Definitions of functional food vary but are essentially based on the foods' ability to enhance the quality of life, or physical and mental performance. Health benefits are best obtained through a varied diet containing fruits, vegetables, grains, legumes and seeds. However, fortified foods and dietary supplements have been marketed and the food industry have made functional food one of their current leading trends. Recently, the number of functional foods that have a potential benefit on health has hugely grown and scientific evidence is supporting the role of functional foods in prevention and treatment of several diseases. Cancer, diabetes, heart disease and hypertension are the most important diseases that can be treated or prevented by functional foods; other diseases are osteoporosis and arthritis. The majority of cancers in the USA have a nutrition / diet component suggesting a great impact of functional food and foods components on incidence and treatment of cancer. Numerous factors complicate the evaluation of scientific evidence regarding the benefit of functional foods such as the complexity of food substance, metabolic changes associated to dietary changes, the frequency of ingestion, combination of ingestion with other functional foods, and the lack of biological markers of disease development.

List of Functional Foods
Quite a number of foods could be considered functional foods. Some of them include:
Acai berry is growing in popularity
Flaxseeds may lower the rate of prostate cancer and are a wonderful stool softener, reducing the risk for constipation.
Goji berry is from Asia
Probiotics have good bacteria
Royal jelly is a functional food with lots of vitamins, minerals, protein, fatty acids, and other substances that have an influence on the immune system, energy, and well being.
Salmon Roe has important fatty acids and phospholipids
Soy sauce is used widely in Japan
Whey protein, derived from milk, is a good supplement for those who have a low intake of protein.

Functional foods stand for a new category of remarkably promising foods bearing properties (i.e., low cholesterol, antioxidant, anti-aging, anticancer, etc.) that have already rendered them quite appealing. There are many classes off functional foods (pro- and pre-biotics, dietary fiber, low fat, etc.), and their definition is occasionally confused with that of nutraceuticals. Consumers' main skepticism regarding functional foods resides in the veracity of health claims and in the low and often inadequate control of their claimed properties.

Is Coffee a Functional Food?
The worldwide use of coffee for social engagement, leisure, enhancement of work performance and well-being is widely recognized. Depending on the quantities consumed, it can affect the intake of some minerals (K, Mg, Mn, Cr), niacin and antioxidant substances. Epidemiological and experimental studies have shown positive effects of regular coffee-drinking on various aspects of health, such as psychoactive responses (alertness, mood change), neurological (infant hyperactivity, Alzheimer's and Parkinson's diseases) and metabolic disorders (diabetes, gallstones, liver cirrhosis), and gonad and liver function. Despite this, most reviews do not mention coffee as fulfilling the criteria for a functional food. Unlike other functional foods that act on a defined population with a special effect, the wide use of coffee-drinking impacts a broad demographic (from children to the elderly), with a wide spectrum of health benefits. However, there is a down side to coffee drinking. Coffee can cause anxiety and blood pressure problems, and it could also cause or aggravate heart rhythm abnormalities. Hence, it is possible to claim that coffee, when limited to one cup a day could be a functional food, but not when 3 or more cups are drank a day, particularly if the coffee is ingested later in the day and causes insomnia.