Can toenails predict heart disease risk?

According to a study published in the June issue of the American Journal of Epidemiology, toenails can help predict heart disease risk in female smokers. Researchers from the University of California San Diego School of Medicine and Harvard University analyzed toenail clippings from 62,641 women who were between the ages of 36 and 61 years and participants in the Nurse's Health Study.

Based on their analysis, between 1984 and 1998, 905 women were found to have heart disease, and, on average, these patients had double the level of nicotine in their toenails as women without this condition. Researchers found no difference in age, body mass index, aspirin use, or history of high cholesterol corresponding to the toenail nicotine levels. Women with the highest levels of toenail nicotine were physically less active, had a lower body mass index, drank more alcohol, and were more likely to have histories of high blood pressure, diabetes, and a family history of myocardial infarction than those with lower toenail nicotine levels.

Toenail nicotine levels represent nicotine uptake from the blood by the nails during growth, and because they grow slowly, toenails provide a more stable estimate of average exposure. "Using toenail nicotine is a novel way to objectively measure exposure to tobacco smoke, and, ultimately, to increase our understanding of tobacco-related illnesses," said Wael Al-Delaimy, PhD, Department of Family and Preventive Medicine, University of California San Diego School of Medicine. "It has advantages over using other biomarkers and could become a useful test to identify high-risk individuals in the future. This would be especially helpful in situations where smoking history is not available or is biased," he said.

The Nurses' Health Study, upon which this retrospective review was conducted, was established in 1976, when 121,700 registered US female nurses, 30 to 55 years old, completed a mailed questionnaire regarding lifestyle factors and medical history, including cardiovascular risk factors, such as smoking, history of diabetes, hypertension, high cholesterol, and dietary variables. The information has been updated every 2 years since, and, in 1982, all participants were asked to provide toenail clippings from their 10 toes for nicotine exposure assessment.

A limitation of the current study is the declining exposure of nonsmokers to secondhand smoke and the decline in active smoking nationwide. Furthermore, the measured exposure in 1982 might have misclassified exposure in later years. However, according to Al-Delaimy, "if anything, such decline in exposure will underestimate the risk we found between toenail nicotine levels and heart disease, [which] means the risk is possibly even higher than reported."