The analysis of data on more than 125,000 men and women followed for an average of 24 years found that those who ate the most whole grains had nearly 40 percent lower odds of developing liver cancer compared to those who ate the least.

There were just 141 cases of liver cancer in the study group, though, so more research is needed to determine why whole grains might be protective, the researchers note in JAMA Network Open.

Although deadly, liver cancer is relatively rare in the U.S., said senior study author Dr. Xuehong Zhang of Harvard Medical School and Brigham and Women's Hospital in Boston.

"The low number of cases is primarily because of the very low incidence of liver cancer in the United States (less than 5 per 100,000 individuals) although the incidence has been rapidly increasing in the past decades," Zhang told Reuters Health. "As expected, we have documented no more than 200 (liver cancer) cases, despite the large sample size and long-term follow-up periods."

The researchers suspected that whole grains might be protective against liver cancer because grains have been found to improve a number of well-known risk factors for the disease, Zhang said in an email.

"Consumption of whole grains and dietary fiber, especially cereal fiber, have been associated with lower risk of obesity, type 2 diabetes, and nonalcoholic fatty liver disease, which are known predisposing factors for (liver cancer)," Zhang said. "Besides improving insulin sensitivity, metabolic regulation, and decreasing systemic inflammation, intake of whole grains and dietary fiber may improve gut integrity, and alter gut microbiota composition, thereby leading to increased production of microbiota-related metabolites including short-chain fatty acids, particularly butyrate."

To look at the possible impact of whole grains, Zhang's team examined data gathered in two long-term studies of nurses and other health professionals. Along with a host of other health measurements, the 125,455 participants had filled-out detailed descriptions of their diets approximately once every four years.

When it came to whole grains, even those who ate the most consumed only about an ounce a day (33.28 g/day), Zhang's team found. The researchers divided participants into five groups based on their average intake of whole grains, as well as components of whole grain, bran and germ. They also looked at total dietary fiber from cereal grains, fruits and vegetables.

After accounting for factors such as age, BMI, physical activity, smoking, type 2 diabetes, alcohol consumption and aspirin use, the researchers found that those who consumed the most whole grains were 37 percent less likely to develop liver cancer compared to those who consumed the least.

Liver cancer risk was also reduced among those who ate the most bran, but not those who had the highest germ consumption. The same was true for the highest cereal grain intake, but not for fruit and vegetable fiber.

Outside experts said that with such a small number of cancers it's hard to have a lot of confidence in the association found by the researchers.

Moreover, those who consumed the most whole grains were also the healthiest study participants overall, said Dr. Robert Brown of NewYork-Presbyterian/Weill Cornell Medical Center in New York City.

They "had lower BMI, engaged in more physical activity, consumed less alcohol, were less likely to be smokers, were more likely to use aspirin and tended to have higher intakes of fruits, vegetables, total folate, multivitamin and dietary vitamin, but less fat, compared to participants who took in the least," Brown said in an email.

Beyond that, Dr. Mariana Lazo of the Johns Hopkins University School of Medicine in Baltimore noted, "it's important to be very cautious in singling out particular food items."

While the study is not strong enough to spark new recommendations with respect to liver cancer and whole grains, "given the overall benefits of whole grains relative to refined grains, shifting your diet away from processed grains is likely helpful to all people, including those at risk for liver cancer," Brown said.