May 13 (HealthDay News) -- Smoking marijuana increases the body's production of a protein that raises levels of blood fats associated with heart attack and stroke, government researchers report.

The finding helps explain the increased risk of such cardiovascular problems found in previous studies of marijuana, said Dr. Jean Lud Cadet, chief of the Molecular Neuropsychiatry Research Branch of the National Institute of Drug Abuse. He is lead author of the report, published in the May 13 issue of Molecular Psychiatry.

"We've done a number of clinical papers reporting on marijuana users," Cadet said. "In the past, we have reported cognitive abnormalities, and a paper in 2005 reported vascular [blood vessel] abnormalities."

The latest paper looked at blood levels of a variety of proteins in 18 long-term heavy users of marijuana and 24 volunteers who had never used the drug. It used a new electronic technology that made such protein measurements possible, Cadet said.

"The idea was that there might be marijuana-induced changes in proteins related not only to the vascular problems we were reporting but also to other medical problems marijuana users suffer from," he said.

The analysis showed significantly higher blood levels of apolipoprotein C-III in the marijuana users than in the nonusers. APOC-III is one of a large family of proteins that interact with fats. It delays the breakdown of triglycerides, fats that can accumulate in blood vessels and promote formation of plaques, leading to blockages that can cause heart attacks, strokes and other problems.

"It is a risk factor for cardiovascular disease," Cadet said. "The study suggests that APOC-III might be one of the risk factors contributing to the medical problems that marijuana users suffer from."

In terms of heart disease and stroke, is smoking marijuana more dangerous than smoking tobacco? "It is very difficult on the basis of our study to say," Cadet said, since there was no comparison between marijuana and tobacco smokers.

"It is an interesting study, but I don't think it is particularly profound," said Dr. Christopher Granger, a professor of medicine at Duke University who has done studies of risk factors in cardiovascular disease. "It looks at a relatively small number of people and the relationship between marijuana and this particular protein in the blood stream. There is a relationship, but there is far from proof that it is cause-and-effect, and if it is cause-and-effect that it would have an effect on the people using marijuana."

The study "does lift us to a new level of understanding about marijuana use, that it may have a real impact on the way people who use marijuana metabolize food, showing that it could have an adverse effect on cholesterol," Granger said.

"I consider this study exploratory, by definition, because of the technique they used, sort of a shotgun approach," said Dr. Kenneth Mukamal, an associate professor of medicine at Harvard Medical School who has studied the effects of marijuana use on cardiovascular disease. "They chose this protein out of a range of thousands of possibilities, so there is a chance you find something just by luck."

The effects of marijuana on the cardiovascular system are still uncertain, Mukamal said. "It has negative effects on heart rate and may even trigger heart attacks," he said. "But in mice, it apparently prevents atherosclerosis. Whether it is good or bad remains a question."

SOURCES: Jean Lud Cadet, M.D., chief, U.S. National Institute on Drug Abuse Molecular Neuropsychiatry Research Branch, Baltimore; Christopher Granger, M.D., professor, medicine, Duke University, Durham, N.C.; Kenneth Mukamal, M.D., associate professor, medicine, Harvard Medical School, Boston; May 13, 2008, Molecular Psychiatry, online