Researchers have discovered a reason why smoking greatly increases the risk of heart disease and stroke. According to a mouse study, nicotine promotes insulin resistance, also called pre diabetes, which is a risk factor for cardiovascular disease.

People with blood glucose levels that are higher than normal but not yet in the diabetes range have pre diabetes. Insulin resistance and pre diabetes usually have no symptoms and a person could have one or both conditions for several years without noticing anything.

Image: model of human insulin.

The study results could explain why cigarette smokers have a high cardiovascular death rate, even though “smoking causes weight loss, which should protect against heart disease,” said the study’s lead author, Theodore Friedman.

Pre diabetes and diabetes are known risk factors for cardiovascular disease. Past studies show that cigarette smokers tend to be insulin resistant, meaning that their hormone insulin does not work properly. To compensate, their blood glucose (sugar) levels become higher than normal but not yet high enough for diabetes. Smokers also have higher rates of diabetes, but it is not clear whether smoking is the cause, because they could have other risk factors, Friedman explained.

Some studies demonstrate that nicotine and cigarette smoking induce high levels of the stress hormone cortisol. “As cortisol excess is known to induce insulin resistance, it has been suggested that glucocorticoids, such as cortisol, are the missing [causative] link between cigarette smoking and insulin resistance,” Friedman said.

The study results suggest this theory is correct, he said. The researchers studied the effects, on 24 adult mice, of twice-daily injections of nicotine for 2 weeks. The mice ate less food than control mice that received injections without nicotine, and they also lost weight and had less fat. Despite this, the mice receiving nicotine developed pre diabetes (insulin resistance). These mice also had high cortisol levels in their blood and tissues.

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References:
1. Theodore Friedman, et al. Endocrine Society.

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