li, & Xiao, 2015b). This system is related to the Biopharmaceutical Classification
Scheme (BCS) commonly used to classify the factors limiting the bioavailability of
pharmaceuticals based on their solubility and permeability characteristics (Dahan, Miller,
& Amidon, 2009; Kawabata, Wada, Nakatani, Yamada, & Onoue, 2011; Lennernas &
Abrahamsson, 2005).
The Nutraceutical Bioavailability Classification Scheme (NuBACS) uses a three-
coordinate system (B*A*T*) to classify a nutraceutical according to the primary factors
that limit its bioavailability (McClements, Li, & Xiao, 2015a). Here, B* is the
Bioaccessibility, A* is the Absorption, and T* is the Transformation of the nutraceutical.
Each coordinate is designated “(+)” if it does not limit bioavailability, and “(-)” if it does.
Additional insights into the precise mechanisms associated with the poor bioavailability
are provided using subscripts, such as limited liberation from the food matrix (L), poor
solubility in the GIT fluids (S), and high susceptibility to metabolism (M) or chemical
degradation (C) (Table 1). As an example, curcumin, which is a highly hydrophobic
anticancer nutraceutical whose bioavailability is limited by poor solubility in GIT fluids,
metabolism, and chemical degradation is classified as B*(-)S A*(+) T*(-)M,C.
One of the major advantages of using this classification scheme is that a common
strategy may be developed to increase the bioavailability of a group of nutraceuticals
with similar properties. For example, food matrices (delivery or excipient systems)
containing digestible lipids could be used to increase the bioavailability of many
lipophilic nutraceuticals, i.e., B*(-)S.
5. Boosting the bioavailability of anticancer nutraceuticals using food
matrix design
Conventionally, the composition and structure of foods is usually only optimized to
enhance their quality attributes, such as appearance, texture, mouthfeel, and taste. More
recently, foods have been designed to improve their nutritional profiles by reducing the
levels of macronutrients believed to have adverse health impacts (such as saturated fats,
digestible carbohydrates, and salt) or to enrich them with food components that are
believed to bring beneficial health effects (such as vitamins, minerals, dietary fibers or
nutraceuticals). In this section, we focus on two different food-based approaches that
may be utilized to enhance the bioavailability, and therefore bioactivity, of anticancer
nutraceuticals (Figure 1).
5.1. Delivery Systems
Delivery systems are specifically designed to contain the anticancer nutraceuticals to
be delivered (Arora & Jaglan, 2016; Gleeson, Ryan, & Brayden, 2016). If the anticancer
nutraceutical is soluble or dispersible in one of the components (e.g., oil, water, or
powder) used to prepare a traditional food then it can often be simply dissolved or
dispersed in this component prior to preparing the final product. Nevertheless, the food
may still have to be carefully formulated to avoid potential adverse effects, such as
undesirable appearance, taste, or aroma caused by the presence of the nutraceutical.
Moreover, the food matrix may have to be carefully designed to avoid the chemical
degradation of the nutraceutical within the product during food manufacturing, storage,
and utilization. In many cases, an anticancer nutraceutical cannot be simply dissolved or
dispersed into a food matrix. Instead, specialized colloidal delivery systems have to be
fabricated that consist of the anticancer nutraceutical encapsulated within a small particle
(Figure 2).
5.1.1. Particle types
Numerous different types of colloidal particles have been developed that could
potentially be used as delivery systems to encapsulate, protect, and deliver anticancer
nutraceuticals (McClements, 2015c; Yao, McClements, & Xiao, 2015) (Figure 3). Some
of the most promising ones for commercial applications are highlighted here.
Microemulsions: Oil-in-water microemulsions contain colloidal particles dispersed in
water that consist of small clusters of surface active molecules (surfactants) that have
their hydrophobic tails located towards the interior and their hydrophilic heads located
towards the exterior (Figure 3A).
Conversely, water-in-oil microemulsions contain
colloidal particles dispersed in oil where the surfactants are organized so that their
hydrophilic heads form the interior and their hydrophobic tails face toward the exterior.
Both types of microemulsions are thermodynamically stable systems that usually contain
relatively small colloidal particles (5 nm < d < 100 nm). O/W microemulsions are most
suitable for encapsulating hydrophobic nutraceuticals, whereas W/O microemulsions are
most suitable for encapsulating hydrophilic ones (Flanagan & Singh, 2006; Spernath &
Aserin, 2006). Because they are thermodynamically favorable, these systems should
form spontaneously when the required components are mixed together. Nevertheless,
some energy often has to be applied to ensure thorough mixing of the ingredients and to
overcome any kinetic energy barriers.
Emulsions: Oil-in-water emulsions (d > 200 nm) or nanoemulsions (d < 200 nm) consist
of emulsifier-coated oil droplets dispersed within water (Figure 3B), and are therefore
most suitable for encapsulating non-polar nutraceuticals inside the hydrophobic interior
of the oil droplets (McClements, 2012; McClements, Decker, & Weiss, 2007).
Conversely, water-in-oil emulsions or nanoemulsions consist of emulsifier-coated water
droplets dispersed in oil, and are therefore more suitable for encapsulating polar
nutraceuticals within the hydrophilic interior of the water droplets. Typically, a relatively
hydrophilic emulsifier is required to form an O/W emulsion, whereas a relatively
hydrophobic one is required to form a W/O emulsion.
In practice, the formation,
stability, and functional performance of emulsion-based delivery systems is highly
dependent on the nature of the emulsifiers used. Emulsions may be made using a variety
of high-energy (high pressure homogenization, microfluidization, and sonication) or low-
energy (spontaneous emulsification or phase inversion temperature) methods
(McClements & Rao, 2011).
The nature of the homogenization method utilized to
prepare an emulsion depends on many factors, including the type of ingredients used, the
required particle size distribution, and the amount of material that should be produced.
Solid lipid nanoparticles: Solid lipid nanoparticles (SLN) are similar to oil-in-water
emulsions or nanoemulsions, but the oil phase is crystallized (Figure 3C) (Guri,
Guelseren, & Corredig, 2013; Mehnert & Mader, 2012).
Crystallization of the oil phase may improve the physical stability of the particles, as well as improving the retention and stability of the encapsulated nutraceuticals. SLNs are usually fabricated by forming an O/W emulsion or nanoemulsion using a high melting point lipid, and then cooling the system to promote crystallization of the lipids.
Hydrogel beads: Hydrogel beads (microgels) consist of spherical particles, typically in
the range of about 1 to 1000 m, which consist of cross-linked biopolymers that trap
water (Figure 3D) (Chen, Remondetto, & Subirade, 2006; Joye & McClements, 2014;
Shewan & Stokes, 2013; Zhang, Zhang, Chen, Tong, & McClements, 2015). Filled
hydrogel beads also contain other types of colloidal particles dispersed within the beads,
such as lipid droplets or liposomes. Hydrogel beads can be formed using a wide range of
different methods depending on the nature of the biopolymers and cross-linking agents
used. Typically, food-grade proteins (such as whey protein, caseinate, or gelatin) or
polysaccharides (such as agar, alginate, carrageenan, pectin, or starch) are used as the
biopolymers. The gelation mechanism used is largely determined by the nature of the
biopolymer utilized and may involve temperature changes (heating or cooling),
desolvation (dehydration or solvent removal), or addition of cross-linking agents (such as
ions, polyelectrolytes, or enzymes). Certain types of hydrophilic nutraceutical can be
directly incorporated into the hydrogel beads interior by mixing them with the
biopolymer solution prior to fabrication.
Conversely, hydrophobic nutraceuticals may
have to be encapsulated into lipid droplets or liposomes prior to being incorporated into
the hydrogel beads. The composition, dimensions, pore size and interactions of hydrogel
beads can be controlled to create delivery systems with different properties.
Liposomes: This type of colloidal particle typically consists of single or multiple
lamellar spherical structures fabricated from phospholipids (Desai & Park, 2005; Fathi,
Mozafari, & Mohebbi, 2012; Mozafari, Johnson, Hatziantoniou, & Demetzos, 2008).
The phospholipids are assembled into bilayers with the non-polar tails of each layer being
in contact (Figure 3E). Liposomes have a hydrophilic interior that can be used to
encapsulate polar nutraceuticals, and a hydrophobic region in the bilayer that can be used
to encapsulate non-polar ones. Liposomes can be formed using a number of different
methods, including microfluidization and rehydration of surface deposited layers
(Mozafari, Johnson, Hatziantoniou, & Demetzos, 2008).
5.1.2. Particle properties
The colloidal particles used as delivery systems may vary appreciably in their
properties, which determines their efficacy at encapsulating, stabilizing, and delivering
anticancer nutraceuticals (Figure 4). Some of the most important particle properties that
can be controlled to obtain specific functional attributes in delivery systems are
summarized below (McClements, 2015c):
Composition: Food-grade colloidal particles suitable for encapsulating anticancer
nutraceuticals can be fabricated from lipids, proteins, carbohydrates, minerals, surfactants
and/or water. The nature of the components used to fabricate the particles plays a major
role in determining their ability to encapsulate, stabilize, and release the nutraceuticals.
The amount of an anticancer nutraceutical that can be encapsulated within a colloidal
particle largely depends on its solubility within the particle interior. Hydrophobic
nutraceuticals can be solubilized within particle domains comprised of non-polar
components such as lipids, surfactant tails, phospholipid tails, or hydrophobic
biopolymers (e.g., zein). Conversely, hydrophilic nutraceuticals can be solubilized with
particle domains comprised of polar components such as water, hydrophilic proteins, and
polysaccharides. The chemical stability of certain nutraceuticals can be enhanced by
encapsulating them in colloidal particles that restrict the molecular motion of reactants
(such as solid lipid nanoparticles) or by incorporating components that protect the
nutraceutical from degradation, such as antioxidants. The gastrointestinal fate of
anticancer nutraceuticals can also be controlled by manipulating the composition of the
colloidal particles, i.e., the rate and extent of their digestion in different regions of the
GIT. For example, colloidal particles comprised of starch may be initially digested by
amylases in the mouth, those made by proteins or lipids may be digested by gastric or
pancreatic proteases or lipases in the stomach and small intestine, and those made by
dietary fibers may not be digested until they reach the colon due to the action of colonic
bacteria.
Dimensions: Colloidal particles can be fabricated with a wide range of dimensions,
ranging from around 10 nanometers (surfactant micelles) to a few millimeters (hydrogel
beads). The dimensions of colloidal particles influence many of the properties of
delivery systems, including their chemical stability, release rate, optical properties,
physical stability (to gravitational separation and aggregation), rheology, release rate, and
gastrointestinal fate. Anticancer nutraceuticals that are prone to chemical degradation
when they are dispersed in water (such as curcumin under basic conditions) tend to
breakdown more rapidly when they are in small rather than large lipid particles (Zou,
Zheng, Liu, Liu, Xiao, & McClements, 2015). This effect can be attributed to the fact
that the fraction of nutraceutical molecules in close proximity to the oil-water interface
increases with decreasing particle size. Conversely, the rate of lipid particle digestion
and nutraceutical release tends to increase as the particle size decreases because then
there is more oil-water interface available for digestive enzymes to adsorb to (Salvia-
Trujillo, Qian, Martin-Belloso, & McClements, 2013a). The particle size also affects the
incorporation of colloidal particles into food products. Larger particles tend to cream or
sediment more rapidly than smaller ones because the magnitude of the gravitational
forces is proportional to the diameter squared. Gravitational separation of particles may
be an important consideration in functional foods and beverages that have relatively low
viscosities. The optical properties of foods depend on the particle diameter (d) relative to
the wavelength of light (): colloidal dispersions go from being transparent when d <<
to turbid/opaque when d , and to visibly distinguishable as separate particles when
d >> . The dimensions of the particles will also affect their perception within the
mouth: a colloidal dispersion with particles less than about 50 m in diameter will feel
smooth in the mouth, whereas one with larger particles will feel gritty or lumpy.
Obviously, these factors have to be taken into account when designing delivery systems
intended for application in commercial food products that must be perceived favorably by
consumers.
Interfacial properties: The colloidal particles used in the food industry as delivery
systems may have different interfacial properties, e.g., composition, thickness, charge,
and hydrophobicity (McClements, 2015c). The interfacial properties also have a major
impact on the chemical stability, physical stability, release rate, optical properties,
rheology, and gastrointestinal fate. For example, the magnitude and sign of the electrical
charge on the surfaces of the colloidal particles determines their ability to interact with
other electrical substances, such as other charged particles, surfaces, or polymers. In
addition, the charge may impact the chemical stability of encapsulated nutraceuticals by
influencing the attraction or repulsion of pro-oxidants to the particle surfaces (such as
cationic transition metals). The surface chemistry and digestibility of the particle surface
influences the ability of digestive enzymes (such as lipases, proteins, or amylases) to
adsorb to them and hydrolyze the particles. Colloidal particles can often be coated with a
substance that is resistant to digestion in one region of the GIT, but that is digested in
another region. This phenomenon can be used to control the release of encapsulated
nutraceuticals in different parts of the GIT, e.g., mouth, stomach, small intestine or colon.
Physical state: Colloidal particles may be liquid, semi-solid, or solid depending on the
materials used to assemble them and the environmental conditions (such as temperature,
pH, and ionic composition). The oils and water phase used to prepare oil-in-water or
water-in-oil emulsions are typically liquid. However, high melting oils can be used to
create solid lipid nanoparticles that have a crystalline interior. The biopolymers (proteins
and polysaccharides) used to prepare hydrogel beads usually form semi-solid gel-like
particles. The physical state of a particle may change appreciably when environmental
conditions are changed within a food or as it passes through the GIT.
5.1.3. Mechanisms of Action
Appropriately designed delivery systems may enhance the oral bioavailability and
bioactivity of anticancer nutraceuticals due to numerous mechanisms of action:
Enhance Dispersibility: Most non-polar nutraceuticals are so hydrophobic that they
cannot simply be dispersed into aqueous-based foods (such as beverages, sauces,
dressings, yogurts, and desserts) because of their extremely poor water-solubility (Donsi,
Sessa, Mediouni, Mgaidi, & Ferrari, 2011). However, if these nutraceuticals can be
encapsulated within colloidal particles that have a hydrophobic core and a hydrophilic
shell (as in oil-in-water emulsions, nanoemulsions, or microemulsions), then they can
easily be dispersed into these aqueous-based foods (Flanagan & Singh, 2006;
McClements, 2012). Conversely, many polar nutraceuticals are so hydrophilic that they
cannot easily be dispersed into oil-based foods (such as butter, margarine, and spreads).
In this case colloidal particles that consist of a hydrophilic core and hydrophobic shell (as
in water-in-oil emulsions, nanoemulsions, or microemulsions) can be used to encapsulate
them.
Enhance Chemical or Biochemical Stability: Many nutraceuticals are highly
susceptible to chemical or biochemical degradation within food products or inside the
GIT (Braithwaite, Tyagi, Tomar, Kumar, Choonara, & Pillay, 2014; Lu, Kelly, & Miao,
2016; Yallapu, Nagesh, Jaggi, & Chauhan, 2015). Delivery systems can be specifically
designed to protect the encapsulated nutraceuticals from degradation by providing a
physical barrier or by containing specific components (such as antioxidants, buffers or
UV-visible absorbers).
As an example, some nutraceuticals (such as curcumin) are more
prone to chemical degradation when they are dissolved in water than when they are
dissolved in oil. Consequently, their chemical stability can be improved by encapsulating
them in delivery systems that have lipophilic domains, such as vesicles, nanoemulsions,
protein nanoparticles, or solid lipid nanoparticles (Zou, Liu, Liu, Xiao, & McClements,
2015; Zou, Zheng, Zhang, Zhang, Liu, Liu, et al., 2016a, 2016b).
Including antioxidants or chelating agents within a delivery system can inhibit the oxidation of unsaturated
lipids (Davidov-Pardo & McClements, 2015; Qian, Decker, Xiao, & McClements, 2012;
Tamjidi, Shahedi, Varshosaz, & Nasirpour, 2014). Delivery systems can be designed so
that chemically unstable nutraceuticals are trapped within colloidal particles with
coatings that physically separate the nutraceuticals from any other components that may
accelerate their degradation.
Enhance Bioaccessibility: Many nutraceuticals have a low oral bioaccessibility
because of their low solubility in aqueous GIT fluids (Arora & Jaglan, 2016;
McClements, Li, & Xiao, 2015b). Delivery systems can be designed to contain digestible
lipids (such as triglycerides) that form free fatty acids and monoacylglycerols that
enhance the solubilization capacity of the mixed micelles for hydrophobic nutraceuticals
(Ozturk, Argin, Ozilgen, & McClements, 2015; Salvia-Trujillo, Qian, Martin-Belloso, &
McClements, 2013b; Salvia-Trujillo, Sun, Urn, Park, & McClements, 2015; Yao,
McClements, & Xiao, 2015).
The type of lipid used is particularly important since it
determines the rates of lipid digestion and mixed micelle formation, as well as the
solubilization capacity of the mixed micelles. The hydrophobic domains in the mixed
micelles (micelles and vesicles) must be large enough to accommodate the nutraceuticals,
so that they can transport them through the mucus layer to the epithelial cells. There is
therefore considerable interest in designing the lipid phase of foods so that they form
mixed micelles in the small intestine that have an appropriate solubilization capacity for
the nutraceuticals being delivered (Yao, McClements, & Xiao, 2015; Yao, Xiao, &
McClements, 2014).
Enhance Absorption: A delivery system may contain specific components that can
enhance the uptake of the anticancer nutraceuticals, such as components that disrupt the
mucus layer, inhibit efflux inhibitors, stimulate active transporters, or increase the
dimensions of tight junctions (Arora & Jaglan, 2016; Gleeson, Ryan, & Brayden, 2016;
McClements, Li, & Xiao, 2015b; Yao, McClements, & Xiao, 2015). Gleeson and co-
workers have recently highlighted the major strategies for increasing the absorption of
nutraceuticals (Gleeson, Ryan, & Brayden, 2016).
Certain components found in foods,
such as bromelain (an enzyme from pineapple) and papain (an enzyme from papaya),
have the potential to disrupt the mucus layer that normally coats the epithelial cells,
which may lead to enhanced absorption of nanoparticles coated with these substances (de
Sousa, Cattoz, Wilcox, Griffiths, Dalgliesh, Rogers, et al., 2015). Other components
present in foods are able to increase the permeability of epithelial cells, including
medium chain fatty acids like caprylic acid (Gleeson, Ryan, & Brayden, 2016). Other
food components may also have the ability to act as permeation enhancers, such as
cationic biopolymers (chitosan) chelating agents (EDTA), surfactants (polyol or sugar
esters of fatty acids) and phytochemicals (such as piperine) (McClements, Li, & Xiao,
2015b).
These intestinal permeation enhancers are believed to increase absorption by
increasing the fluidity of the phospholipid membranes, increasing the dimensions of the
tight junctions, stimulating active transporters, and/or blocking efflux transporters.
Nevertheless, it is important to realize that there may be adverse health effects associated
with altering the normal absorption mechanisms of nutraceuticals, which should be
considered when designing functional foods based on this principle (McCartney,
Gleeson, & Brayden, 2016).
5.2. Excipient Systems
Unlike delivery systems, excipient systems do not necessarily contain any
nutraceutical components (McClements & Xiao, 2014; McClements, Zou, Zhang, Salvia-
Trujillo, Kumosani, & Xiao, 2015). Instead they are designed to boost the bioavailability
and bioactivity of the nutraceuticals present in other foods that they are co-ingested with,
such as fruits and vegetables. Alternatively, they can be designed to boost the
bioavailability of nutraceuticals in dietary supplements or drugs (Salvia-Trujillo &
McClements, 2016b). Typically, the composition and structure of the excipient food
matrix is carefully controlled so as to improve the bioaccessibility, absorption, or
transformation profile of a nutraceutical-containing substance that is co-ingested with it.
Many of the approaches developed to improve the bioavailability of nutraceuticals using
delivery systems can also be used with excipient systems.
A considerable research effort has recently been carried out with the objective of
developing excipient systems based on emulsions that are suitable for utilization in the
food, dietary supplement, or pharmaceutical industries (McClements & Xiao, 2014;
McClements, Zou, Zhang, Salvia-Trujillo, Kumosani, & Xiao, 2015). These excipient
emulsions consist of small lipid droplets dispersed in an aqueous medium.
The
composition, size, and interfacial properties of the lipid droplets are optimized based on a
number of factors: (i) the ability to extract and solubilize lipophilic nutraceuticals from
food matrices (such as fruits and vegetables); (ii) the ability to be rapidly digested within
the GIT; (iii) the ability to rapidly form mixed micelles that solubilize the lipophilic
nutraceuticals released from the food matrices; (iv) the ability to protect the
nutraceuticals from chemical or biochemical degradation within the GIT. Additional
components may also be added to the aqueous phase of the excipient emulsions to
improve their efficacy, including antioxidants, chelating agents, buffering agents,
biopolymers, or permeation enhancers.
Recent research has shown that excipient emulsions can be used to increase the
bioavailability of carotenoids in a range of produce, including mangoes (Liu, Bi, Xiao, &
McClements, 2016), carrots (Zhang, Zhang, Zou, Xiao, Zhang, Decker, et al., 2015,
2016a; Zhang, et al., 2016b), yellow peppers (Liu, Bi, Xiao, & McClements, 2015), and
tomatoes (Salvia-Trujillo & McClements, 2016a). The composition and structure of the
excipient emulsions has to be carefully optimized to effectively enhance carotenoid
bioavailability (Figure 5). Studies have shown that excipient emulsions containing long
chain triglycerides (LCT) are more effective than those containing medium chain
triglycerides (MCT) at improving the bioaccessibility of carotenoids (Zhang, et al.,
2015). This effect was attributed to the fact that digestion of LCT by lipase leads to the
generation of long chain fatty acids that enhance the solubilization capacity of the mixed
micelle phase by increasing the dimensions of the hydrophobic domains (Cho, Salvia-
Trujillo, Kim, Park, Xiao, & McClements, 2014; Salvia-Trujillo, Qian, Martin-Belloso, &
McClements, 2013b).
Conversely, the hydrophobic domains formed by the medium
chain fatty acids resulting from MCT digestion are too small to accommodate large
carotenoids. Studies with carrots showed that the bioaccessibility of the carotenoids
increased with increasing fat content in the excipient emulsions co-ingested with them
(Zhang, et al., 2016b). Moreover, decreasing the size of the fat droplets in the excipient
emulsions was found to increase carotenoid bioaccessibility, which was attributed to their
higher surface area and rate of digestion, leading to quicker mixed micelle formation and
nutraceutical solubilization (Zhang, et al., 2016a).
Excipient emulsions have also been used to improve the bioavailability of curcumin
from turmeric powder (Zou, Liu, Liu, Liang, Li, Liu, et al., 2014; Zou, Zheng, Liu, Liu,
Xiao, & McClements, 2015). A higher amount of curcumin reached the simulated small
intestine when the turmeric powder was co-ingested with an excipient emulsion, which
was attributed to a number of effects. First, some of the curcumin was transported into
the hydrophobic interior of the small lipid droplets. Second, the chemical stability of the
curcumin was higher within the hydrophobic interior of the lipid droplets than in the
surrounding aqueous phase. Third, the mixed micelles formed after digestion of the lipid
droplets were able to solubilize the non-polar curcumin molecules in the aqueous GIT
fluids.
The concept of excipient foods is fairly new, and a considerable amount of research
is still required in this area. Nevertheless, it is a promising approach for improving the
bioavailability characteristics of many anticancer nutraceuticals in functional foods,
dietary supplements, and pharmacological drugs.
6. Conclusions
787 There is growing interest in using specific nutraceuticals commonly found in foods
as chemo-preventative agents. Cell culture and animal studies suggest that ingestion of
these nutraceuticals may inhibit certain types of cancers. Nevertheless, many anticancer
nutraceuticals cannot simply be incorporated into foods because of their poor solubility,
stability, and bioavailability characteristics. There is a considerable potential to create
functional food products designed to overcome these challenges, and therefore increase
the potential efficacy of anticancer nutraceuticals. These foods may be designed to
contain the nutraceuticals themselves (delivery systems) or they may be designed to boost
the bioavailability of nutraceuticals in other foods (excipient systems). Despite their
potential, a considerable amount of research is still needed to demonstrate the efficacy of
nutraceuticals, and the ability of delivery or excipient systems to enhance their bio-
efficacy.
Unlike pharmaceuticals, which are typically taken in a well-defined dose at a
particular time, nutraceuticals are obtained from numerous sources at levels that vary
from person to person and from time to time as part of a complex diet. In addition, the
beneficial effects of nutraceuticals may arise from taking relatively low levels over an
extended period.
Consequently, it is often difficult to carry out clinical studies (double
blind randomized feeding trials) using humans to prove their efficacy. Nevertheless, it
may be possible to gain some insights into their efficacy using long-term animal feedings
studies where the diet is carefully controlled. Having said this, there is also considerable
skepticism about the bioactivity demonstrated by certain nutraceuticals established using
in vitro and in vivo assays.
For instance, recent articles have criticized the evidence
suggesting that curcumin has a high bioactivity due to its ability to interfere with many
types of assays used to measure biological activity (Baker, 2017; Nelson, Dahlin, Bisson,
Graham, Pauli, & Walters, 2017). This may at least partially account for the lack of
efficacy that curcumin has demonstrated in many clinical trials.
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