A study published online in the British Medical Journal shows for the first time that breathing other people's smoke raises the risk of developing glucose intolerance, the precursor to diabetes.
The US research also shows that overall, white Americans are more susceptible to this effect than African-Americans.
Researchers examined 4572 men and women in four US cities, dividing them into four categories of smoking status: ranging from those who smoked, to those who had neither smoked nor breathed in other people's smoke. The study focussed only on those who were white or African-American.
The authors then tracked how many participants developed glucose intolerance - where the body can no longer produce enough insulin to regulate blood sugar - over 15 years of follow-up.
The study found that smokers had the highest risk, with 22% of them getting the disease over the study period. Non-smokers who had no exposure to second-hand smoke had the lowest risk, with less than 12% developing the condition.
But 17% of those who had never smoked themselves but were subject to second-hand smoke also developed glucose intolerance - higher than the 14% risk rate in the group who had previously smoked and given up.
Those breathing second-hand smoke are exposed to many toxins, say the authors. And the chemical reactions which produce second-hand smoke mean that some of those toxins may be at even higher concentrations than the levels breathed in directly by smokers. If one of these toxins particularly affects the pancreas - the organ which produces insulin - this may explain the findings, they suggest.
Until now, it had not been known that those breathing second-hand smoke faced an increased risk of diabetes, say the researchers. More studies are now needed, they conclude.
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