Food Composition Tables
The first category of food constituents are intermediary metabolites formed by digestion of lipids, polysaccharides and proteins. Most of these compounds are common to all living organisms and identical to human endogenous metabolites, thus they cannot generally be used as dietary biomarkers because of their common identity and the impossibility to trace their dietary origin. The possible exceptions are the essential amino acids, essential fatty acids, along with most vitamins, and minerals5 which cannot be produced by humans and must originate from external dietary sources.
The second category of food constituents are those metabolized through transformation by host tissues. Food compounds that are not useful for basic metabolism or which do not correspond to familiar endogenous metabolites are treated as xenobiotics. Examples of exogenous food constituents include polyphenols, alkaloids, carotenoids, chlorophylls, artificial colors, artificial flavors, natural volatiles for flavoring/aroma and Maillard reaction products formed during cooking. The human body maintains a complex defense system consisting of dozens of enzymes and membrane transporters to recognize these foreign and potentially toxic chemicals and to neutralize them by rapid biotransformation and/or elimination. Although not many have been well characterized, host-transformed metabolites retain many of the features of their parent compounds and consequently these exogenously derived metabolites can be quite useful as specific food biomarkers.
The third category of food metabolites are those transformed through microbial metabolism. Microbes have a very different set of enzymes than mammals and given that there are more than 1000 different species of microbes in the human gut () there is an enormous diversity of enzymatic processes that act on food-derived compounds. Certain microbial metabolites can be useful as food biomarkers, although due to the complex relationship between food source, predominant gut microbial species and resulting food metabolites (), microbial metabolites should be treated with some caution when used as food biomarkers.