Take 120,000 female nurses, add another 150,000 to the mix, simmer over four decades, and what do you get? A fruitful recipe for discovering groundbreaking insights into health, in areas ranging from oral contraceptives and breast cancer to diabetes and heart disease. I’m referring to the ongoing Nurses’ Health Study (NHS), which recently celebrated its 40th anniversary.
Long-time readers of the UC Berkeley Wellness Letter and this website have likely noticed that we often refer to this study. Initiated in 1976 by researchers at the Harvard School of Public Health and Brigham and Women’s Hospital in Boston, NHS is now the longest running study of health and wellness ever done. It is particularly vital, since back then most studies focused on men’s health.
NHS is actually more than one study. It has branched out to include additional cohorts of women (NHS2 began in 1989 and NHS3 in 2010). Collectively, the studies have generated more than 1,200 scientific publications and have helped shape public health recommendations, notably the Dietary Guidelines for Americans and the Physical Activity Guidelines for Americans. (Men, don’t feel left out: The male counterpart of NHS, called the Health Professionals Follow-Up Study, was launched in 1986.)
In brief, NHS involves participants' filling out questionnaires over the years about their demographics, lifestyle habits (diet, exercise, smoking), disease risk factors (if they have high blood pressure, for example), environmental exposures, mental status, and other variables. They also provide blood, saliva, urine, and even toenail samples. The researchers then try to connect the dots to see what differences there may be between participants who develop illnesses and those who do not.
A strength of this “prospective cohort study” is that it follows people over time, yielding more accurate results than so-called retrospective studies, which rely on participants to remember what they did in the past (something people are notoriously bad at doing).
Still, it’s an observational study, meaning that it reveals associations but does not prove causality. And because it includes well-educated and mostly white women, the results may not generalize to all women— though women of more diverse races and ethnicities, and even some men, are now included. The studies have also been criticized for overanalyzing the data and finding associations that may have occurred just by chance, no matter how much the researchers try to control for such problems.
Here’s just a tiny sampling of what we know today thanks to NHS, much of which we now take as a given:
- Trans fats, from partially hydrogenated vegetable oils, increase the risk of cardiovascular disease. NHS was the first to make this connection, which was subsequently confirmed in other studies. As a result, trans fats have been mostly eliminated from the food supply.
- Most cases of cardiovascular disease can be prevented by not smoking, getting regular exercise, maintaining a healthy weight, consuming moderate alcohol, and following a diet low in saturated and trans fats, refined grains, and sugary drinks and rich in vegetables, fruits, whole grains, and unsaturated fats.
- Women can reduce their breast cancer risk by limiting weight gain, limiting alcohol, eating more vegetables, and reducing duration of hormone therapy (estrogen plus progestin). NHS has also identified other breast cancer risk factors, including having dense breasts and benign breast disease.
- Key risk factors for colorectal cancer include red and processed meat, alcohol, obesity, and smoking. Exercise, aspirin, vitamin D, calcium, and folate seem to be protective.
- Being overweight is the main risk factor for type 2 diabetes. Maintaining a healthy weight—together with eating healthfully and following other healthy lifestyle habits—can prevent about 90 percent of diabetes cases.
Commenting in a Harvard press release, Walter Willett, M.D., Ph.D., one of the leading NHS investigators, said, “the Nurses’ Health Studies have provided evidence that allows individuals, health care providers, and policy makers to reach informed decisions. This would have all been impossible without the amazing commitment of the participants, who have shared their personal information over the last four decades.”
I’d like to extend my personal gratitude to these trailblazing women—94 percent of whom have stuck it out the entire 40 years. None of them, I bet, could have imagined the extraordinary contributions they would be making to the field of public health and women’s health in particular. Want to be one of them? NHS3 is still recruiting nurses (and student nurses), ages 19 to 51, who live in the U.S. or Canada—men included.
To learn more about NHS, you can read for free the entire issue of the American Journal of Public Health devoted to the study’s 40th anniversary.
Also see 3 Types of Research Studies.