Antioxidants Uncovered

Dietary antioxidants are vitamins, minerals, carotenoids, polyphenols and other compounds found mostly in plant foods. Many antioxidants give foods their vibrant color, such as the deep-red of tomatoes and the blue-purple of blueberries. Antioxidants prevent free radicals from attacking cells and damaging DNA. Research suggests that free radicals are involved in the development of a number of degenerative diseases, including cancer, cardiovascular disease, cognitive impairment, Alzheimer’s disease, immune dysfunction, cataracts and macular degeneration.

In search of definition

Antioxidants are complicated. Some appear to confer health benefits and some do not, and some have mixed results. Remember the clinical intervention trial that revealed beta-carotene supplementation may be a contributing factor to increased risk of lung cancer in smokers?

Although many plant compounds have been loosely lumped together into the category of antioxidants, they are not interchangeable and differ from each other in sites and mechanisms of action. Some compounds can act as antioxidants under one condition and as pro-oxidants in another. It is unlikely that a common health benefit can be linked with each antioxidant. The Panel on Dietary Antioxidants and Related Compounds, Institute of Medicine (IOM), National Academy of Sciences (NAS), attempted to define dietary antioxidants in order to establish dietary reference intake levels (DRIs).

The panel developed a proposed definition of a dietary antioxidant as a substance in foods that significantly decreases the adverse effects of reactive oxygen species (ROS), reactive nitrogen species (RNS), or both, on normal physiological function in humans. The substance should be found in human diets, measured in foods commonly consumed, and must show, in humans, to decrease the adverse effects of free radicals.

Top four

Many compounds have been identified in foods that are potentially vital for health, but many lack enough solid research to back them up. Thus, the IOM panel narrowed its sights on beta-carotene and other carotenoids, vitamin C, vitamin E and selenium.

Beta-carotene, as well as other carotenoids like alpha-carotene, lycopene, lutein and zeaxanthin, and cryptoxanthin, are found in many fruits and vegetables. A large body of evidence links higher blood concentrations of beta-carotene and other carotenoids from foods with lower risk of several chronic diseases. But the IOM reported that the evidence cannot be used to establish a requirement for beta-carotene or carotenoid intake, because the effects may be due to other substances found in carotenoid-rich food or to other factors related to increased fruit and vegetable consumption.

Found mostly in seafood and organ meats, selenium is an essential component of several enzymes that remove ROS. Research points out antioxidant effects of selenium supplementation on reducing the risk of disease development. The DRI range for ages 2 to 70 for selenium is 20 to 55 µg per day.

Found abundantly in fruits and vegetables, vitamin C has many important roles, including acting as a cofactor in the biosynthesis of a number of different compounds, effectively quenching a variety of ROS and RNS, preventing oxidative damage to lipids and DNA in some cases, and potential regeneration of other body antioxidants. The DRI range for ages 2 to 70 for vitamin C is 15 to 90 mg per day.

Vitamin E, a group of fat-soluble molecules in eight different forms with similar structures, occurs naturally in plant and animal sources. It possesses antioxidant activity and can protect unsaturated lipids in the body, but can act as a pro-oxidant in vitro under certain circumstances. Research supports its health-promoting effects. The DRI range for ages 2 to 70 for vitamin E is 6 to 15 mg per day.

More beneficial antioxidants

Other antioxidant compounds not addressed by the IOM/NAS panel include flavonoids, phenols and polyphenols, phytoestrogens, lipoic acid, and food additives. The panel found that the published literature and comprehensive food composition data on some of these potentially important dietary substances was insufficient at the time of the review and DRIs were not established, but ongoing research may warrant intake recommendations in the future.

Though DRIs have not been established for many important antioxidants, a growing body of research supports their health benefits, at least in their food form. “The foods that are generally rich in antioxidant activity are the same foods that are associated with health benefits with regard to disease prevention for many different chronic diseases,” says Susan T. Mayne, Ph.D., professor of epidemiology, Yale School of Public Health, New Haven, CT. “These foods include fruits and vegetables, but of note, tea and coffee are also fairly rich in antioxidants. It is not known, however, whether benefits associated with consuming antioxidant-rich foods are driven by the antioxidant activity or other mechanisms. The clinical trials of isolated antioxidant nutrient supplements have had disappointing results and some have shown adverse effects. So, the bottom line continues to be highly consistent with prior recommendations, that consuming foods rich in antioxidant nutrients, such as fruits and vegetables, is a prudent approach for health promotion and disease prevention.”

While the science of antioxidants poses many complexities, the take-home message that people should search for a wide variety of food sources rich in a rainbow of antioxidants seems to be well understood.

Sharon Palmer is a registered dietitian with 16 years of experience in health-care and foodservice management. She writes on food and nutrition for newspapers, magazines, websites and books. Palmer makes her home in Southern California and can be reached at [email protected].