ATLANTA — People who consume three or more alcoholic drinks a day have a 40 percent greater risk of developing colorectal cancer, an analysis published today shows.

The study of a half million people shows for the first time an unmistakable relationship between alcohol consumption and the nation's second deadliest form of cancer.

While some studies have suggested a correlation between beer drinking and colorectal cancer, this study shows that all types of alcohol — liquor, wine and beer — can increase the risk.

"Our study was one of the largest studies, and it shows that no matter the type of alcohol, the risks increase with moderate or more drinking," said Dr. Eunyoung Cho of Brigham and Women's Hospital in Boston, who examined eight studies conducted in Europe and North America over 16 years. The analysis is published in Annals of Internal Medicine.

While alcohol consumption already has been linked to cancers of the breast, ovary and esophagus, the link between colorectal cancer and drinking had been inconclusive.

Cho's research specifies a threshold of danger. People who drink two drinks or fewer a day showed only a slighter higher risk of colorectal cancer, whereas risk increases substantially for those who have three or more drinks a day.

"It all adds up," said Dr. Herman Kattlove, a medical editor and oncologist at the American Cancer Society. "We've known it, but this brings it all together: Alcohol does increase the risk of colorectal cancer."

Cho said she and other researchers do not yet understand just how alcohol affects cells. One theory is that as alcohol breaks down in the body, it degrades folate, a chemical that protects against colorectal cancer. Another theory is that alcohol may damage DNA.

The American Cancer Society estimates that colorectal cancer will kill 56,730 people this year, second to lung cancer's death toll of 160,440.

Dr. John Kauh, assistant professor of hematology at Emory University in Atlanta, said it is still difficult to know if it's the alcohol that causes the higher risk.

Heavier drinkers may eat an unbalanced diet that could be lacking in folate.

"Whether it's a reflection of the alcohol or the lack of nutrients that could be associated with it, we don't know," he said.

Detected early, colorectal cancer is largely curable. The cancer typically is slow-growing.

The American Cancer Society recommends regular screening for people age 50 and older and for those who have family history of the disease or chronic inflammatory bowel disease. The most reliable method is a colonoscopy, experts say.

"We can prevent almost everyone from dying from colorectal cancer by early screening," said Kattlove of the American Cancer Society.

Kattlove said he did not think the new report would prompt the cancer society to recommend that people avoid alcohol. Some studies have shown that alcohol, particularly red wine, may help reduce heart disease.

"The take-home message is always to moderate alcohol intake," Kattlove said. "I wouldn't say cut it out because nearly every major study says that in small quantities, alcohol has a positive effect."