Adherence to healthy lifestyle and risk of gestational diabetes mellitus
Prospective cohort study
Adherence to healthy lifestyle and risk of gestational diabetes mellitus: prospective cohort study by Culin Zhang and colleagues (BMJ 2014;349:g5450).
Objective—To quantify the association between a combination of healthy lifestyle factors before pregnancy (healthy body weight, healthy diet, regular exercise, and not smoking) and the risk of gestational diabetes.
Design—Prospective cohort study.
Setting—Nurses’ Health Study II, United States.
Participants—20 136 singleton live births in 14 437 women without chronic disease.
Main outcome measure—Self reported incident gestational diabetes diagnosed by a physician, validated by medical records in a previous study.
Results—Incident first time gestational diabetes was reported in 823 pregnancies. Each lifestyle factor measured was independently and significantly associated with risk of gestational diabetes. The combination of three low risk factors (non-smoker, ≥150 minutes a week of moderate to vigorous physical activity, and healthy eating (top two fifths of Alternate Healthy Eating Index-2010 adherence score)) was associated with a 41% lower risk of gestational diabetes compared with all other pregnancies (relative risk 0.59, 95% confidence interval 0.48 to 0.71). Addition of body mass index (BMI) <25 before pregnancy (giving a combination of four low risk factors) was associated with a 52% lower risk of gestational diabetes compared with all other pregnancies (relative risk 0.48, 0.38 to 0.61). Compared with pregnancies in women who did not meet any of the low risk lifestyle factors, those meeting all four criteria had an 83% lower risk of gestational diabetes (relative risk 0.17, 0.12 to 0.25). The population attributable risk percentage of the four risk factors in combination (smoking, inactivity, overweight, and poor diet) was 47.5% (95% confidence interval 35.6% to 56.6%). A similar population attributable risk percentage (49.2%) was observed when the distributions of the four low risk factors from the US National Health and Nutrition Examination Survey (2007-10) data were applied to the calculation.
Conclusions—Adherence to a low risk lifestyle before pregnancy is associated with a low risk of gestational diabetes and could be an effective strategy for the prevention of gestational diabetes.
Why do the study?
Gestational diabetes is defined as glucose intolerance with onset or first recognition in pregnancy. It affects 2-6% of pregnancies, but multiple population based studies have shown increases in the occurrence of gestational diabetes in the past decade.  
Gestational diabetes generally resolves after the baby is born, but it is linked to adverse health outcomes in women and their offspring. Women have an increased risk for type 2 diabetes after pregnancy, and children born after a pregnancy with gestational diabetes have a higher risk of developing childhood obesity and glucose intolerance in early adulthood.
Prevention of gestational diabetes could be an important strategy in curbing the obesity and diabetes epidemic in this and future generations. We already know that maintaining a healthy body weight, adopting a healthy diet, regular physical activity, and abstinence from cigarette smoking are each associated with a lower risk for gestational diabetes. However, we do not know exactly what the combined effect is of these four lifestyle risk factors on preventing gestational diabetes. This study sought to answer this question.
What did the authors do?
The authors undertook a prospective cohort study of 14 437 women who did not have any underlying chronic diseases. Cohort studies are observational studies that follow a group of people over time to see what happens to them. Researchers collect data about the people participating in the study and look for patterns between a particular variable and an outcome. In this study researchers used data from a well known established cohort, the Nurses’ Health Study II in the United States. The cohort was established in 1989 by enrolling 116 671 female nurses aged 24-44 at the start of the study from 15 states in the United States. These women answered an extensive questionnaire on medical, diagnostic, and prescription history; occupational status; family history of diabetes; gravidity; height and weight; and a range of other characteristics. After that, they answered questionnaires twice yearly to update lifestyle characteristics and health related outcomes.
Physical activity was assessed on the main questionnaire in 1989, 1991, and 1997. Participants were asked to report the average amount of time they spent each week on the following moderate to vigorous recreational activities: bicycling, calisthenics or use of a rowing machine, lap swimming, squash or racquetball, jogging, running, tennis, or other aerobic exercise. Women were also asked to report the amount of time they spent each week walking—that is, walking or hiking outdoors, including walking to work—and to categorise their usual walking pace outdoors as easy or casual (slower than 2.0 mph), normal (2.0-2.9 mph), brisk (3.0-3.9 mph), or very brisk or striding (faster than 4.0 mph). Their total physical activity before pregnancy was calculated as the sum of the minutes spent each week in each activity, including brisk or very brisk walking. Participants were also asked to self report whether they smoked and how much they weighed twice a year. Self reporting can be a source of bias so the researchers objectively checked the weight in a subset of the participants, and this was found to be sufficiently accurate.
Beginning in 1991 and every four years thereafter, participants were asked to complete a questionnaire to measure average dietary intake over the past year. To assess the quality of the diets the researchers created a summary diet score based on the US Department of Agriculture Healthy Eating Index, designed to measure adherence to US dietary guidelines. They computed scores for adherence to the index before pregnancy from each food frequency questionnaire cycle occurring before a given pregnancy. They included 10 of 11 components of the index in their diet score: higher intakes of vegetables, fruit, nuts, whole grains, polyunsaturated fatty acids, and long chain omega 3 fatty acids and lower intakes of red and processed meats, sugar sweetened beverages, trans fats, and sodium. Participants’ scores for each of the individual components ranged from 0 to 10 based on their level of intake, with 10 representing optimal dietary behaviour. The overall diet score ranged from 2.5 (worst) to 77.5 (best).
The outcome of interest, gestational diabetes, was assessed in the main questionnaire. This was also self reported, and the researchers checked its accuracy by a validation study among a subgroup of the Nurses’ Health Study II cohort. They were able to confirm 94% of self reported gestational diabetes events by medical records.
In cohort studies, researchers calculate the probability of developing the outcome of interest for a given variable. These researchers were interested in the joint effect of four lifestyle risk factors: a healthy body weight, adherence to a healthy dietary pattern, regular exercise, and abstinence from smoking. For these four risk factors the researchers categorised each participant into different risk groups. They used the following definitions. Not being overweight or obese was classified as a body mass index (BMI) <25. Participants in the upper two fifths in their scores before pregnancy on the Alternate Healthy Eating Index-2010 were considered to be in the lower risk category for adherence to a healthy diet. Low risk usual exercise was defined as an average of at least one half hour a day of vigorous or moderate physical activity for five days a week (150 minutes a week), including brisk walking, in keeping with published US guidelines. Women who reported being current non-smokers in the questionnaire preceding the pregnancy were counted as low risk for smoking status. For each of the four factors, researchers created a binary variable, with participants receiving 1 if they met the criteria for low risk and 0 otherwise.
What did the study find?
During the 10 years of follow-up, they found 823 pregnancies with gestational diabetes among the 20 136 eligible singleton pregnancies in 14 437 women (93% white). As we already knew, the four risk factors were all individually associated with gestational diabetes. The associations remained significant after adjustment for other risk factors of gestational diabetes such as maternal age, parity, and family history of diabetes.
When the researchers looked at participants who had more than one low risk factor, they found that the risk of gestational diabetes went down with each additional low risk factor (see figure). 1 If we assume that women who had no low risk factor had a risk of 1.0, then women with four low risk factors had a relative risk of developing gestational diabetes of less than 0.2. To phrase it more simply: women at low risk for all four lifestyle factors had more than an 80% lower risk compared with those without any of the low risk factors.
What are the strengths and limitations of this study?
A strength of this study is its size: it is large and has a relatively long follow-up. All data were collected prospectively. It is also the first study to look at the combined effect of the four modifiable risk factors. The biggest limitation of observational studies is that we can draw conclusions only on relations and not on causations. In this study it is tempting to speculate that modifying all the four risk factors would decrease an individual’s risk of developing gestational diabetes by 80%. It is possible, but we would need a randomised controlled trial to prove that.
Another aspect to look at is generalisability of the population. The participants were all nurses, who are perhaps more likely to seek medical attention, and this is reflected in the relatively high overall rate of gestational diabetes of 5.6%. Another limitation is that most were white and well educated, so it is problematic to apply the results directly to other populations.
What does the study mean?
The data indicate that adherence to a healthy lifestyle in the period before pregnancy is associated with a substantially lower risk of gestational diabetes. However, we do not know if tackling these risk factors in an individual patient will translate into a lower risk of gestational diabetes. However, as there is no large trial in progress to assess this it might be the best evidence we have to recommend these lifestyle modifications to pregnant women to ward off gestational diabetes.Wim Weber, European research editor, The BMJ
Correspondence to: firstname.lastname@example.org
Competing interests: None declared.
Provenance and peer review: Commissioned; not externally peer reviewed.
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Cite this as: Student BMJ 2014;22:g6713