Web Date: November 9, 2012
Dietary Fiber Data Misses Class Of Compounds
Common wisdom says to eat plenty of fruit and veg, to pump our bodies with antioxidants, which form in the body as it digests plants’ polyphenols, vitamins, and carotenoids. But according to a Spanish food chemist, nutritionists have overlooked perhaps half of all dietary polyphenols (J. Ag. Food. Chem., DOI: 10.1021/jf303758j).
Fulgencio Diego Saura-Calixto at the Institute of Food Science, Technology and Nutrition, a national research center in Madrid, first became aware of these neglected compounds in the late 1980s, when looking at the amount of dietary fiber in carob pods. Researchers had already determined that the plant’s dietary fiber made up 40% of its dry weight. But Saura-Calixto found that the polysaccharides and lignin assumed to be the fiber’s constituents actually accounted for just less than half of that 40%.
Saura-Calixto discovered that the remaining 21% of the plant’s weight consisted of nonextractable polyphenols, or NEPP. These compounds don’t dissolve in organic solvents, such as methanol and acetone, used to extract polyphenols from dietary fiber for use in chromatographic or biological assays. NEPPs tend to be big molecules. They include tannins, some proanthocyanidins, and hydrolyzable polyphenols. Acidic solutions or ones containing enzymes, rather than organic solvents, can dissolve and extract them. But because these polyphenols don’t dissolve in conventional solvents, most researchers, nutritionists, and doctors are unaware of their existence, Saura-Calixto says.
These ignored compounds could affect gastrointestinal health, Saura-Calixto argues in the new paper, and deserve researchers’ attention.
The body isolates these molecules much better than chemists can, he says. Instead of being absorbed by the small intestine, where most extractable polyphenols break down, most of these long-chain molecules are digested by bacteria in the lower gastrointestinal tract, according to other researchers’ studies in vitro and with rats. Saura-Calixto argues that when polyphenols are metabolized in the lower intestine, antioxidants are released and go on to affect gastrointestinal health, including perhaps preventing diseases such as colon cancer. Lab studies over the past ten years, in cell cultures and animal models, have shown that in the colon, NEPPs produce active metabolites that alter expression of genes linked with tumor growth. Saura-Calixto hopes other scientists will study how cells metabolize NEPPs.
“There are big gaps in what we know is in food,” says Jeremy Spencer, an expert in metabolism chemistry at the University of Reading, in the U.K. But Spencer says that extracting NEPPs to study general cell metabolism may not be worth the effort. “If you’re using any cells other than the gastrointestinal cells, you won’t see anything happening,” he says. He agrees that knowing more about the class of molecules would be useful to understand diseases of the lower intestine.
Saura-Calixto intends to use what he is learning about NEPP to develop a dietary supplement. “Our aim is to continue to prove the health properties of these compounds,” he says.
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