At least 34 different nutrients build, maintain and protect our bodies and keep us healthy. But most of us take in insufficient levels of certain nutrients.
"Americans who do not consume the recommended five servings of fruits and vegetables a day are likely to have suboptimal levels of certain nutrients that increase their risks for important chronic diseases," says Dr. Kathleen Fairfield of the Maine Medical Center and author of a review on vitamins and minerals in the Journal of the American Medical Association.
How can you tell if you're not getting enough of a vitamin or mineral? "You can't," says David Katz, a professor of medicine at the Yale School of Public Health. "Routine tests will not show inadequate intake." The only real way to find out is to keep track of what you eat over a period of time and go over the records.
If you find you're not getting enough nutrients, should you take supplements? If you aren't going to change your eating habits, yes, says Katz, "but it's vital to remember the following: supplement, not substitute. There is no substitute for a healthful dietary pattern."
Here's the lowdown on nutrients U.S. diets often lack:
Why you need it: Vitamin A is important for vision, gene expression, cellular differentiation, growth, immune function and healthy bones, teeth and hair.
How much: Men: 3,000 international units; women: 2,310 IU.
Tolerable upper level for retinol (vitamin A found in animal products and supplements): 10,000 IU for both men and women.
Getting enough: Liver, eggs and whole milk are good sources of vitamin A, but it's also added to skim milk and other fortified products like cereal. Brightly colored fruits and vegetables, such as carrots (one: 8,666 IU), kale, spinach (1/2 cup: 11,458 IU) and cantaloupe (1 cup: 5,411 IU) contain carotenoids, which are precursors for vitamin A.
Why you need it: Vitamin E, an antioxidant, counteracts cell damage, reducing risk for heart disease and cancer. It's also important for the formation of red blood cells and muscles.
How much you need: 15 milligrams for men and women.
Tolerable upper level: 1,000 mg.
Getting enough: Many E-rich foods are also packed with fat and calories. Americans get 12 percent of their vitamin E from mayonnaise and salad dressing, and 9.5 percent from oil. Almonds (1 ounce: 7.4 mg) and hazelnuts (1 ounce: 4.3 mg) are relatively rich in vitamin E.
Why you need it: Vitamin C, an antioxidant, counteracts cell damage from smoking and pollution and helps protect against cancer. It helps the body absorb iron, strengthens blood vessels and maintains gums.
How much you need: 90 mg for men; 75 mg for women.
Tolerable upper level: 2,000 mg.
Getting enough: Fruits (especially citrus) and vegetables are the best food sources. Orange: 70 mg; green pepper (1/2 cup): 60 mg; broccoli (1/2 cup, cooked): 51 mg.
Why you need it: Magnesium helps muscle and nerve functions, steadies heart rhythms, supports the immune system, strengthens bones and regulates blood sugar.
How much: Men: 400-420 mg; women: 310-320 mg.
Tolerable upper level of supplemental magnesium: 350 mg.
Best sources: Green leafy vegetables (1/2 cup cooked spinach: 78 mg), legumes, nuts and seeds (1 ounce cashews: 74 mg) and whole grains (1/2 cup cooked oat bran: 44 mg).
Why you need it: Calcium maintains strong bones, and plays a role in blood clotting, blood pressure and muscle and nerve function.
How much you need: 1,000 mg for men and women ages 19-50; 1,200 mg for men and women ages 51 and up.
Getting enough: Dairy products are your best bet (stick to low or no-fat). Skim milk (1 cup): 302 mg; plain nonfat yogurt (8 ounces): 452 mg.
Why you need it: Potassium is necessary for muscle contractions (including heartbeat), transmission of nerve impulses and balancing fluid and electrolytes. Diets rich in potassium lower blood pressure.
How much you need: 4,700 mg per day — which more than 90 percent of Americans don't meet. Experts suggest potassium supplements, but check with your physician first.
Getting enough: Potassium is found in milk, fruits, vegetables and grains and is particularly high in potatoes (610 mg) and bananas (422 mg).
Charles Stuart Platkin is a nutrition and public-health advocate. Write to firstname.lastname@example.org.