It might seem like you hear it five times each week: The "five-a-day" rule for eating fruits and vegetables, which are loaded with disease-fighting antioxidants such as vitamins C and E and selenium, beta-carotene and Coenzyme Q10.

You probably know, too, that a healthful diet contains limited amounts of fat and sugar and instead emphasizes whole grains, legumes and fish.

But try as you might to follow sound eating principles, your diet may still lack crucial vitamins and minerals. "It's hard to maintain a truly healthful diet with the hectic lives we lead these days," says T. Franklin Williams, M.D., a geriatric medicine professor at the University of Rochester and scientific director with the American Federation for Aging Research.

Does your diet contain all of the following recommended nutrients?

• Beta-carotene. Found in orange, yellow and leafy green vegetables, this antioxidant, like others, helps the body to fight cancer by attacking destructive free radicals. Beta-carotene might also protect against cardiovascular disease and memory loss, but more studies are needed to confirm the association.
• Vitamin A. Liver, whole eggs and whole milk, as well as carrots, cantaloupe and sweet potatoes are examples of foods rich in vitamin A. Other foods, including breakfast cereals, are fortified with the vitamin, which seems to fight cancer and heart disease. Supplementation is rarely necessary and usually not recommended because of the vitamin's potential to reach toxic levels and cause fatigue and problems with the skin, hair, liver and spleen.
• Vitamin C. This antioxidant, found in citrus fruits, berries, peppers, tomatoes and potatoes, is being studied not only for its cancer-fighting potential, but also for its possible memory-enhancing effect. But the vitamin can have a down side at high levels, causing some people to experience chronic diarrhea and kidney stones.
• Vitamin E. Vegetable oils, nuts and green, leafy vegetables contain this antioxidant vitamin, which is being studied for its potential to reduce the risk of heart attacks as well boost immunity and protect against several types of cancer. Recent studies suggest that a vitamin E-rich diet can help protect some people against Alzheimer's disease.
• Selenium. This mineral has antioxidant properties that, according to some studies, could decrease the death rate from lung, colorectal and prostate cancers and might promote heart health.