Nutrition researchers at the U-M and University of California, Los Angeles, have found another reason why health-conscious consumers might consider adding a beta-carotene capsule to round out a balanced diet that includes raw fruits and vegetables.
Clinical studies show that beta-carotene in the pure supplement form is absorbed into the blood stream more efficiently than beta-carotene in raw fruits and vegetables, the researchers say. It appears that the presence of pectin and fiber in raw fruits and vegetables reduces the absorption levels.
Beta-carotene, a precursor substance that the body can convert to vitamin A, is an efficient antioxidant that wipes out free radicals, thus helping protect against strokes and cancer.
Cheryl L. Rock, assistant professor of community health and a scientist in the Program in Human Nutrition at the School of Public Health, and Marian E. Swendseid, professor emeritus of nutritional science at UCLA, reported their findings last year in the American Journal of Clinical Nutrition. Rock and Swendseid noted that this is a promising, but small initial study that we would like to see replicated with a larger sample and different food components.
The researchers based their conclusions on a study of seven subjects whose food intake was controlled at two points and the results checked in a series of blood tests taken over eight days.
On two occasions, the subjects ate 500-calorie meals that were free of pectin. The meals included either macaroni and cheese or yogurt and a sweet roll, and a 25 milligram capsule of beta-carotene. At one of the meals, which were consumed three weeks apart, the research participants also were given 12 grams of citrus pectin.
After each meal, the researchers collected blood samples at eight, 30, 48 and 192 hours (eight days).
Beta-carotene taken orally is absorbed relatively slowly, they noted. The peak absorption levels occur between 2448 hours following a dose.
While there was a wide variation in individual absorption rates, the pattern of reduced absorption with pectin was consistent, the researchers said. We found the mean absorption level when pectin was added to the meal was about half of the level without pectin.
The researchers said the study suggests that if absorption of the concentrated, readily absorbed pill form of beta-carotene is blocked by pectin, there would be even less absorption of the vitamin A precursor that exists naturally in pectin-rich raw fruits and vegetables.
The researchers speculate that pectin interferes with the absorption of beta-carotene by slowing down the digestive process. Pectin delays gastric emptying and disrupts the formation of micellesthe mix of partially digested food, bile components and lecithin that eventually enters the small intestine to be metabolized, they explained.
At least one study done in the late 1970s suggested that cooking fruits and vegetables breaks down pectin and enhances absorption, but the extent to which it helps has yet to be determined. We are in the process of screening candidates for such a study at the University right now, Rock says.
Do their findings so far suggest that consumers should forget about fruits and vegetables altogether and do their vitamin shopping at the pharmacy?
Absolutely not, the researchers stress. Beta-carotene supplements cannot substitute for fruits and vegetables, which contain many other compounds and vitamins required by the body. We also need some fiber to enhance digestion.