WASHINGTON (AP) - That hot cup of coffee may do more than just provide a tasty energy boost. It also may help prevent the most common type of liver cancer.

A study of more than 90,000 Japanese found that people who drank coffee daily or nearly every day had half the liver cancer risk of those who never drank coffee.

The American Cancer Society estimates that 18,920 new cases of liver cancer were diagnosed in the United States last year and some 14,270 people died of the illness. Causes include hepatitis, cirrhosis, excess alcohol consumption and diseases causing chronic inflammation of the liver.

Animal studies have suggested a protective association of coffee with liver cancer, so the research team led by Monami Inoue of the National Cancer Centre in Tokyo analysed a 10-year public health study to determine coffee use by people diagnosed with liver cancer and people who did not have cancer.

They found the likely occurrence of liver cancer in people who never or almost never drank coffee was 547.2 cases per 100,000 people over 10 years.

But for people who drank coffee daily the risk was 214.6 cases per 100,000, the researchers report in this week's issue of the Journal of the National Cancer Institute.

They found that the protective effect occurred in people who drank one to two cups of coffee a day and increased at three to four cups. They were unable to compare the effect of regular and decaffeinated coffee, however, because decaf is rarely consumed in Japan.

It's the caffeine in coffee that makes some people nervous and it has been shown in other studies to prompt mental alertness in many people. Some studies have suggested caffeine aggravates symptoms of menopause or intensifies the side effects of some antibiotics. Heavy caffeine use has been linked to miscarriage. But studies have also shown that a skin cream spiked with caffeine lowers the risk of skin cancer in mice.

"It's an excellent, interesting and provocative study and their conclusions seem justified," commented Dr. R. Palmer Beasley of the University of Texas M.D. Anderson Cancer Center in Houston.

"It will provoke a lot of new work here," said Beasley, who was not part of the research group.

While the study found a statistically significant relationship between drinking coffee and having less liver cancer, the authors note that it needs to be repeated in other groups.

And the reason for the reduction remains unclear.

However, Inoue's team noted that coffee contains large amounts of antioxidants and several animal studies have indicated those compounds have the potential to inhibit cancer in the liver.

In their study, the team also looked at green tea, which contains different antioxidants, and they found no association between drinking the tea and liver cancer rates.

"Other unidentified substances may also be responsible" for the reduction in cancers, they said.

A separate study reported in the same issue of the journal reported no relationship between drinking caffeinated coffee or tea and the rates of colon or rectal cancer.

However, that analysis did find a 52 per cent decline in rectal cancer among people who regularly drank two or more cups of decaffeinated coffee.

In that study a team led by Karin Michels of Brigham and Women's Hospital in Boston analysed data from two large studies - the Nurses' Health Study of women and the Health Professionals' Follow-up Study involving men. The analysis of nearly two million person years found 1,438 cases of colorectal cancer.

While they did not find any association between cancer rates and consumption of caffeinated coffee or tea, people who regularly drank two or more cups per day of decaffeinated coffee had about half the incidence of rectal cancer as those who never drank decaf.

The rate of rectal cancer was 12 cases per 100,000 person-years among those who consumed two or more cups of decaffeinated coffee per day. For those who never drank decaffeinated coffee, the rate was 19 cases per 100,000 person-years.

That difference may, however, be due to differences in lifestyle, the researchers commented, suggesting that drinkers of decaffeinated coffee might be more health conscious overall.

The Japanese study was funded by the Ministry of Health, Labour and Welfare of Japan. The U.S. study was funded by the National Institutes of Health.