The body is under constant attack from oxidative stress. Oxygen in the body splits into single atoms with unpaired electrons. Electrons like to be in pairs, so these atoms, called free radicals, scavenge the body to seek out other electrons so they can become a pair. This causes damage to cells, proteins and DNA.
Free radicals are associated with human disease, including cancer, atherosclerosis, Alzheimer's disease, Parkinson's disease and many others. They also may have a link to aging, which has been defined as a gradual accumulation of free-radical damage, according to Christopher Wanjek, the Bad Medicine columnist for Live Science.
Substances that generate free radicals can be found in the food we eat, the medicines we take, the air we breathe and the water we drink, according to the Huntington's Outreach Project for Education at Stanford University. These substances include fried foods, alcohol, tobacco smoke, pesticides and air pollutants.
Free radicals are the natural byproducts of chemical processes, such as metabolism. Dr. Lauri Wright, a registered dietitian and an assistant professor of nutrition at the University of South Florida, said, "Basically, I think of free radicals as waste products from various chemical reactions in the cell that when built up, harm the cells of the body."
Yet, free radicals are essential to life, Wanjek wrote in 2006. The body's ability to turn air and food into chemical energy depends on a chain reaction of free radicals. Free radicals are also a crucial part of the immune system, floating through the veins and attacking foreign invaders.
The danger of free radicals
According to Rice University, once free radicals are formed, a chain reaction can occur. The first free radical pulls an electron from a molecule, which destabilizes the molecule and turns it into a free radical. That molecule then takes an electron from another molecule, destabilizing it and tuning it into a free radical. This domino effect can eventually disrupt and damage the whole cell.
The free radical chain reaction may lead to broken cell membranes, which can alter what enters and exits the cell, according to the Harvard School of Public Health. The chain reaction may change the structure of a lipid, making it more likely to become trapped in an artery. The damaged molecules may mutate and grow tumors. Or, the cascading damage may change DNA code.
Oxidative stress occurs when there are too many free radicals and too much cellular damage. Oxidative stress is associated with damage of proteins, lipids and nucleic acids, according to an article in the Pharmacognosy Review. Several studies throughout the last few decades have suggested that oxidative stress plays a role in the development of many conditions, including macular degeneration, cardiovascular disease, certain cancers, emphysema, alcoholism, Alzheimer's disease, Parkinson's disease, ulcers and all inflammatory diseases, such as arthritis and lupus.
Free radicals are also associated with aging. "The free radical theory of aging states that we age because of free radical damage over time," said Wright. Free radicals can damage DNA's instructional code, causing our new cells to grow incorrectly, leading to aging.
Symptoms of oxidative stress
According to a 2010 article in Methods of Molecular Biology, there are no officially recognized symptoms of oxidative stress. According to naturopathic doctor Donielle Wilson’s website, however, symptoms include fatigue, headaches, noise sensitivity, memory loss and brain fog, muscle and joint pain, wrinkles and gray hair, vision trouble and decreased immunity.
Testing for free radicals
It is not possible to directly measure the amount of free radicals in the body, according to Rice University. According to a 2000 article in theAmerican Journal of Clinical Nutrition, there are indirect methods of measuring oxidative stress, usually involving analysis of the byproducts of lipid peroxidation. The article warns that all methods should "should be used with caution because of the lack of accuracy, validity or both."
The more recent article in Methods of Molecular Biology states that kits for testing oxidative stress are increasingly available, though their accuracy and validity are still under scrutiny.
Antioxidants and free radicals
Antioxidants keep free radicals in check. Antioxidants are molecules in cells that prevent free radicals from taking electrons and causing damage. Antioxidants are able to give an electron to a free radical without becoming destabilized themselves, thus stopping the free radical chain reaction. "Antioxidants are natural substances whose job is to clean up free radicals. Just like fiber cleans up waste products in the intestines, antioxidants clean up the free radical waste in the cells," said Wright. Well-known antioxidants include beta-carotene and other carotenoids, lutein, resveratrol, vitamin C, vitamin E, lycopene and other phytonutrients.
Our body produces some antioxidants on its own, but an insufficient amount. Oxidative stress occurs when there is an imbalance of free radicals and antioxidants (too many free radicals and too few antioxidants), according to the Pharmacognosy Review.
Antioxidants can be acquired through diet. "Antioxidants are plentiful in fruits and vegetables, especially colorful fruits and vegetables," said Wright. "Some examples include berries, tomatoes, broccoli, spinach, nuts and green tea."
Antioxidants became well known in the 1990s when scientists began to realize the possible effects of free radicals on cancer development, atherosclerosis and other chronic conditions. During the subsequent decades, scientists have conducted many studies on the effects of antioxidants with mixed results. Wright gave a few examples. "A six-year trial, the Age-Related Eye Disease Study (AREDS), found that a combination of vitamin C, vitamin E, beta-carotene and zinc offered some protection against the development of advanced age-related macular degeneration," she said.
On the other hand, Wright mentioned that a beta-carotene trial among Finnish men who were heavy smokers found an increase in lung cancer among those taking beta-carotene supplements.
Scientists do not completely understand the mixed results from the trials or the exact mechanism that makes antioxidants effective or ineffective against free radicals, but according to Wright, the study results suggest that it is more effective and potentially safer to get antioxidants through whole foods rather than supplements.
Free radicals and exercise
According to an article in Biochemical Society Transactions, intense aerobic exercise can induce oxidative stress. Burning fuel in high-intensity cardio exercise causes chemical reactions that make free radicals form at a faster rate. This isn't an excuse to skip the gym, however. According to an article in the American Journal of Clinical Nutrition, frequent exercise training seems to reduce the oxidative stress initially brought on by exercise. This is because regular physical exercise enhances antioxidant defenses.
Spurred by the concern that intense exercise could cause oxidative stress, several studies were conducted to look at the effects of antioxidant supplementation for athletes. The American Journal of Clinical Nutrition article said that supplementing high intensity exercise with antioxidant supplements produced no beneficial effects, however. Regular exercise alone was enough to build up antioxidant defenses against the initial exercise-induced oxidative stress.
Therefore, out of shape and infrequent exercisers who do a spontaneous bout of intense physical activity may invoke oxidative stress, while those who are consistently active should not worry.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.