Phytochemical bioavailability

The absorption and transport processes of many of the phytochemicals present in food are complex and not fully understood, and prediction of their bioavailability is problematic. This is particularly true of the lipid-soluble phytochemicals. In this chapter the measurement of carotenoid bioavailability will be discussed. The carotenoids serve as an excellent example of where too little understanding of food structure, the complexity of their behaviour in foods and human tissues, and the nature and cause of widely different individual response to similar intakes, can lead to misinterpretation of study results and confusion in our understanding of the relevance of these (and other) compounds to human health. 

The mucosa of the GIT represents an interface between the external and internal environments. The expansive surface area is necessary for the efficient hydrolysis of foodstuffs and the absorption of energy and nutrients. The mucosa also influences the systemic availability of non-nutrient compounds in the diet, both beneficial and detrimental. Digestion and absorption of glucosinolates are critical determinants of health benefits (see Chapter 4) Similarly, the bioavailability and health benefits of phytoestrogens, such as genistein (see Chapters 5 and 10) are at least partly dependent on the carrier-mediated processes of absorption associated with the GIT (Oitate et al, 2001). Moreover, the metabolic activities of the mucosa can influence the systemic concentrations and forms of dietary phytochemicals, as exemplified by research with soy isoflavones (Andlauer et al., 2000). 

With investigations of phytochemicals and functional foods, the outcome measure is generally going to be a biomarker of disease, such as serum cholesterol level as a marker of heart disease risk, or indicators of bone turnover as markers of osteoporosis risk. Alternatively, markers of exposure may also indicate the benefit from a functional food by demonstrating bioavailability, such as increased serum levels of vitamins or carotenoids. Some components will be measurable in both ways. For instance, effects of a folic acid-fortified food could be measured via decrease in plasma homocysteine levels, or increase in red blood cell folate. 

Degenhardt, P. and Winterhalter, E., HSCCC a powerful tool for the preparative isolation of bioactive compounds, in Biologically-Active Phytochemicals in Food Analysis, Metabolism, Bioavailability and Function, Pfannhauser, W. et ah, Eds., Royal Society of Chemistry, Cambridge, 2001, 143. 

Phytate (myo-inositol hexaphosphate Fig. 15.3, structure 33) is found in many food species and can be considered as a phytochemical. Its role in the plant is primarily as a phosphate store in seeds, but it is found in other tissues as well, for example, tubers (Harland et al., 2004). Phytate and its hydrolysis products are anti-nutrients that chelate metal ions and thus reduce their bioavailability (Persson et al., 1998 House, 1999). This is particularly a problem with cereal grains, but pre-processing can improve mineral absorption from these foods (Agte and Joshi, 1997). There is some concern that high phytate foods could also contain higher levels of toxic heavy metals caused by natural accumulation. Plants also contain phytate-degrading enzymes that can also influence metal ion bioavailability (Viveros et al., 2000). 

Ren Y, Waldron KW, Pacy JE and Ellis PR. 2001. Chemical and histochemical characterization of cell wall polysaccharides in almond seeds in relation to lipid bioavailability. In Pfannhauser W, Fenwick GR and Khokhar S, editors. Biologically Active Phytochemicals in Foods. Cambridge, UK Royal Soc Chem, pp. 448-452. 

However, the effect of piperine on SULT and flavonoid status across the life cycle remains to be investigated. Induction of phase II metabolism appears to decrease the bioavailability and accelerate the excretion of flavonoids. For example, Siess et al.115 and Walle et al.116 reported flavones induced rat hepatic UGT activity in HepG2 and Caco-2 cells. This induction of UGT enhanced quercetin glucuronidation in Caco-2 cells. In addition to inducing UGT activity, the flavone chrysin inhibits hepatic SULT-mediated sulfation of acetaminophen and minoxidol." The impact of chrysin on the capacity of COMT action toward flavonoids has not been examined. Further, the effect of age on phase II modulation by piperine and chrysin has not been reported. Thus, information on the relationship between age and intake of flavonoids and other phytochemicals that also affect phase II metabolism is required. 

Echinacea has been used for centuries as a medicinal plant. Research from the last two decades shows that Echinacea can enhance the immune system when in vitro and in vivo indicators are used in the model investigations. Further investigations, e.g., bioavailability and clinical studies, are needed to thoroughly understand the immune-enhancing activity of Echinacea. In addition, further research is needed to characterize processing methods that do not significantly reduce the levels of phytochemicals. 

The bioactivity, bioavailability, and toxicology of phytochemicals need to be carefully assessed by in vitro and in vivo studies (Schieber et al., 2001)... 

Oleszek, W. Sitek, M. Stochmal, A. Cheeke, P., Antioxidant properties of Yucca schidigera products. In Biologically-active Phytochemicals in Food. Analysis, Metabolism, Bioavailability and Function., Pfannhauser, W. Fenwick, G. R. Khokhar, S. Eds. Royal Society of Chemistry 2001 pp 303-306. 

Holst, B. and Williamson, G. 2004. Methods to study bioavailability of phytochemicals. In Phytochemicals in health and disease, ed. R. Fenwick, 25-56. New York Marcel Dekker. 

Before considering the bioavailability of phytochemicals in a food matrix and relating these to biological effects in humans, it is important and necessary to understand that phytochemicals are mostly minor plant constituents whose concentration varies considerably according to, for example, seasonal and agronomic factors, the variety, age, and part of the plant examined. Such variability can lead to serious problems of interpretation of results from epidemiologal studies or human intervention trials if the dose of the phytochemical applied is not determined. 

The ubiquitous flavonoids may be cited as an example of the manifold factors affecting the bioavailability of phytochemicals thus, the bioavailability of flavonoids differs between food sources, predominantly due to the nature of the attached sugar(s). The chemical structure (glycoside or aglycone) of a phytochemical will affect intestinal absorption and first-pass metabolism and will, therefore, modulate its pharmacokinetics [4,6]. 

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