Researchers said it was possible that the nicotine in tobacco bound to melanin, which gives skin its colour. The darker the skin, the greater the amount of melanin — and perhaps the more nicotine is stored. The researchers, whose study appears in Pharmacology, Biochemistry and Behaviour, noted the small number of people used in the study and said the findings should be considered preliminary.

But the research may shed light into why some people appear more affected by nicotine than others. For the study, the researchers, led by Gary King, a professor of biobehavioural health at Penn State, looked at 150 African-American smokers. They measured the volunteers' levels of melanin and cotinine, a byproduct of nicotine. They also surveyed the volunteers about their smoking to assess how strong their habit was. Those people with the most melanin were found to smoke the most and have the most cotinine in their system, and to have the highest level of dependence on tobacco.

Strokes can go unnoticed

You might think that the best way to know if someone ever had a stroke would be just to ask. But researchers have found that among older people, at least, accurate information on the subject is not always likely to be forthcoming. The researchers report that while many patients say they have never had a stroke, MRIs of their brains often tell a different story.