© 2000 by Oxford University Press
Journal of the National Cancer Institute, Vol. 92, No. 8, 597-599,
April 19, 2000
© 2000 Oxford University Press
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Nurses’ Health Study: Nurses Helping Science and Themselves
As the Nurses’ Health Study nears its 25-year anniversary, a glance back at the contributions made to women’s health by more than 100,000 nurses reveals an impressive legacy. Early findings focused on the effects of oral contraceptives and smoking on major illnesses in women. As time progressed, the study has broadened to look at the effects of dietary and other lifestyle factors on women’s health. Now, nearly 400 publications later, the Nurses’ Health Study is still active with more than 50 publications in 1999 alone.
It all began with the birth control pill. Frank E. Speizer, M.D., of Harvard Medical School’s Channing Laboratory in Boston, wanted to determine the long-term health effects, if any, of oral contraceptives. So, in 1976, as head of the Nurses’ Health Study (NHS) research group, Speizer sent a letter and two-page questionnaire to 121,000 married, female registered nurses 30 to 55 years old. The letter accompanying the questionnaire stated "we believe that registered nurses—because of their knowledge, training, and interest in health issues—will provide a higher response rate as well as more complete and accurate information than other groups."
This belief has certainly been borne out with time. Over the years, more than 90% of the nurses in the study have responded to questionnaires sent every other year, faithfully recording answers to detailed questions about their illnesses, vitamin intake, sense of well-being, amount of physical exercise, and consumption of meat and various fruits and vegetables.
So what have the researchers found? The most striking finding, according to one of the lead investigators, JoAnn E. Manson, M.D., Dr.P.H., professor of medicine at Harvard Medical School and chief of preventive medicine at Brigham and Women’s Hospital, is the vital role of lifestyle factors in the prevention of chronic diseases in women, and presumably in men as well.
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"The overall message is that heredity isn’t destiny," she said. "We largely control our risk of these major diseases through our lifestyle practices. The findings are really striking to us."
For example, Manson explained that the NHS found that the vast majority of heart attacks in women are preventable through relatively simple lifestyle modifications—exercising regularly, maintaining healthy weight, and making dietary changes such as reducing saturated fats (found in meat and high-fat dairy products like butter) and trans fatty acids (found in baked goods containing partially hydrogenated vegetable oils, such as pastries), increasing fiber and fruits and vegetables, and not smoking. In addition, data from NHS and other studies suggest that a large percentage of colon cancer is preventable with early screening—fecal occult blood testing, sigmoidoscopy, and/or colonoscopy. Lifestyle factors may prevent at least 80% of adult-onset diabetes and 70% to 80% of heart disease and stroke.
Lifestyle Changes
Graham Colditz, M.D., Dr.P.H., professor of medicine at Harvard Medical School and one of the early members of the NHS Research Group, believes that a unique aspect of the study is the constant updating of the lifestyle changes of the nurses.
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"Probably the biggest difference between the nurses and other cohorts is that we take repeated measures of lifestyle every 2 years," said Colditz. "This really separates us out from other studies which generally take a one-time baseline measure and then follow the population forward. This is very important when factors such as postmenapausal hormones, and various components of the diet change over time."
Since 1976, for example, the researchers have found far more use of combined hormones (estrogen plus progesterone) compared with estrogen alone, a reduction in red meat consumption, and weight gain among the nurses. And it appears that these changes can have important effects on a woman’s health: Combined hormone use is associated with an increased risk of breast cancer; lower red meat intake is associated with lower colon cancer risk; and weight gain is associated with increased risks of several major diseases—heart disease, stroke, adult-onset diabetes, postmenopausal breast cancer, and colorectal cancer.
Manson finds the weight gain trend disturbing. "The prevalence of obesity is increasing substantially in children, adolescents, and adults in the United States," she said. "Children are now beginning to develop adult-onset diabetes."
The findings from the NHS show that the type of dietary fat consumed is more important than the total amount of fat. "Monounsaturated fats from plant oils and polyunsaturated fatty acids from fish and certain vegetable oils are beneficial in terms of heart disease, and perhaps cancer and diabetes," said Manson. "It’s the saturated fats and trans fatty acids that are associated with higher disease risks."
Other findings from NHS:
• Researchers found that breast cancer risk increased with an increasing number of drinks per day.
• No association was found between total fat intake and breast cancer risk.
• Folate and folic acid were found to be associated with lower risk of heart disease, colorectal cancer and perhaps other forms of cancer.
• The risk of pancreatic cancer falls rapidly after stopping cigarette smoking.
• Women who have used oral contraceptives for more than 5 years have half the risk of ovarian cancer.
• The current use of hormone replacement therapy in women up to age 71 is associated with a lower risk of heart attack.
• No association was seen between dietary calcium intake among postmenopausal women and the risk of osteoporotic fractures.
Limitations
One of the limitations in the study of this group of nurses is that of any observational study—the researchers cannot control the lifestyle factors they want to study. Randomized trials are the gold standard to test risk factors for disease.
"However, the Nurses’ Health Study plays an important role in generating hypotheses that are then tested in randomized trials," said Manson. Research generated by NHS and other observational studies, for example, contributed to the initiation of the Women’s Health Initiative, a clinical trial focusing on strategies for preventing heart disease, breast and colorectal cancer, and osteoporosis in postmenopausal women (see sidebar, p. 598).
And even though compared with the general population the nurses are generally white, thinner, more health-conscious, more highly educated, and have better access to medical care, the homogeneity can be an advantage. "If you look at nurses who smoke versus nurses who don’t smoke, or nurses who eat vitamins versus those who don’t, those biases tend to cancel out when you’re looking at comparisons within the group," said Terry Manolio, Ph.D., an epidemiologist from the National Heart, Lung, and Blood Institute.
Cohort Comparisons
Have the results from NHS been consistent with other reports? A publication last year showing a lack of association between fiber and colon cancer, for example, flew in the face several previous studies. Manson thinks that the relationship between fiber and colon cancer is complex, that there is probably not a strong association, and that if there is one, it may depend on the type of fiber. Colditz agreed and also noted the NHS found a weaker association than other groups between diets high in fruits and vegetables and lower health risks.
The Nurses’ Health Study II began in 1989 with an eye toward resolving the long-term health effects of oral contraceptives and clarifying the role of exercise and diet in cancer. A second cohort of more than 116,000 female nurses aged 25 to 40 are now faithfully responding to detailed inquiries every 2 years about their diet, hormone use, medical problems, exercise habits, and work environment. A cohort of children of the younger nurses was added to examine adolescent diet, activity, and excess weight gain.
The commitment of nurses is legendary. Lois Breen, R.N., of Pittsfield, Mass., who was contacted in 1976 to participate in the study after she graduated from Massachusetts General Nursing School, seems to typify their dedication. "It’s been great to feel that you’ve contributed a little bit to something. I will definitely keep on doing it. I think that everyone who does it feels very strongly about it," she said.
And there are no signs that the high productivity of the two research groups is waning. This year more than 20 articles have already been submitted, are in press, or have been published.
"The uniqueness of the Nurses’ Health Study is that it began a long time ago and because of the foresight of the researchers they have the opportunity to evaluate a lot of risk factors," said Louise Brinton, Ph.D., epidemiologist at National Cancer Institute’s Division of Cancer Epidemiology and Genetics in Bethesda, Md. "Over time they have incorporated a lot of different elements and modified the questionnaire to address new topics. The cohort continues to be rich with findings and opportunities."
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