Cycling to break the cycle of diabetes in pregnancy: the role of exercise in the prevention of gestational diabetes mellitus

Gestational diabetes mellitus (GDM) affects an increasing number of women worldwide, with the recurrence rate of this condition being as high as 69%. To date, few randomised controlled trials have examined the effects of regular antenatal exercise on the prevention of GDM and no previous studies have specifically sampled women with a history of GDM in a prior pregnancy, who are at high-risk of recurrence. Unfortunately, the barriers to exercise experienced by this population of women are often exacerbated, since they have existing children to care for, along with other household/work responsibilities and a lack of time. Given that low adherence rates to exercise has been highlighted as a limitation in some previous studies reporting regular exercise to be ineffective in preventing GDM, consideration must be given to the design of exercise interventions for the pregnant woman. Supervised, home-based exercise may effectively remove the issues of child care availability and transportation faced by pregnant women consequently enhancing opportunities for exercise engagement. However, the specific format and intensity of exercise to safely optimise benefits for health and fitness for pregnant women is not known.

The first study of this thesis (Chapter 2) examined how the addition of brief, self-paced, higher intensity intervals to traditional continuous moderate intensity exercise affected energy expenditure and the enjoyment of exercise in pregnant women. This was important to determine given that any additional energy expenditure resulting from higher intensity intervals may be meaningless if enjoyment is compromised, since long term adherence would likely be low. Twelve healthy pregnant women in their third trimester (30 ± 1 weeks gestation; age 35 ± 6 y; BMI 27.1 ± 4.3 kg/m2) performed either(i) continuous stationary cycling (CONT) at a steady power output equivalent to 65% age-predicted heart rate maximum or (ii) interval cycling (INTV) consisting of continuous cycling at the same power output as CONT, but with the addition of six 15-sself-paced higher intensity efforts performed at regular intervals. Mean cycling power output, heart rate, oxygen consumption and energy expenditure were higher during INTV compared with CONT (P < 0.05), suggesting that the addition of as little as six15-s intervals (90 s in total) to a 30 min bout of continuous moderate intensity exercise effectively increases overall energy expenditure. Despite this overall higher intensity, there was no difference in the mean rate of perceived exertion between conditions, while the enjoyment of exercise was higher with INTV (P = 0.01).

Given that interval-type cycling was found to effectively increase energy expenditure and enhance the enjoyment of exercise compared with traditional continuous cycling in pregnant women, the second study of this thesis (Chapter 3) was a randomised controlled trial that investigated the effects of a 14-week supervised, home-based exercise program of interval stationary cycling on the recurrence of GDM in women with a history of this condition in a previous pregnancy. The secondary aims of the study included examining the effects of the exercise intervention on maternal cardiovascular fitness, body anthropometrics, psychological well-being and obstetric outcomes. From June 2011 to July 2014, 172 women with a history of GDM (age 33.7 ±4.0 y; BMI 26.1 ± 5.3 kg/m2) were randomised into either an exercise intervention (EX; n = 85) or standard routine antenatal care (CON; n = 87) at 13 ± 1 weeks gestation. The intervention involved 14 weeks of supervised, home-based exercise on a stationary cycle ergometer, performed three times a week for 30 – 60 min. Overall compliance to the intervention was 83%. Based on an intention-to-treat analysis, the recurrence of GDM was similar between groups (EX, 40.5%; CON, 40.0%), and there was no difference in the overall degree of glucose tolerance or insulin response to a 75 g oral glucose tolerance test between groups post-intervention (28 ± 1 weeks; P > 0.05).However, significant improvements in maternal cardiovascular fitness were observed in EX, resulting in higher fitness in EX compared with CON post-intervention (P = 0.001).In addition, psychological distress (depression, anxiety and stress) assessed by the Depression Anxiety and Stress Scale21 was significantly reduced in EX compared with CON post-intervention (P < 0.05). Importantly, there were no differences in obstetric and neonatal outcomes observed between groups suggesting that the exercise intervention had no adverse effects on fetal well-being during pregnancy.

 Overall, the findings of this thesis suggest that a regular program of supervised, home-based exercise commenced from the first trimester of pregnancy does not prevent GDM in women with a history of the condition. However, the intervention was successful in overcoming barriers to exercise in pregnancy, promoting high adherence to the exercise program and providing important benefits for maternal cardiovascular fitness and psychological well-being. Despite the lack of effect of the intervention on GDM recurrence observed here, future studies are needed to determine whether the commencement of a regular program of exercise earlier in pregnancy, or even prior to conception affect the incidence of GDM.