Here are three reasons why following a healthy diet is important:

1. to maintain health by preventing loss of muscle strength, bone mass, and vitamin deficiency states;
2. to prevent diseases such as heart attacks, strokes, obesity, osteoporosis, and certain cancers; and
3. to help control and/or treat chronic diseases and conditions such as high blood pressure, diabetes mellitus, sleep apnea, and celiac disease.

The body requires carbohydrates, fats, proteins, vitamins, and minerals to maintain healthy organs, bones, muscles, and nerves, and to produce hormones and chemicals that are necessary for the proper function of organs.

Vitamins and minerals are naturally occurring substances that are essential for the growth and function of the body. Vitamins and minerals are both necessary (in small amounts) for normal chemical reactions (metabolism) in the body.

Obesity and heart attacks are major public-health problems in the United States and other countries. Therefore, most dietary recommendations are aimed at preventing these two diseases. Obesity comes over time by eating more calories than the body burns. Obesity, in turn, can contribute to the development of many diseases such as heart disease, diabetes, sleep apnea, liver disease, arthritis, high blood pressure, gout, gallstones, and certain cancers.

To lose weight or maintain a healthy weight, it helps to eat more low-energy-dense foods. Low-energy-dense foods (such as vegetables and fruits) contain few calories per unit volume of food so that one can eat a large volume of it (for example, lettuce) without taking in many calories. One should also eat less of the high-energy-dense foods such as fats, egg yolks, fried foods, sweets, and high-fat salad dressings. Foods with a high energy density also often have high cholesterol and saturated fat content. One should also eat less of those foods that provide calories but little other nutrients, such as alcohol and many packaged snack foods.

The Dietary Guidelines for Americans, published in 2010 by the United States Department of Agriculture (USDA), contains guidelines for healthy diets based upon review of scientific studies for people above 2 years of age. These guidelines recommend that a healthy diet should:

• balance calories with physical activity to manage weight;
• consume more of certain foods and nutrients such as fruits, vegetables, whole grains, fat-free and low-fat dairy products, and seafood;
• consume fewer foods with sodium (salt), saturated fats, trans fats, cholesterol, added sugars, and refined grains.

MyPyramid is an online animated program to help a person customize his/her diet by choosing proper foods and portion sizes based on the individual's age, sex, and activity level. The key objectives of the MyPyramid Plan are to help a person get the most nutrition (proteins, vitamins, and minerals) out of the recommended number of daily calories and to achieve a balance between food intake and physical activity to maintain a healthy weight. The MyPyramid Plan recommendations include:

• Make half your grains whole.
• Vary your veggies.
• Focus on fruit.
• Get your calcium-rich foods.
• Go lean with protein.
• Find your balance between food and physical activity.

Vitamins and mineral supplements are important both in preventing deficiency states as well as in preventing diseases. Most diseases resulting from vitamin deficiencies such as scurvy (vitamin C deficiency), night blindness (vitamin A deficiency), and beriberi (thiamine deficiency) occur mainly in third-world countries and are almost nonexistent in the United States. But certain special populations in the United States can develop vitamin or mineral deficiencies, and thus require dietary supplements. For example,

• severely malnourished alcoholics can develop nerve damage from thiamine deficiency;
•  individuals lacking sun exposure can develop bone disease from vitamin D deficiency;
• pernicious anemia is a condition associated with nerve damage, which can result from vitamin B12 deficiency; and
• people with celiac sprue can also develop vitamin deficiencies, as well as iron deficiency.

For these special populations, vitamin supplements are important to prevent these deficiencies.

Vitamin supplements are used to prevent deficiencies and also to prevent diseases. Certain vitamin supplements (such as folic acid, vitamin B6, and B12) have been used to lower blood levels of homocysteine, which may help prevent heart attacks. Folic acid fortification in cereals and vitamin supplements has been found to decrease the risk of birth defects in the developing fetus in pregnant women.

Diets low in simple sugars are important in controlling blood glucose levels in people with diabetes mellitus. When the condition cannot be adequately controlled by diet alone, medications (sometimes including insulin) are required.

• The DASH diet is recommended to lower blood pressure. If dietary measures alone are not sufficient, medications are frequently prescribed by doctors (sometimes in combination) to lower blood pressure.
• A gluten-free diet is the primary treatment for celiac disease (celiac sprue). Since people with celiac sprue may have difficulty absorbing nutrients and vitamins, some people with this condition may also need calcium, iron, and vitamin supplements.
• Therapeutic lifestyle changes (TLC) are important treatments for high blood levels of cholesterol, especially the "bad" ( LDL) cholesterol. When TLC are not sufficient, then medications are usually indicated to lower blood lipid levels.