Study results were published in the online edition of the American Journal of Epidemiology, April 2008, and also appear in the June print edition.
"Using toenail nicotine is a novel way to objectively measure exposure to tobacco smoke, and ultimately, to increase our understanding of tobacco-related illnesses," explained Wael Al-Delaimy, Ph.D., Department of Family and Preventive Medicine, UC 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."
Researchers analyzed toenail clippings from 62,641 women between the ages of 36 and 61 years collected for the Nurses' Health Study cohort. Between 1984 and 1998, 905 women were diagnosed with heart disease and, on average, those diagnosed with heart disease had double the level of nicotine in their toenails than women without heart disease.
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 toenail nicotine levels were physically less active, had a lower body mass index, drank a higher amount of alcohol, and were more likely to have histories of high blood pressure, diabetes and family history of heart attack than women with lower levels.
The toenails have an advantage over other existing biomarkers of smoking because they grow more slowly. The levels detected in the nails represent nicotine taken up from blood by nails during growth. The slow growth rates of toenails provide a more stable estimate of average exposure, which is most relevant for illnesses related to tobacco smoke, such as heart disease. Using toenail samples in epidemiological studies is also an attractive concept because they're easy to collect and store for relatively low cost.
A limitation of this study is the declining exposure of nonsmokers to secondhand smoke and the decline in active smoking nationwide. The measured exposure in 1982 might have misclassified exposure in later years. "If anything, such decline in exposure will underestimate the risk we found between toenail nicotine levels and heart disease," said Al-Delaimy. "That means the risk is possibly even higher than reported."
This study was funded by grant 12548 from the Flight Attendants Medical Research Foundation.
About the Nurses' Health Study
The Nurses' Health Study was established in 1976, when 121,700 registered female nurses, 30 to 55 years old and residing in one of 11 United States, completed a mailed questionnaire regarding medical history and lifestyle factors. This information has been updated every 2 years since 1976. Cardiovascular risk factors, such as smoking, history of diabetes, hypertension, high cholesterol, and dietary variables, have been updated regularly. In 1982, all participants were asked to collect toenail clippings from their 10 toes and to send them in the provided envelope.
At the beginning of the Nurses' Health Study participants were asked at what age they started smoking and at what age they stopped if they were not smoking. If they were active smokers, they were asked how many cigarettes they smoked per day. The smoking status question was repeated in each 2-year cycle including 1982. Passive smoking was recorded only in the 1982 questionnaire.
For exposure assessment of active smokers, those who reported being current smokers in the 1982 questionnaires were considered as active smokers. Passive smokers were those who reported exposure to tobacco smoke from the home or work environment in the 1982 questionnaire and were never smokers in the 1982 questionnaire. Never exposed are those who were never smokers up to the 1982 questionnaire and reported not being exposed to tobacco smoke from others in 1982. Past smokers are those who reported being smokers previously but were not active smokers in 1982.
About UC San Diego Department of Family and Preventive Medicine
The UC San Diego Department of Family and Preventive Medicine ranks first nationally among peer departments in federal research dollars granted. It is composed of a unique blend of interdisciplinary researchers and clinicians dedicated to conducting ground-breaking research that influences public policy and conquers disease. The department also offers a broad-based curriculum to UCSD undergraduates, medical students, residents, and physicians across the country, and provides health education and clinical care to the underserved community.