Considerable debate in the dietary supplement community has focused on the human body's ability to absorb bioflavonoid compounds. These substances are present in both foods and dietary supplements and may possess a variety of physiologicaily relevant properties. Such properties include inhibition of cancer development, inhibition of estrogen formation (aromitization), reduction of heart attack risk, prolongation of drug effects, increased bone density, antioxidant effects. Interestingly, it's the antioxidant capability that seems to be a primary factor in some of these other biological effects. Earlier scientific work on the topic seems to point to a general inability of the human intestines to absorb certain bioflavonoid substances. Newer research, however, seems to be reaching the opposite conclusion-at least when the flavonoids are consumed via whole-food sources.

As an example, the general consensus from current research investigating a grapefruit/onion/apple bioflavonoid, quercetin, is that humans absorb about 20% of an oral dose. In contrast, earlier research suggested that less than 1% is absorbed into the blood­stream. Although 20% is not nearly as high as the absorption of macro nutrients like protein, carbohydrate and fat, this percentage is still higher than that of some accepted micronutrients (e.g. non-heme iron). Additionally, the fact that these substances can induce physiological effects in humans is further evidence that they are, indeed, absorbed. Case in point quercetin was found to be partly responsible for the much­publicized Seldane (an allergy drug) deaths of 1998. Victims were taking their medication as prescribed, yet they were dying of toxic overdoses induced by co­consumption with grapefruit juice.

It needs to be stated that there are thousands of bioflavonoid compounds with widely varying physiological effects. Grapefruit juice is just one example. Soy foods have gained widespread popularity as anti-cancer and anti-heart disease foods; they are even being shown to be potential estrogen replacement alternatives. It stands to reason then that the bioflavonoids contained therein must be absorbable. Nonetheless, research needs to be done on the extent to which each bioflavonoid is absorbed.

A final point that should be made about the "bioflavonoid absorption controversy" is that consumers and educators must use discretion when reaching conclusions about bioflavonoid research. In vitro studies that find impressive effects (e.g., estrogen inhibition) may not translate into equally impressive in vivo results. Many factors come into play when a substance is ingested that cannot always be adequately controlled in "test tube studies." Intestinal absorption, gender, hormonal environment, diet, activity level, age, etc., are all factors and thus require population-specific studies on living people to ascertain effectiveness for each person.

In conclusion, although evidence is mounting that bioflavonoids may be important, previously unrecognized nutrients, it is difficult to make blanket statements about their absorption. Finding peer-reviewed, unbiased information on a specific bioflavonoid compound of interest is necessary before making decisions regarding its consumption.