We often hear that a food or beverage is “high in antioxidants” but what exactly is an antioxidant? Why would we want to eat foods that are a high in them and which foods are the best sources?
What is an Antioxidant?
Simply put, an antioxidant is a molecule that keeps another from oxidizing. Oxidation is chemical reaction that transfers a hydrogen atom or electrons from one substance to an oxidizing agent. This process can produce something called a “free radical” or “reactive oxygen species” which can start a chain reaction in a cell that can cause damage or death to the cell.
Different Types of Antioxidants
Antioxidants are classified into two categories, depending on whether they are soluble in water or in fat (lipids). In general, water-soluble antioxidants protect the inside of the cell and the blood plasma, while fat-soluble antioxidants generally protect cell membranes from lipid peroxidation (rancidity).
What do Antioxidants do?
Oxidation reactions are not bad in and by themselves, in fact they are necessary for life, but it is the production of these “free-radical” byproducts that are thought to be linked to cell damage that may result in cancer and coronary heart disease. Chemically, antioxidants act as “free radical scavengers” and prevent these harmful reactions from occurring.
Antioxidants in the food industry were originally used to prevent fats from going rancid [German JB et al].
Vitamin A, C and E are naturally occurring antioxidants contained in foods and that when present in insufficient amounts, may lead to something called “oxidative stress” and damage to cells.
What is Oxidative Stress?
A common “reactive oxygen species” is hydrogen peroxide (H2O2); yes the same substance that is used in first aid. The “hydroxyl radical” (-OH) that forms from hydrogen peroxide and other similar “reactive oxygen species”, is very unstable and will react quickly with almost any biological molecule, in order to become stable. These oxidants can damage cells by starting chemical chain reactions such as lipid peroxidation (essentially making the fat in cells rancid), or by oxidizing DNA or proteins.
Oxidative stress is thought to be linked to a wide range of diseases but it is unclear if oxidants trigger the disease or if they are produced as a result of the disease; a byproduct of tissue damage.
The oxidation of the so-called “bad” low density lipoprotein (LDL) oxidation appears to trigger the process of atherosclerosis or “hardening of the arteries”; underlying heart disease. Oxidative damage to DNA is believed to be linked to cancer.
Antioxidants in Foods
Research indicates that diets high in fruit and vegetables (rich sources of antioxidants) reduce the risk of several chronic diseases [USDA, 2010] and prevent approximately 30% of cancer deaths [Hiatt RA et al]. Other foods, such as grains, legumes and nuts are also good sources of antioxidants.
Effect of Cooking and Storage on Antioxidants in Foods
Some antioxidants, such as lycopene (a type of carotenoid from tomatoes) and ascorbic acid (Vitamin C) are destroyed by long-term storage or prolonged cooking. Other antioxidant compounds are more stable, such as the antioxidants in foods such as whole-wheat cereals and tea.
Side Effects of Antioxidants – in Mineral Metabolism
Foods that are high in antioxidants (i.e. are strong reducing agents) may bind in the gastrointestinal tract to trace minerals such as zinc and iron and major minerals such as calcium; preventing them from being absorbed. Some examples of these reducing agents are;
- oxalic acid (found in high quantities in cocoa beans and chocolate), spinach, turnip and rhubarb
- phytic acid found in high quantities in whole grains, corn and legumes (such as chickpeas, lentils, pinto beans, etc)
- tannins (found in tea, beans and cabbage)
What about taking Antioxidant Supplements?
Early studies seemed to indicate that antioxidants taken as supplements (pills / capsules) might be beneficial, but large clinical studies suggest that not only do they not seem to have benefit; they may even be harmful [Bjelakovic G et al].
Which Foods are Highest in Antioxidants?
In a landmark study by Wu et al, the “total antioxidant capacity” of both lipophilic (fat soluble) and hydrophilic (water soluble) antioxidant components in over 100 different foods were measured; including fruits, vegetables, nuts, dried fruits, spices and cereals. These are listed, below. You will notice that some foods are in both categories, which means they have both hydrophilic (water soluble) and lipophylic (fat soluble) antioxidants.
It is important to keep in mind that there are several factors that may affect the antioxidant capacity of foods, including processing of the food, genetics, season, and growing conditions.
Foods highest in hydrophylic (water soluble) antioxidants per serving included:
Wild blueberry, red kidney beans, pinto beans, cultivated blueberry, cranberry, artichoke, blackberry, prunes, strawberries, raspberries, Red Delicious and Granny Smith apples, pecans, sweet cherries, black plums, Gala apples, walnuts, Golden Delicious and Fuji apples, Deglet Noor dates, Green and Red Anjou pears, hazelnuts, navel oranges, figs, Haas avocadoes, broccoli raab, red cabbage, pistachio nuts, Medjool dates, navy beans and red grapes
Foods highest in lipophylic (fat-soluble) antioxidants per serving included:
Haas avocado, navy beans, pinto beans; small red beans, black-eyed peas, broccoli raab, black beans, raspberries, cranberries, spinach, quick oats, Brazil nuts, prunes, blackberry, orange peppers, oranges, walnuts, figs, cashews , yellow pepper, pat bran cereal, old-fashioned oats, pistachios, pecans, artichoke, hazelnuts, corn, pear, red and green leaf lettuce and pumpkin.
Consuming a diet rich in fruit and vegetables. legumes and nuts is an easy and safe way to be assured of getting plenty of antioxidants. Remember though, that some of these antioxidant-rich foods may bind nutrients such as iron, zinc and calcium so be sure to not take those foods together. For example
- drink tea or matcha (powdered green tea) between meals to prevent the binding of the tannins in the tea to iron and zinc
- remember that while spinach is a good source of iron, the oxalic acid in it will bind it, making it largely unavailable to the body
- to offset the binding of phytic acid in legumes and whole grains, be sure to have rich sources of iron and zinc (such as red meat, oysters, etc) at meals that do not contain large amounts of these foods
With berry season just around the corner, why not indulge in antioxidant rich strawberries, raspberries, blueberries and blackberries? Then, anticipate summer with its fresh cherries, black plums, red and green Anjou pears and plenty of fresh salad with green and red leaf lettuce and a handful of nuts on top. On a cold fall night, settle into a bowl of three-bean chili with pinto, black and kidney beans or a veggie curry with chickpeas and pumpkin.
- Bjelakovic G; Nikolova, D; Gluud, LL; Simonetti, RG; Gluud, C (2007). “Mortality in randomized trials of antioxidant supplements for primary and secondary prevention: systematic review and meta-analysis”. JAMA 297 (8): 842–57.
- German, JB (1999). “Food processing and lipid oxidation”. Advances in experimental medicine and biology. Advances in Experimental Medicine and Biology 459: 23–50.
- Hiatt RA, Rimer BK. A new strategy for cancer control research. Cancer Epidemiol Biomarkers Prev 1999;8(11):957–64
- Knight, JA (1998). “Free radicals: Their history and current status in aging and disease”. Annals of clinical and laboratory science 28 (6): 331–46.
- US Department of Agriculture and US Department of Health and Human Services. Dietary guidelines for Americans, 2010. http://health.gov/dietaryguidelines/dga2010/dietaryguidelines2010.pdf
- Wu X, Beecher GR, Holden JM, Haytowitz DB, Gebhardt SE, Prior RL “Lipophylic and Hydrophylic Antioxidant Capacities of Common Foods in theUnited States” J Agric Food Chem 2004. 52, 4016-1037