National dietary guidelines have been historically based on the type of food and amount of nutrients needed for a body to properly function. Many functional foods (e.g., fresh fruits and vegetables, whole grains) are already part of current dietary guidelines. In addition, optimal levels of the bioactive food components in functional foods have not been established. 

Generally, a high-fiber, low-fat diet rich in plant foods (including 5 to 9 servings of fruits and vegetables per day) is recommended to promote good health and help reduce the risk of a variety of diseases and conditions (e.g., cancer, cardiovascular disease).

Common functional foods from plant sources include:

• Oats. An ideal source of soluble fiber that may lower cholesterol levels and reduce the risk of heart disease.
  
  
• Soy. Associated with lowering cholesterol levels (although this is now under debate) and reducing the risk of cardiovascular disease, cancer and when fortified with calcium, osteoporosis. There is speculation that soy may help alleviate menopausal symptoms.
  
  
• Flaxseed. A source of omega-3 fatty acids and the phytochemical lignan. It may be associated with lowering of cholesterol levels and anti-estrogen effects, such as reducing the risk of cancer.
  
  
• Tomatoes. A rich source of the phytochemical lycopene, which acts as an antioxidant and may be associated with reducing the risk of some types of cancer.
  
  
• Garlic. Contains the phytochemical allicin, which may stimulate certain enzymes in the body and have the effect of reducing the risk of cancer. It might also be associated with lowering blood pressure and levels of cholesterol, as well as helping to prevent bacterial and viral infections.
  
  
• Cruciferous vegetables. Vegetables from the cabbage family that naturally contain many different phytochemicals (e.g., indole, isothiocyanate) associated with potentially reducing the risk of certain cancers.
  
  
• Citrus fruits. These fruits contain a variety of healthy substances, including vitamin C, folate, fiber and various phytochemicals.
  
  
• Cranberry. This berry has been associated with a reduced risk of urinary tract infections. Flavonoids in cranberry likely prevent bacteria from attaching to the walls of the bladder. Cranberry juice also may help prevent stomach ulcers and gum disease. 
  
  
• Curry. A mixture of several spices that may help stop the growth of some cancer cells.
  
  
• Dark chocolate. Some research appears to indicate a decline in blood pressure after eating just under 4 ounces of dark chocolate a day. Phytochemicals such as flavonoids appear to be responsible for this effect. Milk chocolate does not appear to have the same effect, and it should be noted that most types of chocolate contain high levels of fat and calories.
  
  
• Tea. Both green and black teas may contain various types of phytochemicals (e.g., polyphenols, flavonoids) that are associated with a reduced risk of cancer. For centuries, members of the Hispanic community have used mate tea to treat disease.
  
  
• Wine, grapes. The skin of purple grapes contains the phytochemical resveratrol, which may help prevent heart disease and cancer. Fermented products that have spent significant time in contact with the skin of these grapes (e.g., red wine) also are a good source of resveratrol.  Moderate consumption (one or two 4-ounce glasses) of red wine also may help raise a person’s “good” HDL cholesterol while lowering “bad” LDL cholesterol. 

Common functional foods from animal sources include:

• Fish. Fatty fish, such as salmon and tuna, are good sources of omega-3 fatty acids, which may lower the risk of cardiovascular disease. The American Heart Association (AHA) recommends two servings per week of these types of fish.
  
  
• Dairy products. Dairy products are a good source of essential nutrients (e.g., calcium). Certain yogurts or other cultured products are also a good source of probiotics, which can help balance bacteria in the digestive tract and may help prevent gastrointestinal disorders. Consuming dairy products is associated with lowered risks of osteoporosis.
  
  
• Beef. Beef is a good source of conjugated linoleic acid (CLA), a fatty acid that may help prevent certain cancers.

Most functional foods provide their health benefits in regular serving size amounts. Simply eating a healthy, balanced diet may lessen the risk of many diseases or conditions, as well as promote good health. A balanced diet should include appropriate daily servings of whole grains, fruits, vegetables, meat, fish, poultry and dairy products.

Daily physical exercise and maintaining a healthy lifestyle are also important components to overall health. Functional foods can not offset poor health habits (e.g., smoking, lack of physical activity). Nor are they a cure for many diseases or conditions. Many health problems are affected by a variety of factors beside diet (e.g., genetics, stress, lifestyle choices). Thus, functional foods may be best suited for assistance in prevention or in mild cases of disease rather than as treatment.

Although dietary supplements exist for certain phytochemicals, obtaining these substances through foods is preferred. In some cases, supplements do not provide the same level of benefit as when it is consumed in food (e.g., lycopene, flavonoids). In other cases, the supplements have not been proven safe. For example, drinking green tea is generally considered safe but green tea supplements may actually be harmful. Also, beta-carotene in supplement form has been associated with an increased risk of lung cancer in smokers.