However, antibiotic treatment to eradicate Helicobacter pylori, a common bacterial infection associated with stomach ulcers and cancer, slows the growth of precancerous stomach tissue, the authors of the second paper report.

Dr. Steven Thomas and associates at the University of Bristol in the UK conducted a review of clinical trials that focused on the effects of nutritional supplements in patients with cancer or precancerous tissue. They excluded studies used to treat complications, as well as studies that used synthetic retinoids, vitamin analogues, herbal supplements, and polysaccharide K.

The final analysis included 25 trials on nutritional studies in people with cancer and 34 trials in people with precancerous conditions. The quality of the trials was generally low, the authors note.

Overall, Thomas and his team found little evidence to support claims that nutritional supplements had any effect, either beneficial or harmful.

Thomas's group cautions that "we should not maintain the notion that nutritional interventions can be promoted because at least they will do no harm."

On the other hand, "encouraging a healthy diet is certainly important." Many cancer patients and those with precancerous conditions will live a long time, and if they have a poor diet, they could eventually die of diet-related diseases. But physicians should not make this a priority for patients undergoing cancer treatments.

They also advise doctors to let patients know about the lack of evidence of any benefit of diet supplements.

In the second paper, Dr. Mitchell Gail from the National Cancer Institute in Bethesda, Maryland, and investigators in the U.S. and in China report the results of a study to reduce the high prevalence of advanced precancerous stomach conditions in patients in Shandong Province, China.

In 1994, tests were performed on 3,365 subjects ages 35 to 64 years old. The investigators found that only 0.18 percent had normal stomach linings.

The 2,258 subjects who tested positive for H. pylori infection were randomly assign three treatment groups:

- Omeprazole (sold in the U.S. as Prilosec and Zegerid), which reduces stomach acids and heartburn, and the antibiotic amoxicillin (sold in the U.S. as Amoxil, Trimax and other trade names)

- A supplement containing vitamin C, vitamin E and selenium

- A supplement containing garlic

These patients were also given a placebo.

The 1107 subjects without H. pylori infection were randomly assigned to receive supplements with vitamins, garlic or placebo.

In 1999 and 2003, visualization of the stomach and biopsies were performed for all patients.

Gail's group found no evidence that vitamin or garlic supplements favorably altered the precancerous stomach tissue or reduce the number of subjects with stomach cancer, severe gastritis, intestinal tissue abnormalities or average disease severity scores.

In contrast, they found that H. pylori treatment lead to statistically significant reductions in the prevalence of severe gastritis, intestinal abnormalities, stomach cancer, and the severity and progression of precancerous stomach tissues.

These two reports illustrate the current status of chemoprevention: "hard to summarize, many negative findings, but some hopeful nuggets of progress," Dr. John A. Baron from Dartmouth Medical School in Lebanon, New Hampshire, writes in a related editorial.