It’s hard to walk through the grocery store without encountering at least one product that touts the health benefits of antioxidants. Marketed as the enemy of “free radicals” (those sometimes pesky molecules that can damage cells by causing oxidative stress), antioxidants are commonly found in fruits, vegetables, nuts, whole grains, and fish, among other foods. “Antioxidant” is also a marketing buzzword printed on labels for everything from soft drinks and supplements to frozen fruits and flu relief. But what are antioxidants, really? What’s the science behind their benefits? And can there be too much of a good thing? Here are some answers.

1. Some antioxidants are essential, others are a bonus. Many types of food components can act as antioxidants, but only a few are essential. The human body needs the essential ones but can’t produce them on its own, which means they need to be acquired through dietary sources. The essential antioxidant nutrients include vitamins A, E, and C and certain carotenoids (red, orange, and yellow organic pigments), such as provitamin A. These antioxidant nutrients are needed to support life. They help prevent oxidation, which can cause damage in cell DNA, lipids, and proteins. Some micronutrients, such as selenium, are needed for the function of antioxidant enzymes in cells. Other dietary antioxidants include natural colors, tannins, lipids, and peptides.
2. Non-nutrient antioxidants can also affect the human body. In addition to preventing oxidative stress, essential antioxidant nutrients have regulatory functions and are involved in maintaining other biological processes. Similarly, nonessential antioxidants may have direct antioxidant effects, but it is likely that their non-antioxidant functions are more important. These actions include inducing cellular antioxidant defenses; changing the gut microbiota; and inhibiting enzymes and cell-signaling processes relevant to inflammation, cancer, cardiovascular disease, and other chronic diseases.
3. You can have too many antioxidants. There is an optimal level of antioxidant consumption — too little yields no benefits, and excessive consumption could lead to toxicity or increased risk of cancer. Usually, this would mean taking high levels of dietary supplements or excessive levels of specific antioxidant-rich foods. Overconsumption of green tea extract can be toxic, and antioxidant supplementation is not advised during certain chemotherapy treatments because some drugs work by inducing oxidative stress in cancer cells. Also, some antioxidants may interfere with the absorption and metabolism of certain therapeutic drugs.
4. It is hard to say how many non-nutrient antioxidants you should consume. While we have dietary recommendations for nutrient antioxidants, it is difficult to give precise guidance for the amount of non-nutrient antioxidants needed to improve health. We know most people in the U.S. do not eat enough antioxidant and nutrient-dense fruits, vegetables, nuts, and whole grains. Increased consumption of these foods is associated with reduced risk of some cancers and heart disease. However, the chemical complexity of antioxidants, limited understanding of their distribution in foods, and limited evidence from human intervention studies prevent precise intake recommendations.
5. The way you absorb and metabolize non-nutrient antioxidants is unique. There’s a high degree of variability in how individuals metabolize dietary polyphenols (plant-based nutrients). Age, diet, gut microbiota, and expression of metabolizing enzymes affect how these compounds are extracted from foods during digestion, broken down, and transformed to be more easily absorbed or excreted. Scientists, including several research groups at UW–Madison, are still working to understand how all of this affects the health-promoting properties of antioxidant-rich foods.
6. Antioxidants are important for reducing food waste. Fresh fruits and vegetables are among the most wasted food because of their perishability. Non-essential antioxidants are part of the process for how foods brown. Enzymatic browning starts with the polyphenol oxidase enzyme, which combines polyphenols to create brown, unappealing, and bitter pigments. Apples, avocados, bananas, and pineapples may go bad because of the activity of this enzyme on the antioxidants. Inactivation of this enzyme by heat or pressure can reduce formation of these pigments and extend the shelf-life of foods. For products that undergo long-term storage, lipid oxidation can lead to nutrient loss and unappealing off-flavors. Antioxidants already present in the ingredients or added to food can help prevent wasted food by blocking lipid oxidation.

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Brad Bolling is associate professor in the Department of Food Science and the Fritz Friday Chair of Vegetable Processing Research.

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This article was posted in Fall 2022, Food Systems, Front List, Health and Wellness and tagged antioxidants, Brad Bolling, Food science, free radicals, oxidative stress.