Free Radicals: Definition, Causes, Antioxidants, and Cancer

What Exactly Are Free Radicals and Why Are They Important?

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Free radicals are highly reactive and unstable molecules that are produced in the body naturally as a byproduct of metabolism (oxidation), 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 the mutations that can lead to cancer. Antioxidants in the foods we eat can neutralize the unstable molecules, reducing the risk of damage.

We will look at the structure, causes, and effects of free radicals, as well as what you should know about antioxidant supplements 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 electron to stabilize themselves—a process that can cause damage to DNA and other parts of human cells. This damage may play a role in the development of cancer and other diseases and accelerate the aging process.

Types of Free Radicals

There are many types of free radicals, though 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) in the environment.

Free radicals can be produced both by carcinogens and 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 cancer increases with age, even when people have few exposures to cancer-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 in the home
  • 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 an 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 reactions that occur which result in free radicals obtain an electron are done in the presence of oxygen.

The process is actually much more complicated, and a vicious circle in essence. When one free radical "steals" an electron from a molecule, that molecule is then missing an electron (becomes a free radical), and so on. Free radicals can damage not only DNA (nucleic acids), but proteins, lipids, cell membranes, and more in the body. Damage to proteins (protein cross-linking and more) and other body components may cause disease directly.

How Free Radicals Can Cause Cancer

Damage done to genes in the DNA may result in genes that produce ineffective proteins; proteins needed to be watchkeepers over the cells of the body. Some of these mutations may involve genes known as tumor suppressor genes.These genes code for proteins which function to repair damages in DNA or cause cells which are damaged beyond salvage to be removed through a process of apoptosis (programmed cell death).

Oncogenes are genes that code for proteins that promote the growth of cells. Normal genes in the body called "protooncogenes" are important in promoting growth of a baby during pregnancy and transiently produce proteins that aid in tissue repair. Mutations in these genes (which are then oncogenes) results in the continuous production of proteins that promote growth of a cell.

Most often, it is a series of mutations in both tumor suppressor genes and oncogenes that leads to cancer. Damage (mutations) to tumor suppressor genes allows a damaged cell to survive unrepaired (abnormal) and damaged oncogenes promote the growth of that damaged cell. The result is—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 with 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. 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, for that matter—with your oncologist. There are a few different concerns that have been raised.

One is with regard to people receiving cancer treatments such as chemotherapy and radiation therapy. Some vitamin supplements can reduce the effectiveness of cancer treatments, and the reason for this makes sense when you consider the mechanism of these treatments. Some treatments for cancer, such as radiation, create free radicals in an effort to kill cancer cells. In this setting, the use of antioxidants could, in theory, decrease the effectiveness of treatment. In this setting, antioxidants may help protect the cancer cells you are trying to kill.

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

A few different studies published in 2019 support this concern.

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.

Antioxidant supplements (not diet)

Taking antioxidant supplements (not diet) may actually worsen a person's prognosis with some cancers.

Free Radicals and Cancer

Cancer is usually caused by a series of mutations that result in the uncontrolled growth and relative immortality of the cell. 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. As noted earlier, however, getting these antioxidants in supplement form has not been found to be effective, and most oncologists recommend dietary sources of these nutrients. In addition, ​some vitamin and mineral supplements may interfere with cancer treatments.

Free Radicals and Aging

There are several theories describing why our bodies age and free radicals are included in one of those theories. Rather than free radicals being responsible for aging-related changes alone, however, it's likely that normal aging is related to a number of different processes in the body.

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 itself, but not in sufficient quantities alone. It's important to note, however, that since free radicals are produced during normal cellular processes, people may "do everything right" and still develop cancer.

Reducing your exposure to free radicals include both avoiding their sources (carcinogens) 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 as a result of normal metabolism in the body. That said, adopting a healthy diet rich in a wide variety of antioxidants is an excellent start.

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Article Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial policy to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  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

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