|Anti-oxidants in food: what are they, how are they measured, and what are they really doing?|
By Matthew Picklo
The scientific literature is full of reports regarding the health benefits of anti-oxidants. Many packaged foods sport labels that emphasize the amount of anti-oxidants inside the product. But just what are these "oxidants" that are being counteracted? How are the anti-oxidant levels in foods measured? And what are they really doing after they are ingested? The first two questions are relatively simple to answer. The last one is more complex.
Under healthy conditions, our normal cells generate "oxidants," mostly through the processes that produce energy in our cells. During cellular respiration, a small amount of the oxygen used gets converted to "Reactive Oxygen Species" - ROS - like hydrogen peroxide, the peroxide you can buy at the drug store. Until recently, it has been thought that production of these molecules were harmful to cells--the cost of living in a 21 percent oxygen atmosphere! Indeed, cells have defense mechanisms to reduce the levels of ROS Some of these defenses include chemicals like vitamin C (ascorbic acid) and vitamin E (tocopherol) and enzymes that metabolize peroxides. New data show that these ROS actually transmit signals within and between cells.
The amount of reactive molecules present in a cell is a balance between ROS production and ROS metabolism. Today, researchers are investigating how the cells regulate this balance. Evidence shows that this balance is regulated in a more complex fashion than previously thought.
ROS may have good effects that are only now becoming appreciated. Our immune cells produce ROS to kill bacteria during infection. Interestingly, our immune cells also make hypochlorous acid, or bleach, as a defense to kill bacteria. Recent data indicate that small amounts of ROS may be necessary for adapting muscle to exercise. A German study demonstrated that vitamin E and/or vitamin C supplementation may blunt exercise adaptation in people. So while overproduction of ROS has been related to many diseases including heart disease and Alzheimer's disease, certain levels or forms of ROS may be beneficial.
So how is the anti-oxidant capacity of foods measured? The most common method perhaps is the "Oxygen Radical Absorbance Capacity" (ORAC) test that was developed by USDA-ARS scientists. Essentially, the ORAC measures the ability of a food nutrient to neutralize ROS. A value is provided that represents the neutralizing activity for 100 grams of the food. A food with a higher ORAC value is able to neutralize more radicals than food with small ORAC values. The Agricultural Research Service of the USDA published in 2007 the ORAC values for 277 common foods as a collaboration between the ARS Nutrient Data Laboratory, Beltsville Human Nutrition Research Center and the ARS Arkansas Children's Nutrition Center in Little Rock, Arkansas and can be accessed at http://www.ars.usda.gov/SP2UserFiles/Place/12354500/Data/ORAC/ORAC07.pdf
So how can foods or the nutrients found in foods be "anti-oxidant"? Research has shown that foods can have direct and indirect anti-oxidant effects. Vitamin C and Vitamin E are present in many foods, are able to react directly with ROS present in the body and can be considered direct anti-oxidants. Vitamin C and vitamin E have high ORAC values. Other nutrients, such as the mineral selenium can be considered an indirect anti-oxidant. Selenium is necessary for the activity of peroxide metabolism enzymes. Selenium supplementation in selenium-deficient people increases peroxide metabolism. Isothiocyanates, found in cruciferous vegetables like broccoli and cauliflower, are a good example of indirect anti-oxidants. Isothiocyanates stimulate production of anti-oxidant proteins. This form of indirect anti-oxidant may not be identified by an ORAC test.
On the other hand, some anti-oxidant food components may have a high ORAC value, but may not directly neutralize ROS in the body. The ORAC test is a test tube measure only. Nutrients have to be released from the food and absorbed from the gut. During this time, nutrients are undergoing their own metabolism. Only a small fraction of some nutrients actually are available for use by the body. An example of high ORAC value food components that are metabolized are polyphenols found in fruits like grapes, berries, and apples. In many cases, these polyphenols give color to the fruit. These polyphenols, when concentrated are very good anti-oxidants when measured by an ORAC test. However, when we eat berries and other fruits, the polyphenols are metabolized in our gut to compounds that have low ORAC value and only a very small fraction of the parent polyphenol gets into our blood. Yet, consuming these polyphenols have anti-oxidant effects in the body. How? New data indicate that the metabolites of the polyphenols are bioactive and anti-oxidant.
Understanding how foods and nutrients can prevent disease and promote health is a very active area of research of the Grand Forks Human Nutrition Research Center that benefits all Americans. The roles of anti-oxidants and ROS in disease prevention and promoting health are now being expanded as research into this area grows.