Exercise: An Alternative Therapy for Gestational Diabetes
Raul Artal, MDTHE PHYSICIAN AND SPORTSMEDICINE - VOL 24 - NO. 3 - MARCH 96
In Brief: Exercise can play a significant role in managing blood glucose levels in women who develop gestational diabetes and in women with type II diabetes who become pregnant. Because contracting muscles help stimulate glucose transport, exercise can help women control gestational diabetes without insulin. After constraints typical of pregnancy are taken into account—such as soft-tissue laxity and fetal status—an exercise program can be tailored to the individual needs of patients. Although fit, active women may tolerate more strenuous exercise, relatively sedentary women may benefit most from non-weight-bearing exercises. Moderate workouts appear to be safe for most women who have gestational diabetes.
One long-accepted therapeutic principle of diabetes management is that physical activity should be combined with an appropriate diet. This cost-effective approach has not been applied to women who have gestational diabetes, primarily out of fear that exercise could affect the developing fetus, but also because of a lack of proven safety and efficacy for the woman. However, obstetricians and other primary care providers can include exercise as an important component of controlling gestational diabetes.
Causes and Effects of Gestational Diabetes
Gestational diabetes—a carbohydrate intolerance of variable severity with an onset or first recognition during pregnancy—involves a relative resistance to insulin. During pregnancy, catabolic stress hormones trigger an increase in fuel delivery to the mother's circulation. Hormone activity results in decreased fasting blood glucose levels and increased postprandial blood glucose levels and is in part responsible for increased insulin resistance. Gestational diabetes—which affects about 3% to 6% of pregnant Americans (1)—develops when the circulating insulin does not counteract the lowered insulin sensitivity.
All the factors responsible for maintaining normal glucose tolerance are affected in pregnancy and further altered when the pregnant woman has diabetes. Insulin sensitivity is impaired, glucose uptake by the muscles and splanchnic organs is decreased, and hepatic glucose production is suppressed.
These effects are similar to the deficiencies found in women who have non-insulin-dependent (type II) diabetes mellitus. In fact, gestational diabetes may represent heretofore undiagnosed type II diabetes. Therefore, an exercise protocol for women who have traditional gestational diabetes can also be applied to women with type II diabetes who become pregnant. However, because of exercise-related hypoglycemia and the uncertain effects of previously injected insulin, an exercise program for pregnant women with insulin-dependent (type I) diabetes may differ or be contraindicated.
An obstetrician should be closely involved in the treatment of all patients who have gestational diabetes. Traditional care for gestational diabetes involves dietary changes and, if necessary, insulin therapy. Incorporating exercise, however, can reduce or eliminate the need for insulin.
How Exercise Helps
Adding exercise to the general management scheme for diabetes has a physiologic rationale. A contracting skeletal muscle can increase its glucose uptake 35-fold. Following exercise, glucose tolerance is increased for variable periods, depending on both insulin and contractile activity. It appears that muscles help regulate the capacity for contraction-stimulated glucose transport.
The process of glucose transport into insulin-sensitive cells has not been studied in pregnancy. But because gestational diabetes is a state of altered insulin sensitivity, exercise is a logical therapeutic intervention.
Experts at the Third International Workshop-Conference on Gestational Diabetes Mellitus agreed with this proposition (2). They concluded that exercise is one of the therapies that may help control hyperglycemia in gestational diabetes. The committee on management strategies recommended the following:
Active women who have gestational diabetes may continue moderate exercise. Also, regular exercise in previously sedentary women shows promise for normalizing blood glucose levels. Many women with gestational diabetes have, in fact, led an inactive life. And because of the increased awareness of wellness and fetal well-being, pregnancy may be the most practical and convenient time to introduce lifestyle modifications such as regular exercise.
Additional studies, however, are needed to determine the effects of cardiovascular fitness training on fetal outcome and on ketone production. For now, exercise prescriptions should be individualized and conducted under careful medical supervision until their effectiveness is established.
Risk Factors and Warning Signs
Prescribing exercise, however, requires knowledge of a pregnant woman's physical capacity, anatomy, and physiologic alterations. Pregnant women who have diabetes may be at higher risk for exercise-related injuries, primarily because many of them have had a sedentary lifestyle that has led to muscle atrophy. The treating physician must take into account each patient's risk factors, physiologic changes, and risk to the fetus.
Because of physiologic adaptations to pregnancy, a woman may or may not be able to continue to train throughout pregnancy. If exercise is not medically or obstetrically contraindicated (table 1), however, previously healthy but sedentary women may safely engage in low-intensity exercise programs to develop cardiorespiratory fitness. Because of potential undesirable effects on the fetus, however, exercise should not be used for weight control in pregnancy, but rather to improve and maintain glucose homeostasis.
The pregnant woman who is alert to the potential hazards of exercise, aware of the warning signs, and in communication with her physician is less likely to experience problems. Therefore, physicians need to educate patients about warning symptoms. Pregnant women should stop exercising and contact a physician if any of these symptoms persist: pain in the back, pubic region, or abdomen; dizziness; shortness of breath; palpitations; faintness; vaginal bleeding or fluid loss; tachycardia (resting heart rate above 100 beats per minute); difficulty with walking; uterine contractions; or absence of fetal movement.
In prescribing exercise for patients who have gestational diabetes, physicians must consider all the physiologic changes related to pregnancy.
Biomechanical alterations. Increased hormonal activity leads to connective tissue laxity and joint instability, rendering joints more susceptible to injury, particularly in previously sedentary women (3). In addition, an enlarged uterus and breasts and increased lordosis can alter balance and strain the hips and back (4). Other concerns include nerve compression syndromes such as carpal tunnel syndrome and, more rarely, separation of the symphysis pubis.
Cardiovascular and hemodynamic response. During and just after pregnancy, maternal blood volume, heart rate, stroke volume, and cardiac output significantly increase (5). Also, hemoconcentration tends to increase during exercise. These changes could significantly reduce cardiac reserve; however, they are compensated by an increase in cardiac output. Of special concern are women whose oxygen-carrying capacity is compromised by anemia and women who are extremely sedentary or are obese. These patients may require a detailed cardiovascular evaluation before beginning even a mild exercise regimen.
Target heart rates have not been established for pregnant women. A better tool for determining appropriate exercise intensity, however, is perceived exertion. Pregnant women should exercise at a level perceived as somewhat hard, which corresponds to about 60% of maximum oxygen uptake (6).
Respiratory response. The enlarging uterus displaces the diaphragm, creating discomfort and dyspnea at rest and during exercise for some women. Pulmonary function at rest, however, is not impaired because of lateral expansion of the rib cage and increased tidal volume. Also, increases in ventilation with mild exercise are less than those in nonpregnant women (7,8). High-intensity exercise during pregnancy results in increased oxygen uptake, decreased pulmonary reserve, and decompensation.
Endocrine factors. Fasting gestational blood glucose levels are reduced and carbohydrate metabolism increased, so hypoglycemia may occur during fasting or during prolonged moderate or strenuous exercise (9). Also, norepinephrine levels increase significantly during physical activity (10), which may precipitate preterm labor in women at risk. In otherwise healthy women, however, exercise does not appear to increase uterine irritability (11).
Fetal response. A primary concern regarding exercise during pregnancy is the potential risk to the fetus. Although risk of fetal injury has not been demonstrated, data are insufficient to support or discount this possibility—particularly in women with gestational diabetes—so exercise recommendations must be made cautiously.
Hyperthermia is of special concern during pregnancy because the fetus has limited ability to dissipate heat. Pregnant women should be cautioned not to exercise in hot, humid weather or when they have a fever. In nonpregnant women, 30 to 60 minutes of strenuous exercise in adverse conditions can increase the core temperature to 38°C, but strenuous exercise for 15 to 20 minutes in any condition usually will not increase temperature above 38°C. Moderate exercise tempered with common sense, therefore, is unlikely to cause teratogenic effects from hyperthermia. However, hyperthermia during embryogenesis could lead to birth defects that are common among infants of women who have gestational diabetes.
The effects of redistribution of blood flow to exercising muscles may be transient or prolonged. There is no evidence that transient effects cause fetal hypoxia and distress; however, prolonged effects may retard fetal growth. These complications, though, are rare and typically tied to strenuous or prolonged exercise combined with inadequate nutrition. Inducing such changes in experimental animals requires a 50% reduction in uterine blood flow (12), which is most likely to occur during strenuous exercise. Such changes may be more prominent in women who have diabetes and vascular changes.
While most of the effects of exercise on the fetus may be transient and insignificant, questions remain about long-term effects. In one study (13), women who continued strenuous exercise throughout pregnancy gained less weight and delivered earlier (by 8 days), and their infants had consistently lower birth weights (by 500 g) compared with sedentary women. A review of a number of studies evaluating mothers who exercised throughout pregnancy confirmed these findings (14). Such studies indicate a need for periodic ultrasound for strenuous exercisers, particularly those who have gestational diabetes.
Developing an Exercise Program
Individualizing an exercise program for women who have gestational diabetes involves assessing health and physical fitness, developing a program specific to the patient's situation, recommending fluid and food intake, and informing the patient about limitations, contraindications, warning signs, and specific concerns. It should be emphasized that most otherwise healthy women with gestational diabetes can safely engage in moderate exercise. We have investigated two protocols, one best suited for home and the other for a hospital- or clinic-based exercise laboratory (table 2).
Home-based program. A clinical protocol previously published by Artal et al (15) seems particularly suited to pregnant women with diabetes who could exercise at home. The patient is initially screened to rule out contraindications. She then undergoes a symptom-limited VO2 max bicycle ergometer test in the lab to determine her work capacity for exercise prescription purposes.
The patient then follows this program: Before breakfast, lunch, and dinner, she rests for 30 minutes while monitoring fetal activity. She monitors fasting and 2-hour-postprandial blood glucose levels. She is instructed not to proceed with exercise if the fetus does not move during the 30 minutes. Also, she must refrain from exercise and notify her physician if the fetus moved less than 10 times in 24 hours or if her plasma glucose was less than 60 mg/dL or greater than 250 mg/dL or if ketonuria was present. In the absence of the above, the patient eats her prescribed meal and then exercises for 20 to 30 minutes on a stationary cycle ergometer at 50% of her maximum aerobic capacity or to a perceived exertion of fairly light to somewhat hard.
Immediately after exercise, the patient rests for 30 minutes and counts fetal movements. The patient should also be aware of uterine activity. If uterine contractions become regular or occur at intervals of 15 minutes or less, she consults her obstetrician. She refrains from further exercise until she consults her obstetrician if fetal movements are too low or if plasma glucose is too low or too high.
The patient is informed of potential complications and signs a consent form. She is also required to keep accurate records of her blood glucose determinations, food intake, physical activities, and fetal movements. Beginning at 32 weeks, fetal heart rate reactivity is checked weekly (nonstress testing). Further fetal testing is done as indicated.
In a recent noncontrolled study, 18 diabetic patients with abnormal fasting glucose levels were managed following the above protocol (unpublished data). Of the 18 patients, 15 completed the study. Three had to be withdrawn from the program, one because she developed mild pregnancy-induced hypertension at 36 weeks, and two others who went into premature labor. One of these patients was subsequently placed on insulin therapy.
The compliance with the prescribed exercise program was over 90%. None of the patients who remained in the study required insulin. Glucose control varied by less than 9%. Patients reported no significant gastrointestinal problems. No episodes of fetal heart rate abnormalities were recorded, and there were no interventions for fetal distress.
Hospital-based program. Recently Bung et al (16) tested a hospital-based program. Forty-one diabetic patients with abnormal fasting glucose levels who failed a diet therapy trial of 1 week were randomized to a protocol of either exercise and diet or insulin therapy and diet.
At enrollment and then every 4 weeks, the subjects in the exercise group underwent a symptom-limited VO2 max test to determine appropriate workload for each patient. This exercise routine ensured a comparable exercise prescription for all subjects. All subjects in the exercise group were instructed to return to the exercise laboratory three times per week to exercise under medical supervision and to avoid a sedentary lifestyle.
In the lab, the subjects exercised on a recumbent bicycle at 50% of their last-determined maximum aerobic capacity. The total duration of the exercise was 45 minutes, divided into three bouts of 15 minutes interspersed with two 5-minute rest periods to facilitate fetal monitoring. This exercise routine was judged to be moderate and to generate an energy demand about 5 to 7.5 times the resting metabolic rate. Before and immediately after the exercise sessions, the subjects' plasma glucose concentrations and blood pressures were measured. Maternal heart rate and uterine activity were continuously monitored during exercise.
Seventeen of the 21 subjects in the exercise group and 17 of the 20 subjects in the insulin group completed the study. The exercise subjects had a compliance rate above 90%.
The average gestational age at delivery was 38.9 + 1.7 weeks (range, 35.5 to 40.5) for the exercise group and 38.2 + 2.0 weeks (range, 33.5 to 40.0) for the insulin group. No episodes of hypoglycemia were recorded, and no significant differences between the groups were observed in the weekly blood glucose determinations. The groups also had comparable complication rates, body composition variables, and fetal health.
Individualized, Supervised Care
As the above studies suggest, in the absence of either medical or obstetrical contraindications, exercise prescription can be an alternative or adjunct therapy for women who have gestational diabetes. With our present knowledge, the exercise regimens and follow-up of these patients must be individualized and medically supervised.
The risks of low-intensity exercise during pregnancy in previously sedentary individuals are minimal and predominantly include soft-tissue injuries. Non-weight-bearing exercises such as stationary cycling, swimming, and arm exercises, therefore, may be most suitable for these patients (17). Walking may be another safe option. Either of the two above-mentioned exercise programs may prove effective, and the regimen of the hospital-based protocol can be adapted for home exercisers who don't need close laboratory monitoring. A standard stationary bike can substitute for a recumbent bike.
Along with the exercise program, women with gestational diabetes should follow a daily balanced American Diabetes Association diet of 30 kcal/kg ideal body weight. Physically active patients will need more calories, and all patients must adjust their diet and insulin to maintain normoglycemia. Both diet and insulin are titrated in relation to glucose control and weight gain. Periodic fetal monitoring is required to ensure fetal well-being. Hypoglycemia is more likely during the first half of pregnancy when the diabetogenic effect of pregnancy is less pronounced, but home glucose monitoring is essential throughout pregnancy.
Additional studies are necessary to validate our observations and evaluate the long-term fetal effects of maternal exercise in infants of mothers who have diabetes.
Dr Artal is a professor and chairman of the Department of Obstetrics and Gynecology at the State University of New York Health Science Center in Syracuse. He is a fellow of both the American College of Obstetricians and Gynecologists and the American College of Sports Medicine, and an editorial board member of The Physician and Sportsmedicine. Address correspondence to Raul Artal, MD, Professor and Chairman, Dept of Obstetrics and Gynecology, State University of New York, College of Medicine, 736 Irving Ave, Suite 3664 West Tower, Syracuse, NY 13210.