Polyphenols Metabolism

In the overall evaluation of the efficacy of wine polyphenols in the prevention of atherosclerosis and heart disease, it is important to determine how well such compounds may be absorbed through the gastrointestinal tract, distributed to various target tissues, and metabolized. Unfortunately, there is limited information in humans, which has led to uncertainty that in vivo, these compounds could express activity of physiologic significance. Because such compounds occur as complex, highly-variable mixtures in plant materials, it is difficult to study their bioavailability and physiologic effects accurately.

Nevertheless, studies continue to investigate the absorption and metabolism of single polyphenolic compounds. In general, the absorption of polyphenols varies depending on the type of food, the chemical form of the polyphenols [(polymerized forms may be less absorbed (Muller & Fugelsang, 1997)], and their interactions with other substances in food, such as protein, ethanol, and fiber (Bravo 1998; Scalbert and Williamson, 2000). It is important to note that polyphenols from wine may be absorbed better than the same substances from fruits and vegetables, because the ethanol may enhance the breakdown of the polyphenols into smaller products that are absorbed more readily (Goldberg, 1995). The elimination of free catechin, consumed as reconstituted red wine, was increased 22% by ethanol in human subjects (Bell et al., 2000). However, since these substances rarely exist as free compounds in plasma (Manach et al., 1995, 1999), the true significance of this observation remains unknown.

Studies in experimental animals and humans indicate that some polyphenols, at least, can be absorbed (Manach et al., 1996; Manach et al., 1995; Ueno et al., 1983; de Vries et al., 1998, Clifford et al., 1996; Maiani et al., 1997; Clydsdale, 1999). As has been described elsewhere, bacteria that colonize the ileum and cecum play a significant role in the metabolism of polyphenols and are important for their absorption (Bravo, 1998; Scalbert and Williamson, 2000). In general, peak blood levels of flavonoids occur between 2 and 3 h after consumption and the elimination half-life varied between 5 and 17 h depending on the particular flavonoid or the food source (van het Hof et al., 1998; Hollman et al., 1996). However, it is important to note that some human studies of polyphenol absorption and metabolism may be misleading due to administration of pharmacologic (4 g) doses (Gugler et al., 1975). Handling of pharmacologic doses may not reflect the normal physiologic mechanisms of absorption and metabolism of dietary flavonoids that would occur with usual dietary intakes.

It should also be mentioned that evidence is accumulating that not all polyphenols need to be absorbed to be effective in reducing oxidative stress in plasma. For example, studies have shown that tannins become complexed in the gut and may not be absorbed, yet are able to reduce plasma levels of peroxides observed after consumption of a high-fat meal (Ursini et al., 1999).

0 0

Post a comment