Levels of nicotine in toenails can help predict risk for coronary heart disease (CHD), a large case-control study in women shows.
The study revealed that the greater the womens toenail nicotine levels were, the higher their risk for CHD - irrespective of smoking history or passive smoking exposure.
Wael Al-Delaimy (University of California, San Diego, USA) and team explored toenail nicotine levels in relation to CHD risk in 62,641 women aged 36 to 61 years who participated in the Nurses Health Study.
"Reported smoking exposure explained only approximately 60% of the variability of toenail nicotine levels, suggesting that [toenail nicotine] measures may reflect different aspects of smoking exposure," they explain.
The researchers analyzed nicotine levels in toenail clippings provided by the women in 1982 and how these related to incident CHD cases at follow-up from 1984 to 1998.
They identified 905 cases of CHD disease during this period and matched each with two control participants without CHD by age and date of toenail collection.
As reported in the American Journal of Epidemiology, there was a dose-response relationship between increasing toenail nicotine level and risk for CHD (p for trend <0.0001). Women in the highest quintile of nicotine concentration had a 3.44-fold increased risk for CHD compared with women in the lowest quintile, after adjustment for other CHD risk factors.
Further analysis showed that each unit increase in continuous log-transformed toenail nicotine levels was associated with a 42% increased relative risk for CHD. The authors say this association was attenuated but still significant after taking into account reported cigarette smoking and passive smoke exposure, at a 12% increased relative risk.
Al-Delaimy et al explain that the slow growth rate of toenails means this measure provides a stable estimate of average smoke exposure. One limitation of the current study could be that both active smoking and nonsmokers exposure to second-hand smoke are on the decline - the measurement in 1982 may have overestimated the womens exposure in later years.
But Al-Delaimy pointed out: "If anything, such decline in exposure will underestimate the risk we found between toenail nicotine levels and heart disease."