The old adage "you are what you eat" takes on added significance when you consider this: every cell in your body is affected by the food you choose. The body needs a balance of diverse nutrients—including minerals, vitamins, amino acids, protein, carbohydrates, water and essential fatty acids—to ensure peak health.

Diet is directly related to an estimated four of the 10 leading causes of death in the United States (heart disease, cancer, stroke and diabetes). So, to stay robust and disease-free, it's important to select a variety of foods that contain essential components. Let's take a brief look at how nutrition can affect your well-being.

Bones and Teeth. Your bones and skeleton keep you together, so you want to keep them strong. This requires an adequate intake of calcium (which is also essential for healthy teeth).

Interestingly, the amount of calcium you consume in the first three decades of life can dramatically influence your future risk for developing osteoporosis—often referred to as a pediatric disease with a geriatric outcome. Osteoporosis, also called brittle bones, is usually not noticeable until later in life. It occurs when not enough new bone is formed or when too much old bone is lost, or both.

One of the best sources of dietary calcium is nonfat milk. Besides providing calcium, milk is rich in vitamin D (it's fortified with 100 IU of vitamin D per cup), which helps the body absorb calcium. Other sources of vitamin D include fortified breakfast cereals and fatty fish such as salmon. Known as the sunshine vitamin, vitamin D can also be synthesized by your body from sunlight. (Note: too much sodium can promote calcium loss.) Other beneficial minerals include phosphorous, which is instrumental in keeping your bones and teeth in good shape, and magnesium, which helps to fight tooth decay by binding calcium to tooth enamel.

Brain. Everything you do and feel depends upon the health of your brain. Nutrition and various neurological states such as depression, insomnia, migraines and mood are intricately related. To give your brain its best chance of functioning properly, you should consume foods that contain a diverse array of nutrients—specifically, thiamin, potassium and vitamins B6, B12 and C, as well as magnesium, among other nutritional substances.

The brain is the richest source of lipids in the human body, and studies reveal that omega-3-polyunsaturated fatty acids are necessary for proper nerve functioning, which has a direct impact on mental function. Nerve cell membranes in the brain are protected by docosahexanoic acid (DHA), a long-chain omega-3 polyunsaturated fatty acid. Adequate amounts of DHA ensure optimal composition of nerve cell membranes in the brain. Vitamin B12 may protect against depression, memory loss, fatigue and possibly dementia. Interesting studies on the antioxidant power of certain foods such as blueberries reveal that age-related memory impairment may be reversed and/or prevented by dietary means. Other substances such as gingko biloba and ginseng may also prove beneficial for neurological health.

Eyes. The trace mineral zinc, highly concentrated in the eye, is believed to help in the prevention of age-related deterioration of the retina by slowing down the disease process. Vitamins A and C also help to prevent macular degeneration by blocking free-radical damage to retinal blood vessels. Research is examining the link between the phytochemicals lutein and zeaxanthin and the prevention of cataracts and age-related macular degeneration. Their antioxidant ability helps to fight sunlight-induced oxidative damage to the retina. People who consume foods high in vitamin A have been shown to have better night-vision than those who don't. Consuming foods high in vitamin C may help protect the lens of your eye from oxidative damage. And some studies have shown that long-term intake of vitamin C can prevent cataract formation. Because significant eye development occurs in-utero and continues during the first year after birth, optimum levels of DHA are crucial for pregnant and lactating women, and consuming fish, canola oil, flaxseed oil and walnuts can add DHA to the diet.

Gastrointestinal System. Nutrition is closely associated with gastrointestinal conditions, including constipation, diarrhea, diverticulosis, irritable bowel syndrome, lactose intolerance and malabsorption, to name a few. All of these may be ameliorated by dietary intervention. One of the most serious gastrointestinal diseases, colon cancer, is highly preventable. Early screening is the key; in fact, when polyps and early-stage cancers are found and removed before they've produced any symptoms, the cure rate can approach 100 percent. Although two recent studies concluded that a high-fiber diet does not prevent colon cancer, both of the studies had extenuating factors that cast doubt on the findings. Health professionals continue to promote the benefits associated with a high-fiber, low-fat diet that is rich in fruit, vegetables, fish, legumes and whole-grain foods.

Heart. As with many disease states, atherosclerosis (hardening of the arteries) can begin early in life. The good news is that this insidious process—plaque building up along artery walls—can be halted, even reversed, if you make healthy changes in your diet and lifestyle. The importance of reinforcing healthy eating habits in our children cannot be overemphasized. Dietary measures to help prevent onset of heart disease include consuming foods high in folate, potassium, and vitamins A, C, and E, as well as foods containing soy protein and those rich in omega-3 fatty acids.

• Folate is believed to reduce levels of an amino acid, homocysteine, that is believed to contribute to the development of arteriosclerosis.
• Omega-3 fatty acids are believed to protect the heart by reducing platelet aggregation (blood clotting), which is associated with increased risk of atherosclerosis.
• Soluble fiber found in whole grains, flaxseeds and oats can help reduce blood cholesterol levels.
• Magnesium is vital for proper functioning of the heart muscle.
• Whole grains, legumes, fish, fresh fruit and vegetables also contribute to a heart-healthy diet.

Prostate. Rich tomato-containing foods, particularly cooked foods like tomato sauce, contain a substance called lycopene that may help to prevent prostate cancer. Evidence also indicates that a diet high in fat and low in intake of fruits, vegetables and soy may increase the risk of prostate cancer development. Soy products containing isoflavones may help to prevent onset of prostate tumors, probably due to the estrogenic effects of the isoflavone genistein. Soy isoflavones are believed to inhibit tumor growth and be beneficial for optimal prostate health by helping to reduce PSA levels in men. Preliminary studies show that the mineral selenium may also protect the prostate.

A Key Discovery. The association between diet and disease goes back to 1915, when Joseph Goldberger, a member of the U.S. Public Health Service, was asked by the U.S. surgeon general to grapple with the crisis of a serious skin disease, pellagra. Goldberger determined that it was due to poor nutrition. In 1937, after Goldberger's death, it was discovered that foods containing niacin, one of the B vitamins, were required to prevent onset of pellagra. This association between diet and health set off a wave of nutrition research that continues to this day.

Dr. Levine is co-director of the Human Nutrition Program at The Rockefeller University in New York City.