Vitamin SupplementsNutrition experts have long recommended that people eat plenty of fruits and vegetables, in large part because they are such good sources of antioxidants. Antioxidants are substances that protect the body against disease by countering the harmful effects of free radicals—highly-reactive forms of oxygen that accumulate in the body as byproducts of a normal metabolic process called oxidation. These particles can injure surrounding cells. Left unchecked, they can damage DNA, potentially causing cancer.

Because of their well-recognized antioxidant properties, vitamins A (beta-carotene), C, and E are often touted for their ability to protect the body against cancer. However, new data have emerged suggesting that cancer patients who take high supplemental doses of vitamins A, C, and E might actually be doing more harm than good.

The most recent research, presented at a meeting of the American Cancer Society by oncologists from Memorial Sloan-Kettering Cancer Center in New York, suggests that cancer patients who take large doses of vitamin C—one gram (1,000 mg) or more daily—might sabotage their treatment by helping cancer cells protect themselves from radiation and chemotherapy.

While these adverse effects have not been proven conclusively, researchers report that there are persuasive biological reasons to think large doses could be harmful. For example, scientists recently discovered that cancer cells contain large amounts of vitamin C, which appears to protect them—just as it protects healthy cells—from oxygen damage to the genes. Several cancer therapies, particularly radiation, work by triggering oxygen damage in cancer cells, helping to cripple and destroy them.

Previous research in mice, conducted at the University of North Carolina’s School of Public Health, has suggested that vitamins A and E may undermine the efficacy of cancer treatment as well.When the DNA of cells in both mice and humans has been damaged beyond repair, the cells undergo a natural process that forces them to destroy themselves. This process is called apoptosis. The mouse study indicated that some antioxidant vitamins may suppress apoptosis, thereby reducing cancer cell death.

Prior to the University of North Carolina study, another mouse study suggested that tumor cells need high amounts of vitamin C to thrive. Moreover, years ago, two human studies found that smokers with high levels of beta-carotene in their blood had a higher risk of lung cancer, compared with smokers with low levels of the antioxidant.

Cancer specialists still recommend eating a diet rich in fruits and vegetables, but many now caution their cancer patients to avoid taking high doses of vitamin supplements. Until we learn more, consider these precautions:

  1. Do not take vitamin supplements unless your doctor has prescribed them.
  2. Avoid taking high doses of antioxidant vitamins during radiation therapy or chemotherapy. After treatment is complete, ask your physician if it is safe to start taking vitamins.
  3. Eat a balanced diet. Doing so should supply enough of the vitamins, minerals, and nutrients the body needs in the natural proportions.