Public Health · 1976
Nurses' Health Study
In 1976, the long-term safety of oral contraceptives was genuinely uncertain. The pill had been approved in the United States in 1960, but the epidemiological tools to study its effects in large populations over decades had not yet been applied systematically to women's health. Frank Speizer, an epidemiologist at Harvard's Channing Laboratory, saw an opportunity. Registered nurses were educated enough to complete detailed questionnaires accurately and numerous enough to generate statistical power for rare outcomes. He enrolled 121,700 married female registered nurses between the ages of 30 and 55 in 1976, creating what would become one of the largest and longest-running prospective cohort studies in medicine.
The initial focus was oral contraceptive safety, but the biennial questionnaire design allowed the study to expand its scope as new questions arose. Diet, physical activity, body weight, smoking, hormone use, and family history were tracked systematically over follow-up that eventually stretched past four decades. Outcome ascertainment relied on medical record review, death certificates, and pathology reports, giving the incidence estimates credibility beyond self-report alone.
The findings on postmenopausal hormone therapy proved particularly consequential. Observational data from the Nurses' Health Study in the 1980s had suggested that estrogen therapy reduced cardiovascular risk in postmenopausal women, a result that drove widespread prescribing of hormone replacement therapy. Later, more rigorous analyses from the same cohort, and especially the Women's Health Initiative randomized trial in 2002, showed that combined estrogen-progestin therapy actually increased cardiovascular events and breast cancer risk. The NHS data had been confounded by the healthy-user effect, a lesson that recalibrated how epidemiologists interpreted observational hormone data.
Walter Willett joined the study as a principal investigator and built out the dietary assessment component with a validated food frequency questionnaire. His analyses established that diet quality, particularly the ratio of saturated to unsaturated fat and the glycemic load of carbohydrate sources, predicted type 2 diabetes and coronary disease risk in women independently of body mass index. Obesity and physical inactivity emerged in parallel as independent coronary risk factors, evidence that shaped public health messaging on lifestyle intervention.
The Nurses' Health Study II enrolled a younger cohort of 116,671 nurses in 1989, allowing examination of exposures like oral contraceptive use in adolescence and early adulthood. NHS3 launched in 2010 with an online platform, extending enrollment to male nurses and those from underrepresented groups. The original cohort has produced more than 1,000 peer-reviewed publications. Its datasets continue to be analyzed for new questions, including circadian rhythm disruption from shift work, which the NHS identified as a risk factor for cardiovascular disease and cancer years before controlled mechanistic work confirmed the pathways.
Key People
- Frank Speizer — Harvard epidemiologist who designed and launched the study in 1976.
- Walter Willett — Subsequent principal investigator; led much of the diet and chronic disease analysis.
- Graham Colditz — Harvard epidemiologist and longtime NHS investigator who led cancer risk analyses.
- JoAnn Manson — Harvard physician-epidemiologist who used NHS data to examine hormone therapy and cardiovascular risk.
Am J Nurs, 1978
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