Why Smoking Increases The Risk Of Heart Disease And Strokes

The study, which will be presented June 11 at The Endocrine Society's 91st annual meeting in Washington, D.C., found that nicotine in cigarettes promotes insulin resistance, a pre-diabetic condition that raises blood sugar levels higher than normal. People with pre-diabetes are at greater risk of developing cardiovascular disease.

Theodore Friedman, MD, Ph.D., chief of the endocrinology division at Charles Drew University, said the findings help explain a "paradox" that links smoking to heart disease.

Smokers experience a high degree of cardiovascular deaths, Friedman said. "This is surprising considering both smoking and nicotine may cause weight loss and weight loss should protect against cardiovascular disease."

The researchers studied the effects of twice-daily injections of nicotine on 24 adult mice over two weeks. The nicotine-injected mice ate less food, lost weight and had less fat than control mice that received injections without nicotine.

"Our results in mice show that nicotine administration leads to both weight loss and decreased food intake," Friedman said. "Mice exposed to nicotine have less fat. In spite of this, mice have abnormal glucose tolerance and are insulin resistant (pre-diabetes)."

Studies have shown that smokers who are pre-diabetic have blood glucose levels higher than normal, but not high enough for diabetes, a known risk factor for heart disease. Smokers also have higher rates of diabetes, but it is not directly clear whether smoking is the cause, because there could be other risk factors, Friedman said.

In the tests, however, the mice receiving nicotine developed pre-diabetes and also had high levels of the stress hormone cortisol, which increases blood pressure and blood sugar. The study's authors were able to partially reverse the harmful effects of pre-diabetes by treating the mice with a drug that blunts the action of nicotine.

"Our results suggest that decreasing insulin resistance may reduce the heart disease seen in smokers," Friedman said. "We anticipate that in the future there will be drugs to specifically block the effect of nicotine on insulin resistance."

New drugs are needed because those that are currently available are not specific enough to completely block nicotine's effects or they have bothersome side effects, said Friedman, whose study is one of 34 being featured at The Endocrine Society's 91st annual meeting..