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Here is the abstract for a prospective randomized controlled study that found that regular exercise starting early in pregnancy can reduce the rate of gestational diabetes (GD) in "overweight" and "obese" women.
The study found that cycling 3x per week for at least 30 minutes each time cut the development of gestational diabetes from 40% down to 22%. That's a pretty impressive difference.
Note that the study did not involve special dietary programs or advice. This study was strictly about the effect of regular exercise on the development of GD. Most studies like this do not differentiate between dietary interventions and exercise interventions, but combine the two under "lifestyle intervention." Yet it's really useful to know what the effect of each is individually. This starts to answer that question.
Another good thing about the study was that it was done with Chinese women. Most GD studies are done on Caucasian women. We need more diversity in GD research, so this is a welcome addition.
Another strength of the study is that the intervention was started early in pregnancy. Most studies start exercise interventions in mid-pregnancy, somewhere in the second trimester. This one started it in the first trimester. It certainly seems logical that starting earlier in pregnancy would result in greater benefits than starting later.
This study also looked at the impact of regular exercise on GD in women of size. Often, exercise and GD studies do not look separately at higher-BMI women. In those studies, there seems to be less preventive impact for average-sized women. I strongly suspect that there is far more impact for higher-BMI women.
One weakness is that the study is fairly small. There were 150 women in the exercise group and 150 in the control group. I'd certainly like to see this study repeated with a larger group. However, it was a randomized controlled study, so that strengthens its findings.
Another weakness was that the groups tended to be more in the "overweight" rather than the "obese" category. I would like to see a study like this done where they see what the effect of regular exercise is differentiated by various classes of obesity.
While the study found slightly lower gestational weight gain among the exercise group, the difference was about 2 kg on average, or slightly less than 5 lbs. Not exactly an earth-shaking difference. Researchers need to focus less on the impact on weight gain, which is a fairly negligible difference in many of these studies, and more on more tangible outcomes like GD rates and other outcomes.
Do note that while the study found slightly lower rates of blood pressure issues, cesareans, and big babies among the exercise group, the difference did not rise to statistical significance. The confidence interval crossed 1.0 for all of these. A bigger study would be needed to know whether regular exercise truly affects those outcomes.
Most research around preventing complications in obese pregnancies centers around efforts that combine multiple interventions, but multiple interventions muddy the research waters.
There have been many trials that tried to lower complication rates in obese women through a combination of limiting weight gain, dietary interventions, caloric restriction, and exercise. Results have been highly inconsistent. Some have shown modest results, while others have shown little or no difference in outcomes.
I think they are trying to cast too broad a net. We need more studies that separate out individual factors more carefully so we can examine the benefits ─ and risks ─ more thoroughly.
Each intervention has potential pitfalls that must be considered carefully. For example, aggressively limiting gain has many risks, including low-birthweight babies and prematurity. As a result, many researchers are re-thinking earlier calls for extremely restrictive gain or weight loss during pregnancy.
Studies on nutritional interventions to prevent GD are a mess, with a recent Cochrane review calling most of the evidence "low" or "very low" in quality. We don't really know if nutritional interventions like a low glycemic diet or caloric restriction are effective or even safe at this point.
Even exercise as an intervention for preventing GD has limited research with uneven quality. As noted above, exercise does not seem terribly effective for preventing GD when considering women of all sizes, but it may be more effective for women of size.
Some research suggests that regular exercise may have other benefits for high-BMI women, like cutting labor length. Still other research suggests that exercise may lower the risk for cesareans in first-time mothers of all sizes. However, exercise seems most useful in lowering the risk for GD. I would love to see further studies done on exercise alone, without caloric restriction or weight gain goals. I would like to see the studies be randomized and controlled, to start early in pregnancy or even before, to have more diverse study populations, and to further differentiate effects by class of obesity.
One potential concern has been whether starting an exercise program in pregnancy would lead to low-birth-weight or premature babies. This kept some doctors in the past from recommending exercise to obese pregnant women, but a recent meta-analysis of studies strongly suggests it does not increase the risk for prematurity.
Exercise is not a magic bullet that will prevent all complications in the pregnancies of women of size, but done reasonably, it does seem like it can moderately reduce the risk for certain complications like gestational diabetes. It certainly seems safer than strong weight gain restrictions or extreme caloric restriction.
I'm all for proactive health actions in people of size, and I think regular exercise is one of the most powerful actions women of size can take for pregnancy.
Let's see more research that more clearly delineates the influence of exercise vs. other factors and reassures us that exercise in pregnancy is indeed safe and beneficial for women of size.
Am J Obstet Gynecol. 2017 Feb 1. pii: S0002-9378(17)30172-2. doi: 10.1016/j.ajog.2017.01.037. [Epub ahead of print] A randomized clinical trial of exercise during pregnancy to prevent gestational diabetes mellitus and improve pregnancy outcome in overweight and obese pregnant women. Wang C, Wei Y, Zhang X, Zhang Y, Xu Q, Sun Y, Su S, Zhang L, Liu C, Feng Y, Shou C, Guelfi KJ, Newnham JP, Yang H. PMID: 28161306 DOI: 10.1016/j.ajog.2017.01.037
BACKGROUND: ...Regular exercise has the potential to reduce the risk of developing GDM and can be used during pregnancy; however, its efficacy remain controversial. At present, most exercise training interventions are implemented on Caucasian women and in the second trimester, and there is a paucity of studies focusing on overweight/obese pregnant women. OBJECTIVE: To test the efficacy of regular exercise in early pregnancy to prevent GDM in Chinese overweight/obese pregnant women. RESEARCH DESIGN AND METHODS: This was a prospective randomized clinical trial in which non-smoking women over 18 with a singleton pregnancy and met the criteria for overweight/obese status (BMI<28 kg/m2; obese, BMI>or = 28kg/m2) and an uncomplicated pregnancy at less than 12+6 weeks of gestation were randomly allocated to either exercise or a control group. Patients did not have contraindications to physical activity. Patients allocated to the exercise group were assigned to exercise 3 times per week (no less than 30 min/session with a rating of perceived exertion between 12-14) via a cycling program begun within 3 days of randomization until 37 weeks of gestation. Those in the control group continued their usual daily activities. Both groups received standard prenatal care, albeit without special dietary recommendations. The primary outcome was incidence of GDM. RESULTS: From December 2014 to July 2016, 300 singleton women at 10 gestational age and with a mean pre pregnancy BMI of 26.78 ± 2.75 kg/m2 were recruited. They were randomized into an exercise group (n=150) or a control group (150). 39 (26.0%) and 38 (25.3%) participants were obese in each group, respectively. (1) Women randomized to the exercise group had a significantly lower incidence of GDM (22.0% vs. 40.6%, p<0.001).(2) These women also had significantly (2) less gestational weight gain (4.08±3.02 kg vs. 5.92±2.58 kg, p<0.001) by 25 gestational weeks and at the end of pregnancy (8.38±3.65 kg vs. 10.47±3.33 kg, p<0.001), and (3) reduced insulin resistance levels (2.92±1.27 vs. 3.38 ±2.00, p=0.033) at 25 gestational weeks. Other secondary outcomes, including (4) gestational weight gain between 25 to 36 gestational weeks (4.55±2.06 kg vs. 4.59±2.31 kg, p=0.9), (5) insulin resistance levels at 36 gestational weeks (3.56±1.89 vs. 4.07±2.33, p=0.1), (6) hypertensive disorders of pregnancy (17.0% vs. 19.3%; odds ratio [OR], 0.854; 95% confidence interval [CI], 0.434-2.683, P=0.6), (7) cesarean delivery (except for scar uterus) (29.5% vs. 32.5%;OR, 0.869; 95% CI, 0.494 -1.529, P=0.6), (8) mean gestational age at birth (39.02 ± 1.29 vs. 38.89 ± 37 weeks gestation; P=0.5); (9) preterm birth (2.7% vs. 4.4%, OR, 0.600; 95% CI, 0.140-2.573, P=0.5), (10) macrosomia (defined as birth weight above 4000 g) (6.3% vs. 9.6%; OR, 0.624; 95% CI, 0.233-1.673, P=0.3) and (11) large for gestational age infants (14.3% vs. 22.8%; OR, 0.564; 95% CI, 0.284-1.121, P=0.1) were also lower in the exercise group compared to the control group, but without significant difference. However, infants born to women following the exercise intervention had a significantly lower birth weight compared with those born to women allocated to the control group (3345.27±397.07 vs. 3457.46±446.00, P=0.049). CONCLUSIONS: Cycling exercise initiated early in pregnancy and performed no less than 30 minutes, 3 times per week, is associated with a significant reduction in the frequency of GDM in overweight/obese pregnant women. And the decrease of GDM is very relevant to the less gestational weight gain before the mid-second trimester. Furthermore, there was no evidence that the exercise prescribed in this study increased the risk of preterm birth or reduced the mean gestational age at birth.