Cancer patients spend billions a year on vitamins and dietary supplements, but there is no proof that these products -- or other nutrition strategies -- are effective for treating or preventing the disease, a new analysis shows.
Researchers who reviewed 59 previous studies assessing a wide range of diet-related approaches found little evidence that specific vitamins, supplements, or foods had any impact on disease-free survival, mortality, or cancer recurrence.
The impact of diet on cancer was either disappointing or could not be determined because most studies conducted to date have been of poor quality, the researchers concluded.
“There is no evidence that dietary modification by cancer patients improves survival and benefits disease prognosis,” they wrote.
Antibiotics vs. Diet Supplements
The analysis was one of two reports on nutrition and cancer, published in the July 10 issue of the Journal of the National Cancer Institute.
In an original study from the National Cancer Institute and China’s Peking University, researchers assessed various interventions thought to prevent stomach cancer or precancerous lesions in Chinese people of Linqu County in Shandong Province, where stomach cancer causes 42 percent of cancer deaths.
The long-time use (around seven years) of either garlic or vitamin E, vitamin C, and selenium supplements did not result in a reduction in precancerous lesions or in stomach cancers. But a two-week course of antibiotics given to kill Helicobacter pylori bacteria in the stomach was associated with a reduction in precancerous lesions.
H. pylori infection is known to be a major cause of stomach ulcers, and it is believed to play a critical role in the development of stomach cancer.
Researchers concluded that the H. pylori treatment reduced the occurrence of precancerous stomach lesions, which could prevent stomach cancers.
Studies Show No Impact
The analysis of previous nutrition and cancer studies included 25 studies in patients with cancer and 34 studies in patients with precancerous conditions.
Steven Thomas, MD, PhD, and colleagues from the U.K.'s University of Bristol concluded that the vast majority of the studies they looked at were poorly designed or poorly executed.
With this caveat, they reported that none of the interventions studied appeared to have an impact on cancer one way or the other.
Specifically, vitamin A supplementation did not appear to be beneficial in studies involving patients with lung, cervical, head and neck, and skin cancers, and leukemia.
Other studies examined multivitamins use by patients with a variety of cancers: beta carotene supplementation in patients with skin, stomach, cervical, mouth, and colorectal cancers; fiber for colorectal cancer; and green tea for esophageal cancer.
“We did not find any conclusive benefit or harm for any of these interventions,” Thomas tells WebMD.
More Study Needed
But that doesn’t mean eating a healthy diet with plenty of fruits and vegetables is not important for people with cancer or for cancer survivors, he adds.
“People with cancer are increasingly living longer as medical treatments improve. We can certainly say that healthy diets are important for general well-being. But it is more problematic to say that a healthy diet is beneficial in treating cancer. There is some evidence that this is the case, but it is not conclusive.”
Nutrition and cancer researcher John A. Baron, MD, of Dartmouth Medical School, tells WebMD that the studies to date have tended to identify dietary interventions that do not benefit cancer patients.
“We actually do know a lot, but it is mostly about what doesn’t work,” he says. “We now know that antioxidant and fiber supplements are not protective against colorectal cancer, for example.”
In an editorial accompanying the study, Baron wrote that while the studies examining diet and cancer have tended to be negative, there are some “hopeful nuggets of progress” in the search for useful diet plans.
He tells WebMD that his own studies strongly suggest that dietary calcium may help prevent colorectal cancer.
“I do believe that we will have more answers in the near future, but we do need better studies to give us these answers,” he says.
By Salynn Boyles, reviewed by Louise Chang, MD
SOURCES: Davies, A.A. and Wei-cheng, Y. Journal of the National Cancer Institute, July 19, 2006; vol 98: pp 961-980. Steven Thomas, MD, PhD, division of maxiofacial surgery, University of Bristol, U.K. Wei-cheng You, Peking University School of Oncology, Beijing, China. John A. Baron, MD, section of biostatistics and epidemiology, Dartmouth Medical School, Lebanon, N.H.