How much nicotine you have in your toenails could predict your risk of developing lung cancer according to a new American study published online last week.

Using data from a large health study, the researchers compared nicotine levels in toenail clippings of male smokers who developed lung cancer over a 12 year period with equivalent male smokers who did not, and found the ones with the highest nicotine levels were 3.57 times more likely to develop lung cancer, regardless of their smoking history.

They said their findings suggest studies that only use smoking history, based on self-reported data, may be underestimating the adverse effect of smoking on health.

For example, if you only use how much people say they smoke as a way to assess lung cancer risk, you may not be measuring how much tobacco smoke and carcinogens they actually inhale into their lungs, leaving open the chance of error coming from respondents either not giving an accurate account, or because some just inhale more than others.

The two study authors were Dr Wael K Al-Delaimy, Associate Professor & Chief, Division of Global Health, Family & Preventive Medicine Cancer Prevention & Control Program at the University of California San Diego (UCSD) Medical Center, and Dr Walter C Willett, Fredrick John Stare Professor of Epidemiology and Nutrition Chair Department of Nutrition, Harvard School of Public Health, in Boston, Massachusetts, and the journal that published their paper is the American Journal of Epidemiology.

They set out to see if nicotine levels in toenail clippings might be an effective biomarker for lung cancer risk.

They used toenail clippings because nails grow slowly and thus provide a more stable measure than say saliva or urine.

For the study, Al-Delaimy and Willett used data on male smokers aged between 40 and 75 years from the Health Professionals Follow-up Study and who, in 1987, had given samples of toenail clippings for nicotine analysis.

They compared 210 men who were diagnosed with lung cancer between 1988 and 2000, with 630 matched controls who did not develop lung cancer.

The found that the mean nicotine level among the men with lung cancer was 0.95 ng/mg, which differed significantly from the 0.25 ng/mg of the men who did not develop lung cancer (P < 0.0001).

The researchers then compared the 20% of men with the highest level of toenail nicotine against the 20% with the lowest level, and found the relative risk of developing lung cancer for the former was a significant 10.5 times higher compared to the latter (95% confidence interval CI ranged from 5.61 to 19.64; while P for trend < 0.0001).

However, this result was before they took into account smoking history, that is how many cigarettes the men said they smoked when they answered questions for the health study.

When Al-Delaimy and Willett adjusted the figures to take into account the men's self-reported smoking history (based on pack- years), the relative risk in the 20% of men with the highest nicotine levels in their toenail clippings was still a significant 3.57 times higher than for the 20% with the lowest levels (95% CI 1.73-7.37; P for trend < 0.0001).

They concluded that:

"… the toenail nicotine biomarker was found to be a strong predictor of lung cancer independent of smoking history, suggesting that the adverse effects of cigarette smoke may be underestimated in studies based on smoking history only."

"Toenail Nicotine Level as a Novel Biomarker for Lung Cancer Risk."
Wael K Al-Delaimy and Walter C Willett
Am. J. Epidemiol. kwq446, first published online 2 March 2011

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