The global obesity epidemic continues unabated, now rapidly expanding to developing countries. Multiple comorbidities and premature mortality are associated with obesity, most frequently diabetes. The associated financial and economical burden is escalating as well. The sedentary lifestyle adopted by many pregnant women because of traditional practices and the current recommendation for gestational weight gain are contributing factors to the obesity and diabetes epidemic.
Physical inactivity is recognized as an independent risk factor for obesity insulin resistance and type 2 diabetes; the physiological and hormonal changes associated with pregnancy magnify this risk. Conversely, evidence and accumulated experience indicate that antenatal lifestyle interventions that include physical activity and judicious dieting could improve the pregnancy outcome and reduce the risk of gestational diabetes and is effective as an adjunctive therapy for diabetes in pregnancy. All major professional organizations, among them American Congress of Obstetricians and Gynecologists (ACOG), American Diabetes Association (ADA), Royal College of Obstetricians and Gynaecologists (RCOG), and Society of Obstetricians and Gynaecologists of Canada (SOGC), recommend lifestyle interventions that include diet and exercise to prevent or manage gestational diabetes or diabetes mellitus.
Introduction
In 2008, it was estimated that 508 million people worldwide were obese and another 1.46 billion people overweight and many more currently . The associated comorbidities, particularly diabetes, and consequences are staggering and the financial burden is of historic proportions. Globally, 366 million are affected by diabetes; it is expected that there will be 552 million diabetics by 2030 . For many women, gestational diabetes is the first manifestation of diabetes.
Several large trials conducted in nonpregnant subjects in China, Finland and United States have proven that adopting a healthy lifestyle that includes weight reduction and physical activity can prevent, manage, or reverse diabetes . Recommendations for the prevention of type 2 diabetes in nonpregnant women include pharmacotherapy, surgery, diet, aerobic exercise, and, most recently, also resistance training . For pregnant women, the only management option available is lifestyle modification. The most effective interventions have combined judicious weight gain and physical activity.
Pathophysiology
Normal pregnancy has been characterized as a diabetogenic event due to hormonal changes, and the progressive insulin resistance begins near mid-pregnancy and progresses through the third trimester to levels that approximate the insulin resistance of patients with type 2 diabetes . Insulin resistance develops at the level of skeletal muscles; thus, exercise is a logical intervention that can counteract this activity. However, caloric restriction also plays an important role in improving insulin sensitivity.
In overweight and obese patients, insulin resistance is further increased in pregnancy. Obese women are at an increased risk of gestational diabetes mellitus (GDM) (odds ratio (OR) 2.6; 95% confidence interval (CI) 2.1–3.4). The increased fat deposition leads to an increase in adiponectin and leptin. Adiponectin regulates insulin sensitivity and glucose homeostasis. Additional weight gain leads to a further increase in adipokines and leptin, which increase the oxidative stress that contributes to the insulin resistance. Adiponectin regulates insulin sensitivity; low levels of adiponectin are associated with beta cell dysfunction. Muscles bind adiponectin which translocates glucose transporter 4 (GLUT4). GLUT4 is responsible for the transport of glucose into cells. Exercise enhances glucose uptake by GLUT4 translocation. Exercise regulates glycemia through two mechanisms:
- 1)
Insulin-stimulated muscle glucose uptake
- 2)
Insulin-independent glucose transport (GLUT4)
The production of leptin by the adipose tissue is also a major cause for insulin resistance. Exercise reduces the leptin levels.
Judicious caloric restriction also plays an important role in enhancing insulin sensitivity. It was demonstrated that obese subjects with impaired glucose tolerance or mild type 2 diabetes after 10 days of caloric restriction and a body weight reduction of only 3.5% will experience a 35% increase in glucose disposal rate during a hyperglycemic clamp, thus enhancing significantly the insulin sensitivity .
The glucose homeostasis is also altered by placental hormones such as human placental lactogen, progesterone, cytokines (tumor necrosis factor alpha (TNFα)), and others. It is also worth noting that fat deposits in the body are used as an energy source, and when large amounts of energy are required the duration and intensity of physical activity can make a difference. The utilization of fat involves several processes: lipolysis, mobilization, transportation, uptake, activation, translocation, and β-oxidation. The process is stimulated by multiple hormones, among them catecholamines. In our previous studies, we have demonstrated that in sedentary pregnant diabetic patients who exercised at 50–70% of their maximal aerobic capacity they elicit a proper catecholamine response, predominantly norepinephrine, sufficient to facilitate the skeletal muscle uptake of glucose .
Adopting the 2009 Institute of Medicine (IOM) recommendations for additional gestational weight gain for overweight and obese pregnant women could further amplify the risk of gestational diabetes as insulin resistance worsens with additional weight gain.
The IOM guidelines for gestational weight gain are based on prepregnancy body mass index (BMI). In the IOM report, it is specified that basing the recommendation on prepregnancy BMI is an approach that reflects imprecision and needs to be considered for the individual woman. Further weight gain, independent of maternal glycemia, is strongly associated with pregnancy complications particularly in overweight and obese women as has been demonstrated by the Hyperglycemic and Adverse Outcome (HAPO) Study , including maternal and fetal complications.
In its 2014 Standards of Medical Care in Diabetes, the American Diabetes Association (ADA) recommends that nonpregnant subjects at a risk of type 2 diabetes be referred to an effective ongoing support program targeting a weight loss of 7% of body weight and increasing physical activity to at least 150 min/week of moderate activity such as walking . The ADA Clinical Practice Recommendations have suggested a 30% caloric restriction for obese women with GDM, noting a minimum 1800 calories/day level since the year 2000. The concerns for potential adverse consequences from ketonuria/ketonemia in pregnancy do not have credible scientific support, unless the patient is at a risk of ketoacidosis.
In the ADA position statement, it is quoted that based on large studies in nonpregnant type 2 diabetes subjects a structured exercise intervention of at least 8-week duration will result in a lower A1C by an average of 0.66% even with no significant change in BMI . In our experience, within 10 days of initiating an exercise program in GDM patients and depending on regimen compliance, 60% of the subjects will attain euglycemia .
The American Congress of Obstetricians and Gynecologists (ACOG), ADA, Royal College of Obstetricians and Gynaecologists (RCOG), and Society of Obstetricians and Gynaecologists of Canada (SOGC) recommend a moderate exercise program for the management of gestational diabetes . In addition, the ACOG recommendations on gestational weight gain emphasize that “individualized care and clinical judgment are necessary in the management of overweight or obese women” and “… for the overweight pregnant who is gaining less than the recommended amount but has an appropriately growing fetus, no evidence exists that encouraging increased weight gain to conform with the current IOM guidelines will improve maternal or fetal outcomes .”
For obese pregnant women, no target gestational weight gain or loss has been established to prevent gestational diabetes; however, we and others have determined that obese pregnant women, particularly class III who gain no weight or even lose up to 10 pounds in pregnancy, show an improvement in glycemic control and experience less maternal comorbidities such as preeclampsia and less fetal morbidities such as macrosomia ( Fig. 1 ).
Lifestyle modification for women at risk of or with gestational diabetes
Historically pregnant women were recommended sedentary lifestyle, and pregnant diabetics were hospitalized for prolonged periods ; only in the past 20–30 years have recommendations changed.
Despite current recommendations, there is still reluctance to prescribe lifestyle modification to obese diabetic pregnant women. Pregnancy should not be a state of confinement or uncontrolled dietary indulgence; in the absence of specific contraindications to exercise (see Table 1 ), pregnant women, including obese and diabetic women, should be encouraged to participate in aerobic and strength-conditioning exercises .
| Absolute contraindications to aerobic exercise during pregnancy |
|---|
| Hemodynamically significant heart disease |
| Restrictive lung disease |
| Incompetent cervix/cerclage |
| Multiple gestation at risk of premature labor |
| Persistent second- or third-trimester bleeding |
| Placenta previa after 26 weeks of gestation |
| Ruptured membranes |
| Preeclampsia/pregnancy-induced hypertension |
| Poorly controlled type 1 diabetes |
| Relative contraindications to aerobic exercise during pregnancy |
|---|
| Severe anemia |
| Unevaluated maternal cardiac arrhythmia |
| Chronic bronchitis |
| Extreme morbid obesity |
| Extreme underweight (BMI < 12) |
| History of extremely sedentary lifestyle |
| Intrauterine growth restriction in current pregnancy |
| Poorly controlled hypertension |
| Orthopedic limitations |
| Poorly controlled seizure disorder |
| Poorly controlled hyperthyroidism |
| Heavy smoker |
Several observational trials have determined that women who engage in regular physical activity prior and during pregnancy lower their risk of developing GDM .
Exercise prescription
The exercise prescription includes the following components: frequency, intensity, duration, and type.
Light physical activity is defined as requiring <3 metabolic equivalents (METs), moderate activities 3–6 METs, and strenuous activities >6 METs. METs are used to quantify the metabolic cost of exercise in terms of oxygen consumption (VO 2 ). One MET obtained at rest is equivalent to 3.5 ml O 2 /kg/min. A brisk walk at 3–4 mph for 1 h will generate approximately 3–5 METs. During moderate physical activity, the subject is still capable of freely conversing and not experiencing respiratory difficulties. Ideally, pregnant women at a risk of GDM should expend between 400 and 600 kcal/day in exercise, at the minimum 300 kcal/day, exercise being performed preferably for 20–30 min after each meal 3 times/day ( Table 2 ) . Most subjects can engage safely in additional physical activities ( Table 3 ).
| Expended energy | Time to expend 200 kcal | |
|---|---|---|
| VO 2 (60%) | 15–17 (kcal/min) | 12–14 min |
| Reference | Study type | Subjects | Activity | GDM relative risk, odds ratio, and 65% CI |
|---|---|---|---|---|
| 1. Dempsey 2004 | Prospective (Questionnaires) | 909 | LTPA a (year before) | Any versus none 1.44 (0.21–0.91) ≥21 MET/week versus none 0.26 (1.0–0.65) |
| 2. Oken 2006 | Prospective (Questionnaires) | 1805 | LTPA (year before) | Vigorous versus none 0.49 (0.24–1.01) |
| 3. Zhang 2006 | Prospective | 21,765 | LTPA (lowest versus vigorous) | Highest versus lowest quantile: 0.77 (0.69–0.94) |
| 4. Rudra 2006 | Case control and prospectively | 688 897 | Strenuous versus light | 0.19 (0.15–0.50) 0.57 (0.24–1.37) |
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