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United States Department of Agriculture

Agricultural Research Service

When do you know when enough is enough?
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Gerald F. Combs Jr.

That our estimates of nutrient requirements are as accurate as they are always has been a source of wonderment for me, given the enormous differences that exist among and between people.

After all, why should my needs to be the same as the next fellow, simply because we are of the same age and gender?

To be very technical about it, they are not. Nutrient needs depend mainly on body size and level of physical activity, but also on genetics and physiological status. This would imply that each individual would need his or her own set of nutritional guidelines. Indeed, the health field is moving in the direction of individuating nutritional guidance in much the same way as other health guidance is delivered.

In the meantime, nutritional information is provided largely on a populationwide basis. This is possible because a 100 years of nutrition research has revealed that the fundamental nutritional needs of humans are similar.

That information provides the basis for the collective reasoning and opinions that are provided by expert committees of the National Academy of Sciences and the U.S. Department of Agriculture to develop our nation's nutritional recommendations and dietary guidelines.

Yet, we hear in one form or another almost daily arguments for certain nutrients as health-promoting agents at levels unrelated to the recommendations of those experts. Is this simply marketing? And if so, is it incorrect or, worse, risky?

The fact is that our scientific thinking about nutrition is changing. Once our concern was limited to the prevention of scurvy and other diseases caused by the lack of certain nutrients. Now, researchers are asking how nutritional status can affect our risks to chronic diseases such as heart disease, cancer and osteoporosis.

While the definitive evidence still is emerging, already it seems that selenium may be able to reduce cancer, that vitamins C, E and the mineral copper are associated with reduced cardiovascular disease and that calcium and, perhaps, copper have roles in preventing osteoporosis.

Incorporating such findings into our national nutritional recommendations is a process that calls for solid evidence and scientific consensus. Both tend to develop along fairly long time lines, in effect protecting consumers from weakly supported ideas.

This built-in tilt toward safety is a good thing, as many nutrients, like so many other things in life, can be overdone. For example:

  • Too much selenium (more than 400 micrograms/day) may cause bad breath, white blotchy nails and brittle hair.
  • Too much vitamin C (more than 1 gram/day) may increase the risk of kidney stones.
  • Too much vitamin A (more than 3,000 micrograms/day) can increase the risk of fetal malformations in pregnant women.
  • Too much zinc (more than 40 milligrams/day) can impair immunity and interfere with the utilization of both iron and copper.
  • Too much copper (more than 10 milligrams/day) may cause gastrointestinal upset.

The best strategy for good nutrition, therefore, is to get only enough of a good thing.

Perhaps, the most consistent research finding in the field of nutrition is that healthy people tend to have highly diverse diets. Their diets contain, over months and years, a wide variety of foods particularly, fruits and vegetables. The message to consume a variety of foods is central to the familiar USDA Food Guide Pyramid. Variety is an option that modern supermarkets, with tens of thousands of food items on their shelves, make available to most Americans.

To decide whether you get enough of those nutrients that can affect your health, start by asking how diverse your diet is and whether you are choosing the three to five servings of fruits and vegetables you should each day. You may want to visit some informative Web sites for information about the dietary guidelines ( as well as the nutrients in the foods you eat (

Having done this, you may decide that nutrient-fortified foods or dietary supplements may be useful. But if your diet is sufficiently diverse, you may decide that enough is enough.

Last Modified: 10/23/2006
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