Smoking may cause insulin resistance

MedWire News: Nicotine may contribute to the increased risk of heart disease in smokers by promoting the development of a pre-diabetic condition called insulin resistance, study results suggest.

Insulin resistance and diabetes are known risk factors for cardiovascular disease. Research has shown that smokers are more likely to be insulin resistant than non-smokers, but it is not clear whether smoking is the cause, as smokers may have other risk factors for the condition.

Some studies have demonstrated 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," said lead researcher Dr Theodore Friedman, from Charles Drew University in Los Angeles, California, USA.

To investigate further, Dr Friedman and team studied two groups of mice that received twice-daily injections of nicotine or placebo (dummy) solution for 2 weeks. All the mice received the same diet.

They found that mice that received the nicotine injections ate less food and produced less fat than the other mice. Mice that received the nicotine injections also lost weight, whereas the other nice did not. Despite this, however, mice that received the nicotine injections developed insulin resistance, which is usually associated with increased fat levels and obesity.

Encouragingly, the researchers were able to partially reverse this harmful effect by treating the mice with the ‘nicotine antagonist’ mecamylamine – a drug that blunts the action of nicotine. Mice with insulin resistance also had high levels of cortisol in their blood, and treatment with mecamylamine lowered levels of the stress hormone.

The findings, presented at the Endocrine Society's 91st annual meeting in Washington, DC, USA, may explain why cigarette smokers have a high cardiovascular death rate, even though, according to Dr Friedman, "smoking causes weight loss, which should protect against heart disease".

He concluded: "Our results suggest that reducing tissue glucocorticoid levels or decreasing insulin resistance may reduce the heart disease seen in smokers.

"We anticipate that in the future there will be drugs to specifically block the effect of nicotine on glucocorticoids and insulin resistance."