Too much of a good thing isn’t always good. This even holds true for healthful foods and essential nutrients. To maintain health, we need to consume all of our essential nutrients in adequate amounts.

We have known for centuries that nutrient deficiencies can result in serious health problems. However, now it is becoming clearer that consuming excessive amounts of even an essential nutrient can be problematic, whether it comes from foods or dietary supplements.

QUESTION: What are the key roles of essential nutrients in prevention of disease?

ANSWER: A balance of essential nutrients is needed in virtually all cells of the body to support their normal function. This is especially true for those cells that protect us from attack by bacteria and viruses as well as cells that inhibit the development of cancer.

Some of these protective cells provide a barrier to invaders, and some are involved in disabling invaders that get across a barrier. For example, the cells lining the intestinal tract and lungs are key barriers between our body and potential invaders in our external environment. Underlying these barrier cells are cells of the immune system that detect and disable most invaders.

An adequate nutrient supply is critical for the function of these barrier cells that provide protection between us and the external environment. Consequently, any nutrient deficiency has the potential to make us more susceptible to disease.

At the other extreme of nutrient intake, there also are concerns. More is not always better. Excessive intake of one or more nutrients can cause problems. Science is gradually uncovering various problems with the excessive intake of some nutrients, and it appears that consuming too much of some nutrients for too long can promote certain types of diseases such as cancer.

Q: If a food or natural chemical in a food shows potential in preventing cancer, is it likely to be helpful in treating cancer?

A: Not necessarily, according to two researchers. Dr. Thomas Wang at the Diet, Genomics, and Immunology Laboratory in Beltsville, Md., studied resveratrol, a compound in grapes that has been found to inhibit the growth of cancerous tumors. When he fed resveratrol to lab animals with cancerous tumors, he found that it first inhibited the growth of the tumors but over time ended up promoting tumor growth.

At the Antioxidants Research Laboratory in Boston, Dr. Jeffrey Blumberg studied supplemental vitamins C and E and beta carotene taken by people during chemotherapy and radiation therapy during human clinical trials. He concluded these supplements support the growth of cancer cells as well as normal cells, indicating antioxidant supplements should not be used during cancer therapies.

Q: What are the benefits or risks of taking a daily multivitamin-mineral dietary supplement?

A: Nutritional supplements can clearly benefit people whose diets are low in calories and limited in specific food groups and overall variety. However, people who consume adequate amounts of wholesome foods from a balanced diet might be better off with limited or only moderate use of supplements.

Perhaps the best rule of thumb to follow for a healthy diet is found in the memorable line from the late comedian Rap Reiplinger, "Not too much, not too little, but just right."

Joannie Dobbs, Ph.D., C.N.S., and Alan Titchenal, Ph.D., C.N.S., are nutritionists in the Department of Human Nutrition, Food and Animal Sciences, College of Tropical Agriculture and Human Resources, University of Hawaii-Manoa. Dobbs also works with University Health Services.