WASHINGTON (Reuters) - Heavy marijuana use can boost blood levels of a particular protein, perhaps raising a person’s risk of a heart attack or stroke, U.S. government researchers said on Tuesday.

A young man smokes marijuana before the 10th annual "Marijuana March" in downtown Toronto May 3, 2008. REUTERS/Mark Blinch

Dr. Jean Lud Cadet of the National Institute on Drug Abuse, part of the National Institutes of Health, said the findings point to another example of long-term harm from marijuana. But marijuana activists expressed doubt about the findings.

Cadet said a lot of previous research has focused on the effects of marijuana on the brain. His team looked elsewhere in the body, measuring blood protein levels in 18 long-term, heavy marijuana users and 24 other people who did not use the drug.

Levels of a protein called apolipoprotein C-III were found to be 30 percent higher in the marijuana users compared to the others. This protein is involved in the body’s metabolism of triglycerides -- a type of fat found in the blood -- and higher levels cause increased levels of triglycerides, Cadet added.

High levels of triglycerides can contribute to hardening of the arteries or thickening of the artery walls, raising the risk of stroke, heart attack and heart disease.

The study did not look at whether the heavy marijuana users actually had heart disease.

“Chronic marijuana use is not only causing people to get high, it’s actually causing long-term adverse effects in patients who use too much of the drug,” Cadet, whose study is in the journal Molecular Psychiatry, said in a telephone interview. “Chronic marijuana abuse is not so benign.”

The marijuana users in the study averaged smoking 78 to 350 marijuana cigarettes per week, based on self-reported drug history, the researchers said.

The researchers said the active ingredient in marijuana, known as THC, seems to overstimulate marijuana receptors in the liver, leading to overproduction of the protein.

RAISING FUTURE RISK

Cadet said higher levels of the protein in marijuana users could raise future risk for cardiac abnormalities, blood flow problems, heart attack and stroke.

People with major medical or psychiatric illness, alcohol dependency and other drug use such as cocaine or heroin were excluded from the study.

A U.S. group supporting legal sales and regulation of marijuana disputed the findings. Marijuana Policy Project spokesman Bruce Mirken said, for example, the study involved people who were extremely heavy users.

“I think the low end was 78 joints a week. That’s 10 or 11 joints a day,” Mirken said in a telephone interview.

“We’re talking about people who are stoned all the time. We’re talking about the marijuana equivalent of the guy in the alley clutching a bottle of cheap wine. If you do anything to that level of excess, it might well have some untoward effects, whether it’s marijuana or wine or broccoli,” Mirken added.

Cadet’s team said the findings suggest long-term harm from marijuana beyond issues such as impaired learning, poor memory retention and retrieval and perceptual abnormalities.

But Mirken said: “Even if you take this finding at face value, it’s not at all clear that it has any relevance to the real world because there is still no data showing higher rates of mortality among marijuana smokers. If this was a significant cause of cardiovascular disease, where are the bodies?”