Free Radicals: Definition, Causes, Antioxidants, and Cancer

What Exactly Are Free Radicals and Why Are They Important?

Free radicals are highly reactive and unstable molecules that are produced in the body naturally as a byproduct of normal metabolism, or by exposure to toxins in the environment such as tobacco smoke and ultraviolet light. Free radicals have a lifespan of only a fraction of a second, but during that time can damage DNA, sometimes resulting in mutations that can lead to various diseases, including heart disease and cancer. Antioxidants in the foods we eat can neutralize the unstable molecules, reducing the risk of damage.

Many different berries held in hands
​Peter Close / Istockphoto.com / Stock Photo

We will look at the structure, causes, and effects of free radicals, as well as what you should know about antioxidant supplements, especially if you have cancer.

Definition and Structure of Free Radicals

Free radicals are atoms that contain an unpaired electron. Due to this lack of a stable number of outer shell electrons, they are in a constant search to bind with another atom or molecule to stabilize themselves—a process that can cause damage to human cells. This damage may accelerate the aging process and can play a role in the development of cancer and other diseases.

Types of Free Radicals

There are many types of free radicals. In humans, the most significant are oxygen free radicals (reactive oxygen species). Examples include singlet oxygen (when oxygen is "split" into single atoms with unpaired electrons), hydrogen peroxide, superoxides, and hydroxyl anions.

Causes/Sources of Free Radicals

You may wonder where free radicals come from in the first place. Free radicals can be produced in a few different ways. They may be generated from normal metabolic processes in the body, or by exposure to carcinogens (cancer causing substances) or other harmful substances in the environment.

Free radicals can be produced both by harmful substances and by the normal metabolic processes of cells.

Free Radicals Due to Normal Metabolic Processes

Our body often produces free radicals in the process of breaking down nutrients to create the energy which allows our bodies to function. The production of free radicals in normal metabolic processes such as this is one of the reasons that the risk of many different diseases increases with age, even when people have few exposures to disease-causing substances.

Free Radicals Due to Exposure to Carcinogens

Exposure to carcinogens in our environment can also produce free radicals.

Examples of some carcinogens include:  

  • Tobacco smoke
  • Ultraviolet radiation
  • Radon
  • Environmental and occupational substances and chemicals such as asbestos and vinyl chloride
  • Some viruses
  • Medical radiation
  • Air pollution

Effect of Free Radicals on the Body: Oxidative Stress

Once free radicals are generated, whether through exposure to a carcinogen or doing the normal processes of body metabolism, they are free to do damage.

The availability of free radicals creates what is known as oxidative stress in the body. The reason it is named oxidative stress is that the chemical reactions that result in free radicals obtaining an electron are done in the presence of oxygen.

The process can involve a series of reactions. When one free radical "steals" an electron from a molecule, that molecule becomes a free radical because it's missing an electron—and so on. Free radicals can damage the body's DNA, which contains genes, as well as proteins, lipids, cell membranes, and more, causing disease.

Free Radicals and Aging

There are several theories describing why our bodies age and free radicals are part of those theories. Free radicals aren't considered fully responsible for aging-related changes—it's likely that normal aging is related to a number of different processes in the body.

How Free Radicals Can Cause Cancer

Damage done to genes in the DNA may result in genes that produce ineffective proteins; some of these proteins are watchkeepers over the DNA integrity.

Some of these mutations may involve genes known as tumor suppressor genes. These genes code for proteins that function to repair damages in DNA or cause cells that are damaged beyond salvage to be removed through a process of apoptosis (programmed cell death).

Most often, it is a series of mutations in tumor suppressor genes and other genes that leads to the formation of a cancer cell.

Antioxidants and Free Radicals

Many of the phytochemicals (plant chemicals) in the foods we eat function as antioxidants. These nutrients function by inhibiting the formation of free radicals and may reduce the damage they would cause in the body. This is thought to be at least part of the reason why a diet rich in vegetables and fruits has been linked with a lower risk of many diseases.

Examples of antioxidants include vitamin E, vitamin A, beta-carotene, anthocyanidins (in berries), epigallacatechin-3-gallate (EGCG) in green tea, and many more.

Antioxidant Supplements

Many studies have found that eating foods rich in antioxidants is associated with a lower risk of developing diseases, including cancer. Unfortunately, simply supplementing your diet with antioxidant supplements does not appear to have the same effect.

An example is lung cancer. Knowing that people who had a higher intake of foods rich in beta-carotene and vitamin E had a lower risk of developing lung cancer, researchers conducted a study in which one group of people took a daily supplement of beta-carotene and the other did not. Results showed that men who smoked and took beta-carotene actually had a higher risk of developing lung cancer.

Antioxidants in People Who Already Have Cancer

For those who are going through treatment for cancer, it is very important to discuss any antioxidant supplements—or any supplements—with your oncologist.

Taking antioxidant supplements may actually worsen a person's prognosis with some cancers and some vitamin supplements can reduce the effectiveness of cancer treatments.

In one study, postmenopausal women with breast cancer who used antioxidant supplements during chemotherapy and radiation were found to have a poorer prognosis. In two separate studies, cell studies suggest that antioxidant supplements (such as vitamin E) may promote the growth and spread of lung cancer.

While antioxidant supplements are often not recommended, most oncologists believe eating a healthy diet containing foods rich in antioxidants is not a problem.

Antioxidants, Free Radicals, and Cancer

Cancer is usually caused by a series of mutations that result in the uncontrolled growth and survival of cancer cells. Since fruits and vegetables in our diet are high in antioxidants, it's thought that this may be one reason why a diet rich in fruits and vegetables has consistently been associated with a lower risk of cancer.

Getting these antioxidants in supplement form has not been found to be effective and some vitamin and mineral supplements may interfere with cancer treatments. Most oncologists recommend dietary sources of these nutrients.

How to Reduce Free Radicals in Your Body

Reducing free radicals in your body includes both reducing the chance they will form and providing your body with antioxidants. The body produces antioxidants, but not in sufficient quantities. It's important to note, that since free radicals are produced during normal cellular processes, people may "do everything right" and still develop cancer or other diseases.

Reducing your exposure to free radicals includes avoiding their sources and providing your body with healthy antioxidants in your diet.

Lifestyle measures to reduce exposure include not smoking, avoiding processed foods, practicing caution with any chemicals you work with at home or on the job, and more.

As far as obtaining a healthy variety of antioxidants in your diet, experts in nutrition often recommend eating a "rainbow of foods" with different color foods often containing different classes of antioxidants.

A Word From Verywell

It's impossible to entirely eliminate the exposure to free radicals, especially those generated as a result of normal metabolism in the body. That said, adopting a healthy diet rich in a wide variety of antioxidants is a good start.

Was this page helpful?
3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Alsharairi N. The Effects of Dietary Supplements on Asthma and Lung Cancer Risk in Smokers and Non-Smokers: A Review of the Literature. Nutrients. 2019. 11(4): 725. doi:10.3390/nu11040725

  2. Jung A, Cai X, Thoene K, et al. Antioxidant Supplementation and Breast Cancer Prognosis in Postmenopausal Women Undergoing Chemotherapy and Radiation TherapyThe American Journal of Clinical Nutrition. 2019. 109(1):69-78. doi:10.1093/ajcn/nqy223

  3. Lignitto L, LeBoeuf SE, Hamer H, et al. Nrf2 Activation Promotes Lung Cancer Metastasis by Inhibiting the Degradation of Bach1Cell. 2019. doi: 10.1016/j.cell.2019.06.003

Additional Reading