What a great question! The jumping-off point for any discussion of antioxidants begins with oxidation—most commonly, the reaction of a substance with the oxygen in the air. It’s a destructive process, writes Robert L. Wolke in What Einstein Told His Cook: Kitchen Science Explained. “Witness the rusting of iron—a pure example of what oxidation can do, even to metals. In the kitchen, oxidation is one of the reactions that makes fats turn rancid. Assisted by enzymes, oxidation is also what makes sliced potatoes, apples, and peaches turn brown.”
To a chemist, Wolke adds, oxidation is a more general chemical process—any reaction in which an electron is snatched from an atom or molecule (a group of atoms joined together by sharing electrons in a chemical bond). “The electron-deprived ‘snatchee’ is said to have been oxidized,” Wolke writes. “In our bodies, such vital molecules as fats, proteins, and even DNA can be oxidized, making them unable to fulfill their critical jobs....”
Well, that’s life.
No, really—that’s life. As long as we’re breathing, our cells use the oxygen in the air we inhale to generate the energy it takes to keep our bodies functioning on a metabolic level. That and other critical processes that involve oxygen create unstable free radicals, which are very good at snatching electrons. “Electrons like to exist in pairs,” Wolke explains, “and a free radical is an atom or molecule that has an unpaired electron desperately seeking a partner.”
Oxidative damage is kept to a minimum by—you got it!—antioxidants, molecules that neutralize free radicals by supplying them with the electrons they need before they snatch them from more important atoms or molecules. We usually think of antioxidants in a human-health context, but they have many industrial uses, such as preservatives in food, cosmetics, rubber, and gasoline.
Antioxidants come in an astonishing array of forms, from vitamins (A, C, and E) and minerals (selenium, manganese) to phytochemicals (more about them in a sec). In a piece called “Antioxidants: Beyond the Hype,” the Harvard School of Public Health provides welcome clarity. “Using the term antioxidant to refer to substances is misleading. It really is a chemical property, namely, to act as an electron donor.... Another big misconception is that antioxidants are interchangeable. They aren’t.”
Each antioxidant, in fact, has its own unique molecular makeup. We make a few of our own, including some enzymes, but we also get them from food—especially plants, including legumes, grains, nuts, and seeds. The theory that our cells produce or absorb antioxidants to protect themselves from oxidative damage is rooted in the free-radical theory of disease, and you may presume that the more antioxidants we have in our bodies (supplements, anyone?), the healthier we will be. Hold that thought; it ties in neatly with another reader’s question, which I’ll address next week.
The term phytochemicals (aka phytonutrients) specifically refers to compounds made by plants (phyto is Greek for “plant”) to combat bacteria, viruses, fungi, and pests—and, in doing so, provide plants with their aromas, flavors, and colors. Some phytochemicals act as antioxidants, but not all: Others may mimic hormones or stimulate enzymes, for instance. Phytochemicals are concentrated in fruits and vegetables but are also found in legumes, whole grains, nuts, seeds, herbs, and spices.
Phytochemicals aren't essential for keeping us alive, like macronutrients (complex carbohydrates, proteins, fats, and dietary fiber) and micronutrients (vitamins and minerals), and they differ greatly in bioavailability, but when we consume them, they may help prevent certain cancers, diabetes, hypertension, and heart disease. Although more than 25,000 phytochemicals have been identified, just a fraction of them have been studied so far.
Carotenoids: Scientists probably know the most about this group of phytochemicals, the red, orange, and yellow pigments in fruits and vegetables. Produce that’s high in carotenoids appears to offer protection against certain cancers, heart disease, and age-related macular degeneration. Specific carotenoids that have been closely examined in this regard include beta-carotene (which converts to vitamin A in the body and is found in green and orange vegetables, including broccoli, sweet potatoes, cantaloupe, winter squash, and carrots); lycopene (tomatoes, red peppers, pink grapefruit, watermelon, guava); and lutein and zeaxanthin (eggs and leafy greens such as kale, spinach, and turnip greens).
Polyphenols: These phytochemicals are found primarily in fruits and plant-based beverages, such as tea, coffee, and wine, but also in vegetables, cereal grains, legumes, and chocolate. Polyphenol research didn’t gather much steam until the mid-1990s; according to a piece in the January 2005 issue of The American Journal of Clinical Nutrition, the main factor that has delayed developments is the “considerable diversity and complexity of their chemical structures.” No kidding. The 518 polyphenols with composition data can be classified into six classes and 31 subclasses. For a more comprehensive take on the vast world of polyphenols, look no further than this 2009 special issue of the journal Molecules.
Eyes glazing over yet? Well, the taxonomy (if that’s the right word) of polyphenols isn’t what I would call user-friendly, but to navigate what you read in the health press (and, these days, even some dinner-party conversations), it’s helpful to know that flavonoids are the most abundant polyphenols in plant foods. Flavonoids can be divided into six classes, one of which is anthocyanins, the pink, red, blue, or purple pigments found in red wine, certain varieties of cereal grains, certain vegetables (eggplant, cabbage, beans, onions, radishes), and, most commonly, berries, cherries, and other fruits. Another class of flavonoids is flavonols, which include quercetin, catechins (including epigallocatechin-3-gallate, or EGCG, the polyphenol most strongly linked to cancer prevention), and tannins (proanthocyanidins), which give tea, red wine, cocoa, and walnuts their astringent quality.
If you are a red-wine drinker delighted at the correlation between moderate consumption and protection against heart disease, then you’ve likely heard of resveratrol, which belongs to yet another class of polyphenols called stilbenes.
Organosulfur compounds: This category of phytonutrients includes glucosinolates from broccoli, brussels sprouts, cabbage, and other Brassica vegetables, as well as allylic sulfides from garlic and onions. Sulfur compounds are also found in grains, wheat germ, oatmeal, and fruits such as figs, papaya, and pineapple.
Organic acids: Whole grains such as oats, rice, and barley contain not just dietary fiber, vitamins, and minerals but potent antioxidant phytochemicals such as ferulic acid, caffeic acid, and even ellagic acid, which is more commonly associated with raspberries, strawberries, and pomegranates. Now that has got me thinking: How could pomegranate seeds on oatmeal be bad?