What are functional foods?
Functional foods are whole foods that naturally contain health-promoting substances or contain added health-promoting substances. Also, functional foods can act in concert with herbal supplements and drugs in the prevention and treatment of disease. In contrast, neutraceuticals are active ingredients isolated from foods and prepared in a medicinal form, and phytochemicals are chemical substances obtained from plants that are biologically active but not nutritive.
The concept of functional foods is not new, as the medical and physiological benefits of some foods have been known for ages. The enhanced interest in functional foods is driven by a confluence of three factors: a greater understanding of the way functional food components exert medical benefits, the desire and capability of the food industry to produce new products based on these concepts, and interest and acceptance by the consumer. The new discipline of functional food science places emphasis in gaining knowledge of how functional food components can affect certain body functions related to health and disease.
Several methods exist to modify a natural (unmodified) food to be a functional food. Any natural food with an active ingredient can be modified to improve its effectiveness as a functional food. For example, the concentration of a particular component that is naturally present in the particular food could be increased to produce beneficial effects. Similarly, a component known to cause a detrimental health effect could be removed. A component not normally found in a particular food but known to produce a beneficial effect could be added. A component whose intake is usually excessive and could cause deleterious health effects can be replaced by a component with beneficial health effects. Finally, the bioavailability or stability of a component known to reduce the disease-risk potential of a particular food could be improved.
The value of functional foods has been recognized by the American Dietetic Association and many other organizations. However, consumers should carefully weigh the health claims and overall value of functional food products. The U.S. Food and Drug Administration (FDA) considers functional foods as foods, not drugs, so manufacturers cannot claim that a functional food can cure, mitigate, treat, or prevent any disease. The Nutrition Labeling and Education Act (1990) does permit some exceptions, known as health claims, which state that a substance included in the diet on a regular basis “may help reduce the risk” of a certain disease. Most companies, however, prepare label claims based on maintaining a normal, healthy structure or function of the human body. Although so-called structure/function statements do not need approval from the FDA, the label must show a disclaimer statement to this effect. An example of a structure/function claim is the statement “helps build strong bones.”
In general, foods in their natural state, such as fruits and vegetables, oily fish, whole grains, and nuts and seeds, are among the best functional foods. They have active ingredients and a fine overall nutritional balance.
Probiotics and prebiotics. Probiotics are beneficial bacteria added to dairy products such as yogurt to improve gastrointestinal health. Prebiotics (oligosaccharide carbohydrates that are abundantly found in artichokes, shallots, and onions) are growth media for beneficial bacteria.
Polyphenols, anthocyanidins, flavones, and tannins. Polyphenols are found in a variety of foods. They have an antioxidant effect, thought to reduce the incidence of cancer and coronary heart disease. Anthocyanidins are found in fruits, catechins in tea, and flavones in citrus. Flavones are widely distributed in fruits and vegetables, and lignans are found in flax, rye, and some vegetables. Tannins are found in cranberries and cocoa.
Dietary fibers. Dietary fibers, those food components obtained from plants that cannot be digested by the body, are classified as either insoluble or soluble. Insoluble fiber consists of plant cell-wall components, particularly cellulose, which form bulk in the diet and promote the regularity of bowel movements. Soluble fiber dissolves in water and thickens to form gels. Soluble fiber consistently has been shown to reduce total cholesterol and LDL (low-density lipoprotein), or bad cholesterol, in the blood. This reduction occurs through the reduced dietary fat and cholesterol uptake of the intestine and through increased fecal excretion of bile acids (which are derived from cholesterol).
Oats and barley contain an important fiber known as beta-glucan, a complex carbohydrate made of glucose units. Oatmeal has become a popular cereal for this reason, and oat bran too is marketed as a cereal or as an ingredient in other foods. Guar gum, pectin, and psyllium also contain abundant soluble fiber.
Omega-3 fatty acids. Omega-3 fatty acids have been found to be beneficial for the prevention of heart disease. (Omega-3 refers to chemical structure.) The omega-3 fatty acids eicosapentaenoic acid and docosapentaenoic acid are found in abundance in fatty fish, such as herring, anchovies, mackerel, salmon, and sardines. Another omega-3 fatty acid, linolenic acid, is found in walnut, soybean, and canola oils. The fish oils have a more protective effect than the plant oils. Omega-3 fatty acids are converted to biologically active compounds such as prostaglandins and leukotrienes, which have anti-inflammatory, antithrombic, antiarrythmic, and vasodilatory effects.
Plant sterols. Plant sterols are similar in structure to cholesterol and are found in the diet as sitosterol, stigmasterol, and campesterol. A compound made from sitosterol, known as stanol ester, is incorporated into a commercial margarine as a cholesterol-reducing agent. Nuts act as antioxidants and have a cholesterol-lowering effect.
Carotenoids. Carotenoids represent a large group of natural pigments found in plants (including yellow and orange fruits and vegetables such as carrots, apricots, squash, and sweet potatoes) and in dark green vegetables such as spinach, kale, and collard greens. The most common dietary carotenoids are alpha-carotene, beta-carotene, and lutein. Because of the nature of their molecules, carotenoids have strong antioxidant activity. Evidence shows that carotenoids have a protective effect against heart disease and some cancers.
Most studies on functional foods have been observational; the great time and expense involved in clinical trials make observational studies the best option. Although there are many types of observational studies of functional foods, all attempt to relate the incidence of disease in a population with the dietary intake of a particular food. Observational studies can provide data from a large number of people in a relatively short period of time at low cost; however, these studies have been criticized for not controlling variables and for being subject to bias. Observational studies, however, can provide a strong indication of trends.
Intervention studies involve assigning participants to control or treatment groups, marked by various types and amounts of functional food components. The groups are followed over time, and researchers note the incidence of disease among groups. In a clinical trial, the groups are randomly assigned. Randomization reduces biases in evaluation of treatment and control groups by making the groups equal in all respects except for the treatments applied.
Before beginning a study, researchers start with basic scientific knowledge regarding functions that are sensitive to modification by food components. These functions could be genetic, cellular, biochemical, or physiologic. Quite often, instead of examining the effect of the component on an outcome, researchers may use a marker that is related to the outcome. For example, researchers could study the effect of an ingredient on cholesterol levels instead of waiting for heart disease to develop. Markers must be able to predict the beneficial or detrimental effects of a food component. The body could respond to the intake of a food component through changes in body fluid levels of certain metabolites or enzymes. Measurement of changes in body tissues, such as extent of narrowing of carotid arteries, can be related to the development of atherosclerosis. Markers need to be sensitive and specific to the disease condition.
Probiotics. Probiotics are bacteria established in the intestinal tract that exert a beneficial effect. The term “probiotics” was coined to contrast with antibiotics, which destroy harmful bacteria. Probiotics are normally added to dairy products as lactobacilli and bifidobacteria. These bacteria promote improved intestinal microbial balance with a reduction in harmful microbes. Probiotics are beneficial in preventing infection and enhancing the immune system. They aid in preventing pathogens from entering the bloodstream through the mucosal epithelial cells by increasing mucin production and reducing permeability.
Probiotics also enhance antibacterial and anti-inflammatory activities of the intestinal epithelium by stimulating synthesis of specialized protective proteins. Clinical studies have shown that probiotics can reduce the symptoms of irritable bowel syndrome and can be beneficial in maintaining remission in cases of ulcerative colitis and pouchitis (inflammation of the intestinal wall). Other studies have shown that probiotics can help prevent necrotizing enterocolitis in infants.
Prebiotics. Prebiotics are indigestible oligosaccharide carbohydrates that can be fermented by lactobacilli and bifidobacteria. Prebiotics, along with probiotics, act in concert to produce the same beneficial results. Prebiotics can serve as fermenting media for probiotic bacteria already in the intestinal tract or in combination with introduced probiotics. Studies are ongoing to determine appropriate conditions for the use of intact cereals as media for the growth of probiotic strains and to develop processing methods to isolate sources of water-soluble fiber that can serve as prebiotics.
Plant sterols. A review article indicated that plant sterols and tree nuts were beneficial for the prevention of coronary heart disease in most clinical trials, while flavonoids in dark chocolate may protect LDL cholesterol from undergoing oxidative modification. Plant sterols are believed to interfere with the absorption of cholesterol from the small intestine by preventing it from dissolving in the micellular structure. Plant sterols are added to margarine products as a cholesterol-lowering agent, but some consumers have concerns about weight gain. A meta-analysis of fifty-nine randomized clinical trials found that plant sterols that were added to milk, orange juice, or yogurt lowered total cholesterol and LDL cholesterol, but did not do so when added to breads or meats.
Nuts. Many clinical studies have shown that the consumption of walnuts, almonds, pecans, pistachio nuts, and macadamia nuts result in lowered total cholesterol and LDL cholesterol. Epidemiological studies found an inverse relationship between nut consumption and the risk of coronary heart disease. Scientists believe that the beneficial effect of nuts comes from their high content of polyunsaturated fatty acids. Nuts may improve endothelial (blood-vessel wall) function too, resulting in better vasodilation. Nuts also may act as antioxidants, reducing LDL oxidation, one of the steps leading to plaque formation.
Polyphenols. Polyphenols, also known as flavonoids, are widespread in commonly consumed foods. Several hundred have been identified in fruits, vegetables, legumes, whole grains, and nuts, and in beverages such as tea, coffee, and wine. Many studies have examined polyphenols; however, many of these studies have been in vitro (in a laboratory) and at high doses. Epidemiological studies with humans have shown a protective effect of polyphenols on reducing fatal or nonfatal coronary artery disease. Epidemiological studies also have shown a protective effect against lung and colorectal cancers.
Procyanidins and isoflavones. Intervention (controlled) studies have shown that procyanidins found in red wine, grapes, cocoa, cranberries, and apples have pronounced beneficial effects on the vascular system, effects including antioxidant activity, decreased platelet aggregation, decreased LDL concentration, and increased HDL concentration. Isoflavones may have effects on bone mineral density and bone mineral content in women who are postmenopausal.
American Dietetic Association. “Functional Foods.” Available at http://www.eatright.org/public.
Institute of Food Technologists. “Functional Foods Expert Report: Opportunities and Challenges.” Describes in detail FDA regulations governing health-related claims on food labels. Available at http://www.ift.orgknowledge-center.
Milner, John. “Functional Foods: The U.S. Perspective.” American Journal of Clinical Nutrition 71 (2000):
1654-1659. An overview of functional foods. Includes discussion of public interest and the means of evaluation.
Roberfroid, Marcel. “Concepts and Strategy of Functional Food Science: The European Perspective.” American Journal of Clinical Nutrition 71 (2000): 1660-1664. Provides an introduction to the science of functional foods and their applications.
Saulnier, Delphine, et al. “Mechanisms of Probiosis and Prebiosis: Considerations for Enhanced Functional Foods.” Current Opinion in Biotechnology 20, no. 2 (2009): 135-141. Discusses the mechanisms and applications of probiotics and prebiotics.
Vita, Joseph. “Polyphenols and Cardiovascular Disease: Effects on Endothelial and Platelet Function.” American Journal of Clinical Nutrition 81 (2005): 292-297. A review of epidemiologic studies that support a relationship between higher intakes of polyphenolic flavonoids and reduced risk of cardiovascular disease.
Increasingly, foods sold in supermarkets come with health claims on their labels. Labels claim that oatmeal and soy help prevent heart disease, that milk and calcium-fortified orange juice fight osteoporosis, and that folate-enriched flour prevents birth defects and disorders. These foods are all functional foods, that is, foods marketed as offering specific health benefits.
There are two main categories of functional foods. The first, and largest, category consists of ordinary foods that contain health-promoting substances. This category essentially includes all fruits, vegetables, and whole grains, soy and other legumes, and numerous other foods such as herbal teas, yogurt, and cold-water fish. When these foods are presented as functional foods, their specific health benefits and healthy constituents are highlighted, constituents such as fiber, vitamins, minerals, and non-nutrient chemicals with potential health benefits.
The second category of functional foods consists of foods that have been enriched with a potentially health-promoting ingredient. Examples include margarine containing stanol esters, orange juice enriched with calcium and other nutrients, and beverages to which echinacea and other herbs have been added.
Some of these functional food products are based on good, solid science. For others, the supporting evidence is weak or speculative. Furthermore, the requirement for good taste sometimes forces manufacturers to limit the amount of herbs and other additives to a level so low that they are unlikely to have any effect.
The following is a list, by condition treated, of some of the more promising functional foods and natural products that are added to food products to create functional foods.
•Cancer prevention. Diindolylemethane (found in broccoli-family vegetables), fish oil (found in salmon and other cold-water fish), flaxseed (contains lignans), folate, garlic, green tea, I3C (found in vegetables in the broccoli family), IP6 (found in nuts, seeds, beans, whole grains, cantaloupe, and citrus fruits), lycopene (found in tomatoes), resveratrol (found in grape skin), selenium, soy foods, turmeric (added to many foods as a preservative), vitamin C, and vitamin E
•Cataracts. Lutein (found in dark-green vegetables)
•Cavities. Xylitol (added to chewing gum and candy)
•Colds and flu. Echinacea (herbal tea) and garlic
•Diabetes. Chromium (whole grains, brewer’s yeast, fortified nutritional yeast, liver) and evening primrose oil
•Diarrhea and other digestive problems. Probiotics (friendly bacteria) found in yogurt
•Ear infections. Xylitol (added to chewing gum and candy)
•Easy bruising. Bioflavonoids (found in citrus fruits, buckwheat, and most fruits and vegetables)
•Eczema. Probiotics (friendly bacteria) found in yogurt
•General nutrition. Fortified grains and beverages
•Heart disease prevention. Alpha-linolenic acid (found in flaxseed oil), calcium (added to beverages; found in milk and other dairy products), garlic, fish oil (found in salmon and other cold-water fish), potassium (found in orange juice, bananas, and other foods), soy products, stanols-sterols (added to margarine and other spreads), fiber (such as in oats), wine and other alcoholic beverages (in moderation)
•High cholesterol. Fiber (found in whole grains and fruits, legumes, and vegetables), garlic, krill oil, soy products, stanols (added to margarine and other spreads)
•Menopausal symptoms. Soy products
•Nausea. Ginger (beverages)
•Osteoporosis. Calcium (added to beverages; found in milk and other dairy products), vitamin D (added to butter, milk, and other beverages), soy foods
•Premenstrual syndrome. Calcium (added to beverages; found in milk and other dairy products) and krill oil
•Ulcerative colitis. Probiotics (friendly bacteria; found in yogurt)
•Urinary tract infection. Cranberry juice
•Vaginal infection. Probiotics (friendly bacteria) found in yogurt
The U.S. Food and Drug Administration (FDA) allows labels on foods similar to those used on dietary supplements. These labels do not require very much scientific validation, and they formally state that the claims made are not approved by the FDA.
In some cases, however, the FDA has specifically authorized higher-level health claims such as “heart healthy.” These claims may be taken as representing scientific consensus. Because food supplementation is such a rapidly growing area, an increasing number of these labels should be expected.
American Dietetic Association. “Functional Foods.” Available at http://www.eatright.org/public.
Bratman, S., and A. Girman, eds. Mosby’s Handbook of Herbs and Supplements and Their Therapeutic Uses. St. Louis, Mo.: Mosby, 2003.
Carlsen, M. H., et al. “The Total Antioxidant Content of More than 3,100 Foods, Beverages, Spices, Herbs, and Supplements Used Worldwide.” Nutrition Journal 9 (2010): 3.
Gobbetti, M., R. D. Cagno, and M. De Angelis. “Functional Microorganisms for Functional Food Quality.” Critical Reviews in Food Science and Nutrition 50 (2010): 716-727.
Lipski, E. “Traditional Non-Western Diets.” Nutrition in Clinical Practice 25 (2010): 585-593.
Roberfroid, Marcel. “Concepts and Strategy of Functional Food Science: The European Perspective.” American Journal of Clinical Nutrition 71 (2000): 1660-1664.
Tapsell, L. C., et al. “Health Benefits of Herbs and Spices: The Past, the Present, the Future.” Medical Journal of Australia 185, suppl. 4 (2006): S4-S24.