Nutrition Hotline
QUESTION: I am a vegan and am thinking of fasting all of next year. I intend to eat just fruits and juices. Are there any problems in doing this? Any suggestions?
MJ, Via e-mail
ANSWER: As nutritionists like to say, "No one food has it all." Granted, you are talking about eating a range of different fruits and fruit juices, but you need a host of nutrients found in a wider variety of foods, including vegetables, legumes, and grains. Long term, fruits and fruit juices alone are not likely to provide you with adequate nutrition, particularly if this is a weight loss plan and you are limiting your calorie intake. I'd consider one year to be "long term" — too long to exist on only fruits and fruit juices.
In the short term (a few days to a week) a fast consisting of nothing but fresh fruits and fruit juices may be fine. However, the primary benefit is psychological: some people feel as though a short term fast gives them a "jump start" on a new or improved dietary regimen. It breaks the old pattern and makes them feel as though they're getting a fresh start.
You don't mention the reason for the fast. If you want to lose weight, are battling food allergies, or have another reason to alter your eating style, it may be helpful to get individual counseling from a registered dietitian. Call The American Dietetic Association's referral service at (800) 366-1655 for the name of a local dietitian. Since you are a vegan, ask for a dietitian with expertise in vegetarian diets and/or alternative nutrition therapies.
QUESTION: Is it the caffeine in coffee and tea that reduces iron absorption?
KC, via e-mail
ANSWER: There is a range of substances in foods that either enhance or inhibit the absorption of dietary iron. In green and black tea, the culprit is tannic acid, which can reduce iron absorption in a meal by half or more. In coffee, it's been more difficult to identify: "I am not sure if we really know what it is in coffee that inhibits iron absorption," says Winston Craig, PhD, RD, professor of nutrition at Andrews University in Berrien Springs, Michigan. In any case, the negative effect of coffee on iron absorption is less than the effect of the tannins in tea. In coffee, it's possible that caffeine, a methylxanthine, is the culprit.
According to Craig, "Tannic acid is polyphenolic, so it has great binding power. Because of tautomerism (hydrogen shifts within the molecule), xanthines such as caffeine appear to have phenolic structure, but the preferred chemical structure in xanthines is the ketone."
The bottom line is that both green or black tea and coffee inhibit iron absorption to varying degrees. If you're concerned about your iron absorption, avoid drinking these beverages during, or within two hours of, meals.
Rossander, L, Hallberg, L, Bjorn-Resmussen, E. 1979. Absorption of iron from breakfast meals. Am Journ of Clin Nutr 32: 2484-2489.
