Carotenoids are a family of fat-soluble plant pigments that provide red and orange colors to fruits and vegetables. Their function is to absorb light in photosynthesis, protecting plants against photosenzitation. Dietary carotenoids are considered to be beneficial in the prevention of a variety of diseases, including certain cancers and eye disorders. The 5 principal carotenoids found in human plasma as a result of ingestion of plants are α-carotene and β-carotene, cryptoxanthin, lutein, and lycopene, but over 600 carotenoids have been identified to date. Carotenoids present in a wide variety of plants are partially concentrated in chromoplasts or chloroplasts in different ways. The extent of release from the food matrix is highly variable depending on whether carotenoids are noncovalently bound to protein or fiber, dissolved in oil (as in corn and palm oil), or in crystalline form (carrots), making their optimal absorption difficult to achieve (Deming and Erdman 1999; Yeum and Russell 2002; Zaripheh and Erdman 2002). In general, the bioavailability of carotenoids has been estimated to vary from 10% in raw, uncooked vegetables to 50% in oil and commercial products (Deming and Erdman 1999). Even when extracted from the food matrix, the bioavailability of carotenoids may be very low as revealed by in vivo studies of capsanthin and capsorubin from paprika oleoresin (Perez-Galvez and others 2003). Since carotenoids are hydrophobic their absorption depends not only on the release from the food matrix but also on the subsequent solubilization by bile acids and digestive enzymes, culminating in their incorporation into micelles. For this reason dietary lipids have been considered to be important cofactors for carotenoid biovailability, particularly in carotenoid-rich fruits that are low in lipids.
In general, the release of carotenoids from plant foods occurs only when the cells in the food matrix are disrupted, as is usually the case during food preparation, processing, and/or mastication, but not during digestion, at least in the ileum of humans (van het Hof and others 1999, 2000a, 2000b; Edwards and others 2002; Faulks and Southon 2005). In addition Zhou and others (1996) suggested that the food matrix, probably pectin-like fibers, and the crystalline form of carotenoids in carrot chromoplasts were the primary factors that reduced the relative bioavailability of carotenoids from carrot juice (so-called “incomplete release”). Serrano and others (2005) concluded that the proportion of β-carotene (and lutein) released by the sole action of digestive enzymes from spinach, chaya (Cnidoscolus aconitifolius), and macuy (Solanum americanum) ranged from 22% to 77%. Following release from the food matrix, the major limiting factor governing the extent of absorption of carotenoids is their solubilization in digesta (Faulks and Southon 2005). It is well known that cooking can increase the extractability of β-carotene from the plant matrix, thereby improving its bioavailability. Daily consumption of processed carrots and spinach by women over a 4-wk period produced an increase in plasma β-carotene concentration that averaged 3 times that associated with consumption of the same amount of β-carotene from the raw vegetables (Rock and others 1998). Livny and others (2003) concluded that significantly more of the β-carotene was absorbed from cooked and pureed carrots (65.1%± 7.4%) than from the raw vegetable (41.4%± 7.4%). One conclusion is that providing cooked and pureed vegetables rather than raw vegetables would appear to be a better approach to providing bioavailable β-carotene from carotenoid-rich foods, which may be applicable for populations who rely on these foods to meet vitamin A requirements.
Polyphenols represent a wide variety of compounds belonging to several classes, for example, hydroxybenzoic and hydroxycinnamic acids, anthocyanins, proanthocyanidins, flavonols, flavones, flavanols, flavanones, isoflavones, stilbenes, and lignans (Manach and others 2005). Phenolic compounds (or polyphenols) are secondary metabolites of the pentosephosphate, shikimate, and phenylpropanoid pathways in plants. These compounds, one of the most widely occurring groups of phytochemicals, are of considerable physiological and morphological importance in plants (Balasundram and others 2006). Polyphenols are abundant micronutrients in our diet and alleged to play several roles in the prevention of degenerative diseases, acting as anti-allergenic, anti-atherogenic, anti-inflammatory, antimicrobial, antioxidant, antithrombotic, cardioprotective, and vasodilatory agents. Derivatives of phenolic acids account for about one-third of the total intake of polyphenols in our diet and flavonoids account for the remaining two-thirds. They have been associated with the health benefits derived from consuming high levels of fruits, vegetables, and wine, mainly as antioxidants (Balasundram and others 2006).
Reported bioavailability of polyphenols is highly variable depending on their structure and conjugation (for example, to sugars): < 0.1% for most anthocyanins (colored flavonoids in berries and red wine), 1% to 5% for quercetin (red wine, apples, and onions), 10% to 30% for flavanones (citrus) and flavanols (red wine, tea, and cocoa), and 30% to 50% for isoflavones (soya products) and gallic acid (red wine, tea, and various fruits) (Scalbert and Williamson 2000). A major part of the polyphenols ingested (75% to 99%) is not found in urine. This implies they have either not been absorbed through the gut barrier, absorbed and excreted in the bile, or metabolized by the colonic microflora or our own tissues. Polyphenols are highly sensitive to the mild alkaline conditions in the small intestine and a good proportion of these compounds can be transformed before absorption (Bermúdez-Soto and others 2007). This has been shown in a recent in vitro study using a digestion Caco-2 cell model by Laurent and others (2007), who obtained evidence that only pancreatic digestion plays a determining role in the bioavailability of phenolic compounds from grape seed extracts. Salivary and gastric digestion had no effect on polyphenol stability, because interactions between proteins (for example, digestive juice proteins and/or brush border cell proteins or enzymes) and phenolic compounds occurred mainly during the intestinal step and decreased their bioavailability. As a consequence, the most abundant polyphenols in our diet are not necessarily those leading to the highest concentrations of active metabolites in target tissues (Manach and others 2005). The bioavailability of phenolic compounds can be also affected by differences in concentration within plant tissues, variations in cell wall structure, location of glycosides in cells, and binding of phenolic compounds within the food matrix (Balasundram and others 2006). Methods of culinary preparation have also a marked effect on the polyphenol content of foods. For example, simple peeling of fruit and vegetables can eliminate a significant portion of polyphenols because these substances are often present in higher concentrations in the skin than in the pulp. Interestingly, Manach and others (2004) commented that the effect of the food matrix on the bioavailability of polyphenols had not been examined in much detail. In a later review by these authors, an extensive variability in the bioavailability and bioefficacy of polyphenols in humans was observed with up to 10-fold variation in the Cmax values for most phenolic compounds. Among several factors that could explain this variability were the food matrix and the background diet (Manach and others 2005). Milbury and others (2002) also concluded that the information on the bioavailability of different flavonoid groups is limited. These authors suggested that anthocyanins appear to be absorbed in their unchanged glycosylated forms by humans and provided measurements of the pharmacokinetic parameters of dietary anthocyanins absorption. In their review, Aherne and O'Brien (2002) concluded that flavonol content in processed foods (canned, glass jars, frozen) from onion, kale, apple, bean, and so on can be significantly lower (approximately 50%) than levels found in fresh products. However, processing of food such as tomatoes may increase flavonol availability (free form flavonol) due to hydrolysis and extraction from food matrix (Stewart and others 2000). The accumulation of quercetin or release of the aglycone form in processed foods can occur during digestion as a consequence of enzymatic hydrolysis of quercetin that has become conjugated during pasteurization and fermentation. Simonetti and others (2005) have shown that flavonoid glucosides such as rutin were absorbed from tomato puree even at low amounts of intake, suggesting that this food was probably a good vehicle for these polyphenolic compounds.
Polyphenols from wine, in particular resveratrol, anthocyanins, catechins, and quercetin, have attracted a great deal of attention. Tannins (complex polyphenols) in the grape berry are located in specialized tissues of the skin and seed, and because of their differential extraction from these matrices during pressing and fermentation (especially in red wines) their presence in wine may not necessarily reflect their relative abundance in the fruit at harvest. This is important when invoking health-related benefits from wines. However, most of the major solutes present in the grape berry at harvest contribute to wine composition. This is the case of resveratrol where higher concentrations are found in red grapes rather than in white varieties, and in red wines (fermented with the skins) rather than in white wines (King and others 2006). Tannins are tightly bound to cellulose and hemicellulose in the cell walls of fresh grapes, but not to pectin (Adams 2006). It was found that only a fraction of the tannin was extracted during winemaking and some of the nonextracted tannin was tightly bound to the insoluble matrix of the grape berry. The capacity of the insoluble matrix to capture tannin can amount to more than 22% of the tannin present in the fruit. This result indicates that tannin binding to the insoluble matrix of grape berries may be an important factor in the ability to extract tannin from fruit during fermentation (Hazak and others 2005). In addition new polyphenols may be formed during processing (van de Wiel and others 2001). Once extracted, the absorption of quercetin, catechin, and resveratrol in humans was almost equivalent in white wine, grape juice, and vegetable juice.
Berries that accumulate large quantities of anthocyanins, pigments associated with the red and blue colors of plant organs (fruits, flowers, and leaves), have been proposed to have important health-related benefits apart from their antioxidant activity. Anthocyanins are composed of 6 anthocyanidin aglycones linked to sugar groups. However, the bioavailability of anthocyanins is very low and their metabolism is still not fully understood (Wu and others 2002). Felgines and others (2003) suggested that anthocyanins in fresh strawberries were glucuro- and sulfo-conjugated in humans and that their absorption was probably affected by the food matrix. On the other hand, in a study by Mazza and others (2002) the absorption of anthocyanins in humans was investigated after the consumption of a high-fat meal with a freeze-dried blueberry powder containing 25 individual anthocyanins. Nineteen of the 25 anthocyanins were detected in blood serum and their presence was directly correlated with an increase in serum antioxidant capacity. These results appear to indicate that anthocyanins can be absorbed in their intact glycosylated and possibly acylated forms in human subjects. In vitro, the exposure to differences in pH, oxygen, and heating combines to greatly reduce raspberry (extracts) anthocyanin availability to the serum fraction, but codigestion with common foodstuffs (such as bread, breakfast cereal, ice cream, and cooked minced beef) may help protect the labile anthocyanins and certainly does not markedly decrease the level of serum bioavailability polyphenols. Results suggests that polyphenols transiently bind to food matrices during digestion, which protects the more labile anthocyanins from degradation; however, the details about the components involved in the process require further attention (McDougall and others 2005). Based on the limited knowledge available on absorption and metabolic fate of phytochemicals found in berries and conflicting results of bioavailabilty studies, Beattie and others (2005) in their comprehensive review have recommended that “…it would be unwise to ascribe additional health promoting benefits from berries beyond those recognized for fruits and vegetables in general.”
Isoflavones (subclass of polyphenols) are phenolic compounds strikingly similar in chemical structure to mammalian female estrogens and occur naturally in plants, predominantly in soybeans, and thus are known as “phytoestrogens.” They are currently heralded as offering potential alternative therapies for a range of hormone-dependent conditions, including some cancers, menopausal symptoms, cardiovascular disease, and osteoporosis (Setchell and Cassidy 1999; Birt and others 2001). Isoflavones occur in different chemical forms: aglycones, ß-glucosides, manolyl-, and acetyl-glucosides. Although in soy foods the predominant form is as glucosides, the concentration and composition vary according to the part of the seed where the isoflavones are found (seed coat, cotyledon, and axis). Food processing can alter the ratio of glucosides and fermentation processes may result in an increase in the levels of aglycones in commercial soy products (Setchell and Cassidy 1999; de Pascual-Teresa and others 2006). In addition, isoflavone glycosides are not absorbed intact across the enterocytes of healthy adults, but require the hydrolysis of the sugar moiety by intestinal ß-glucosidases (Setchell and others 2002).
There is little information about the effect of the food matrix (and its changes along the digestion process) on the bioavailability of isoflavones. Low recovery of isoflavones after in vitro digestion (bioaccessibility) was reported from cookies (22.0%± 14.1%) compared to fruit juice (90.0%± 12.7%) and chocolate bars (99.5%± 0.7%) (de Pascual-Teresa and others 2006). These results were related to the complexity of the sugar/starch/protein matrix in cookies, which may have hindered the extraction of isoflavones. However, these findings were not replicated in a human study (bioavailability), unveiling the difficulties in extrapolating results from in vitro experimentation to humans. Apparently, these differences could be explained because during in vivo digestion the gut microflora degrades isoflavones, in particular, daidzein. Additionally, interactions between released isoflavones and proteins are more likely to happen in vivo than in vitro (as is the case, in general, for all polyphenols) (Cassidy and others 2006).
Various extents of release, partitioning, and stability of the isoflavones occur at different stages of digestion. Sanz and Luyten (2006) using an in vitro method studied custard desserts made with starch or carboxymethylcellulose (CMC) and enriched with a soy germ extract as source of isoflavones. Incubation under simulated mouth conditions did not affect the amount and partitioning of isoflavones (aqueous/fat phase). A lower recovery and different partitioning were found after the stomach incubation, which was associated with the low pH, whereas after the intestine incubation, a higher recovery and an effect on partitioning were found. Regarding the matrix effect, custards containing starch released a significantly higher amount of isoflavones than those made with CMC, probably due to the higher enzymatic resistance of the latter. Finally, the presence of fat significantly increased the bioaccessibility of the aglycone forms, especially of genistein. The complexity in interpreting experimental results is also confirmed in other studies; for example, it has been observed that fractional absorption of isoflavones (as genistein) is influenced by the matrix and chemical composition of the food, and by gender (Birt and others 2001; Hendrich 2002; Faughnan and others 2004). Thus, isoflavones in supplements are likely to be absorbed at a faster rate compared with those ingested within a food matrix (Rowland and others 2003).