One study, a meta-analysis of 59 trials, found little evidence that diet is associated with survival or prognosis. The other study suggests that neither use of garlic nor vitamin supplements delays the progression of precancerous gastric lesions to cancer.
Numerous studies have linked dietary modifications and vitamins to cancer prevention and treatment. But researchers are worried that some dietary changes may not be harmless, a concern echoed by a European Union recommendation to tighten sales of supplements.
Steven Thomas, M.D., Ph.D., of the University of Bristol in the United Kingdom, and colleagues used database searches to identify 59 trials that investigated the effects of a diverse range of nutritional interventions on patients with a previous diagnosis of cancer or precancerous lesions. Trial results were combined using meta-analysis.
The authors suggest that the trials provide little evidence that specific interventions have any effect on disease-free survival, mortality, or recurrence. They say the impact of most nutritional interventions cannot be estimated reliably because of the limited number of trials, many of which are small or of low quality.
The authors write, "The large personal expenditure on supplements and dietary modifications by patients with cancer demonstrates an urgent need to understand their effects on cancer outcomes. This vulnerable group of people need to be better informed as diet is one of the few areas of their lives where they may feel that they have some control."
A second study of Chinese adults showed that garlic and vitamin supplements did not reduce the prevalence of precancerous lesions or gastric cancer, but treatment to kill Helicobacter pylori may limit the progression of these lesions to cancer and reduce their prevalence.
In Linqu County, China, gastic cancer causes 42% of all cancer deaths, and H. pylori bacteria is present in 67% of adults. H. pylori causes gastric cancer.
Wei-Cheng You, M.D., of the Beijing Institute for Cancer Research, Mitchell Gail, M.D., Ph.D., of the National Cancer Institute in Bethesda, Md., and colleagues tested 3,365 Chinese adults ages 35-64 in Linqu County. Beginning in 1995, the subjects were randomly assigned to receive various combinations of three interventions--one-time treatment with antibiotics for H. pylori; long-term vitamin E, C, and selenium supplements; or long-term garlic supplements--or a placebo. The researchers reassessed the patients in 1999 and 2003.
The authors found that one-time use of the antibiotics amoxicillin and omeprazole to treat H. pylori infection reduced the severity and progression of precancerous gastric lesions. The data also suggested, but did not prove, that antibiotic treatment reduced gastric cancer incidence. Long-term vitamin and garlic supplementation had no effect on the incidence of gastric cancer or progression of precancerous lesions.
The authors write, " H. pylori treatment reduces the prevalence of precancerous gastric lesions and may reduce gastric cancer incidence [...] Long-term vitamin or garlic supplementation had no beneficial effects."
In an accompanying editorial examining both studies, John A. Baron, M.D., of Dartmouth Medical School in Lebanon, N.H., discusses reasons the two studies may have come to negative conclusions and suggests changes that could have been made to the study designs. He writes, "Together the two articles in this issue of the Journal well illustrate the contemporary status of chemoprevention: hard to summarize, many negative findings, but some nuggets of progress."