Medical Nutrition Therapy and Lifestyle Interventions

In the summary and recommendations of the Fourth International Workshop-Conference (4) on GDM, it was noted that relatively little information is available to guide evidence-based recommendations for nutrition interventions. Recommendations included the use of the Institute of Medicine (IOM) weight gain guidelines including the minimal gain of 15 lb (7 kg) for obese women (7). It was noted that calorie restriction lowered glycemia, but too severe a restriction produced ketonuria and ketonemia, which are undesirable. Carbohydrate intake should be limited to 35–45% of total calories. Self-monitoring of blood glucose, particularly postmeal monitoring, is recommended, since elevated values are associated with an increase in fetal risks. Exercise may be an adjunct to MNT to improve maternal glycemia; however, the optimal frequency and intensity has not been determined. Insulin is recommended when women are unable to achieve or maintain glycemic goals with MNT alone or show signs of excessive fetal growth. Nutrition therapy should fulfill the minimal nutrition requirement of pregnancy. The food plan should be individualized and culturally appropriate and provided by a qualified individual with experience in GDM management.

In previous years, nutrition therapy research has focused on answering specific questions such as optimal carbohydrate amount or calorie level. Recently, two studies have shown the overall impact of nutrition intervention on GDM clinical care goals and perinatal outcomes.

The American Dietetic Association developed nutrition practice guidelines for GDM and then conducted a randomized prospective study to determine whether the implementation of these guidelines compared with usual care would improve specific clinical outcomes (8,9). Previously developed nutrition practice guidelines for type 1 and type 2 diabetes demonstrated a 1.0% lower A1C or 30–50 mg/dl lower blood glucose levels when implemented (10,11).

The nutrition practice guidelines for GDM were developed by dietitians experienced in GDM management. After reviewing the literature, they developed guidelines for all aspects of nutrition management. MNT was defined as a “carbohydrate-controlled meal plan that promotes adequate nutrition with appropriate weight gain, normoglycemia, and the absence of ketosis” (8). To define “carbohydrate-controlled,” guidelines were developed for the following topics: total amount of carbohydrate, use of foods with sugar, carbohydrate distribution, breakfast-time carbohydrate, glycemic index, fiber, and artificial sweeteners. Guidelines were developed for other aspects of nutrition therapy: individualizing weight gain and caloric needs; monitoring ketone levels; and determining protein, fat, and micronutrient needs. Self-monitoring of blood glucose was recommended for all women with GDM and target pre- and postmeal glucose levels were identified. Criteria were defined for when additional therapy (insulin) was needed.

A randomized prospective intervention trial was conducted at 25 sites across the U.S. that enrolled 215 women who had been diagnosed with GDM. The sites were randomized to provide either Nutrition Practice Guidelines or usual care; 12 sites received the nutrition practice guidelines, and the remaining 13 sites followed their usual care. The findings showed that sites following the nutrition practice guidelines had fewer patients treated with insulin (24.6 vs. 31.7%; P = 0.05), insulin was initiated later at 31.6 weeks compared with 30.4 weeks, A1C was lower at delivery (5.0 vs. 5.2%), and a smaller proportion had A1C above normal at delivery (8.1 vs. 13.6%, P = 0.25) (9).

Within each group were three clinical settings: diabetes clinics, ob-gyn clinics, and other clinics. Comparing the impact of the use of the guidelines in the three clinical settings found that outcomes were improved in ob-gyn and other clinics. In diabetes clinics, the use of guidelines did not change outcomes; these clinics were already implementing practices similar to the practice guidelines (9).

Another study (the Australian Carbohydrate Intolerance Study in Pregnant Women) determined whether treatment of women with GDM reduced the risk of perinatal complications (12). Women between 24 and 34 weeks’ gestation who had GDM were randomly assigned to the intervention group or to routine care. The rate of serious perinatal complications was significantly lower among the infants of the 490 women in the intervention group than among the infants of the 510 women in the routine care group (1 vs. 4%, P = 0.01) (12). Women in the intervention group received an individualized diet plan from a qualified dietitian that took into consideration the woman's prepregnant weight, activity level, and weight gain and were instructed to monitor blood glucose four times per day: fasting and 2 h after each meal. Criteria were developed to determine when insulin was needed. The infants in the intervention group had lower birth weight, lower percentage large-for-gestational-age, and less macrosomia.

Both studies show that implementing nutrition therapy with self-monitoring of blood glucose and criteria for advancing treatment had a positive impact on maternal and infant outcomes. In both studies, the dietitian individualized the food plan. Modifiable components of a food plan include calorie level; amount, distribution, and type of carbohydrate; and amount and type of physical activity. Outcome measures that help guide adjustments to the food plan are: weight changes, blood glucose and ketone levels, and review of food records. This implies the need for follow-up visits to successfully implement nutrition intervention; the nutrition practice guidelines suggest three visits.

The 1990 Institute of Medicine's Nutrition for Pregnancy publication provided the first weight-gain recommendations based on prepregnancy BMI (7). There is a strong correlation between infant birth weight and maternal pregravid BMI for women who begin pregnancy in the normal and underweight categories (7,13). Women with normal BMI (19.8–26.0 kg/m²) were recommended to gain a total of 25–35 lb (11.4–15.9 kg).

For overweight women (BMI 26.1–29.0 kg/m²), the weight-gain recommendation is 15–25 lb (6.8–11.4 kg). Obese women with a BMI >29 kg/m² need to gain 15 lb. As BMI increases, the correlation between infant size and maternal gain weakens to the point that there is no correlation between weight gain and infant size in obese women. Obese women, regardless of weight gain, produce larger infants: 3,593 ± 514 g (14).

Recently, the epidemic of obesity in America has been linked to pregnancy (15). Women often gain too much weight during pregnancy and retain excessive amounts of weight during the postpartum period. Moreover, women who are overweight or obese before pregnancy are more likely to gain more weight related to childbearing (16). Retention of gestational weight gain may contribute to obesity and increase future risk for chronic diseases, including type 2 diabetes in women. About 35% of pregnant women gain within the IOM weight gain recommendation. Also, 22% gain less and ∼43% gain more than the recommended amount (17). Identifying strategies to avoid excessive weight gain is imperative. For women at high risk for excessive weight gain, interventions need to begin in the first trimester. Suggested ways to manage weight gain are recording food intake, plotting weight gain on a graph at each visit, setting behavioral goals, participating in a group, and receiving individually tailored mailings or periodic newsletters (18).

Maternal gestational weight gain and pregravid body size are two of the important factors that influence infant birth weight, whereas gestational age is the strongest predictor. Research on the pattern of maternal gestational weight gain shows that weight gained in the first trimester is more predictive of infant weight than weight gained in the third trimester (19). Daily cigarette smoking is negatively correlated with weight gain. Other factors include socioeconomic factors, ethnicity, and geographic location. Poor nutritional status of the mother is another factor. Maternal age and parity have been shown to have a positive correlation with birth weight.

Calorie intake is also a factor influencing weight gain. The Dietary Reference Intakes (DRI) published in 2001 recommend an increase in calories for pregnancy: no increase in calories in the first trimester, an additional 340 kcal/day during the second trimester, and 452 kcal/day during the third trimester (20). This has been a controversial recommendation. Determining actual calorie intake is always imprecise. As research has improved tools for improved data collection, there is a new awareness that actual calorie intakes are generally underestimated. A wide range of calorie intakes are compatible with successful pregnancy outcomes, ranging from 1,500 to 2,800 calories per day (7). Maternal adaptations to pregnancy that influence calorie needs to meet recommended weight gain include an increase in basal metabolism, decrease in physical activity level, body fatness, climate and living conditions, and prepregnant energy status (20).

The remaining question is “Do the Institute of Medicine and other recommendations for weight gain and calorie intake apply to the woman with GDM?” There is no indication that normal-weight and underweight women with GDM should not follow the IOM weight-gain guidelines and calorie intake. These women need to gain the usual amount of weight and not restrict calories to ensure a normal infant birth weight. But for the overweight and obese woman, who produces larger infants independent of weight gain, there is no consensus regarding calorie and weight gain recommendations.

Restricting calories has been a strategy for controlling weight gain, glucose levels, and avoiding macrosomia in infants of women with GDM. Severe calorie restriction, <1,500 calories per day or 50% restriction, increases ketonuria and ketonemia. In one study, high levels of third-trimester β-hydroxybutyrate resulted in lowered mental developmental index scores and average Stanford-Binet scores (21). This study has been regularly cited to support guidelines that recommend avoiding ketonemia/ketosis due to severe calorie restriction.

Modest calorie restriction, 1,600–1,800 or a 33% reduction in intake, does not lead to ketosis but controls weight gain and glucose levels in obese women and has been more successful. Table 1 summarizes a few notable studies of obese women with GDM who underwent various levels of calorie restriction for glucose control. Many of these studies were small in sample size, were not conducted under strict metabolic conditions, and relied on reported dietary intakes.

Based on the studies in Table 1, American Diabetes Association Clinical Practice Recommendations have suggested a 30–33% calorie restriction for obese women with GDM, noting a minimum 1,800 calorie level since the year 2000 (26,27).

In the outpatient setting, a calorie restriction is often recommended without specific calorie calculation. Using a food record or food recall, the practitioner and patient can identify obvious food selections or portions to change or restrict. To ensure that overrestriction of calories and nutrition is not occurring, the practitioner can monitor urine ketones, food intake, and weights until there is confidence that the nutrient changes are appropriate. Recordkeeping has been shown in numerous studies to be an effective tool to increase adherence to calorie control (28). It also provides the caregiver with more specific information for assessment and counseling.

Calorie formulas have been suggested in articles and guidelines for GDM. Frequently cited are 35–40 kcal/kg desirable body weight for underweight, 30–35 kcal/kg for normal weight, and 25–30 kcal/kg for overweight subjects. However, there is a lack of research to support these formulas. Snyder et al. (29) determined energy intake at ∼2,047 calories (8.56 MJ/day) in all BMI categories in women with GDM. At that level of intake, weight gain was reduced, euglycemia obtained, ketonuria minimal, and birth weight averaged 3,542 g. Their study results support recommendations of 23–25 kcal/kg (pregravid weight) for obese and 30–34 kcal/kg (pregravid weight) for normal-weight women. They also noted that calorie intake was not a significant independent predictor of birth weight (29).

In the nonpregnant state, there is ample strong evidence that controlling total amount of carbohydrate is a strategy for controlling glucose levels (30). Since 1994, the American Diabetes Association has not specified a specific macronutrient percentage for carbohydrate; instead they have suggested that carbohydrate intake be based on individual tolerances and treatment goals. The IOM Food and Nutrition Board recommends 45–65% of total calories from carbohydrate for a healthy diet (20). Carbohydrates are an important dietary source of energy, vitamins, minerals, and fiber content.

The 2002 Dietary Reference Intake Report set a minimum level of 130 g/day for nonpregnant women and 175 g carbohydrate per day for pregnancy; this is an additional 33 g carbohydrate for fetal brain development and functioning. This new minimum recommendation provides an important basis for the level of carbohydrate restriction for women with GDM.

Elevated glucose values, and in particular postprandial glucose elevations, are associated with adverse outcomes in GDM (31). Carbohydrate is the main nutrient that affects postprandial glucose levels. Carbohydrate intake can be manipulated by controlling the total amount of carbohydrate, the distribution of carbohydrate over several meals and snacks, and the type of carbohydrate. These modifications need not affect the total caloric intake level/prescription.

A few nonrandomized trials have examined the effect of varying levels of carbohydrate amount, type, and distribution on blood glucose control and the need for exogenous insulin. In Table 2, the carbohydrate prescription and prevalence of insulin use in observational and nonrandomized studies of GDM are highlighted.

In addition to the total amount of carbohydrate, the type of carbohydrate may also be a factor. In recent years, the glycemic index has received attention as a nutrition intervention to improve glucose control among nonpregnant people. Foods with a low glycemic index (<55) produce a lower postmeal glucose elevation and area under the curve. Foods with a high glycemic index (>70) show higher postprandial values. A meta-analysis of studies using low–glycemic index diets, in nonpregnant people with diabetes, found an additional 0.4% lowering of A1C (38). The ADA Standards of Medical Care state that the glycemic index can provide additional benefit to total carbohydrate control, at least for nonpregnant individuals.

Research using the glycemic index in pregnancy has found that pregnancy does not change the glycemic index values of specific foods (39). Using a low–glycemic index diet or foods will reduce the glucose level after eating. Three studies using a low–glycemic index diet have shown reduced birth weight and increased small-for-gestational-age birth weights in pregnant (nondiabetic) women (40–42). To date, no intervention studies using the glycemic index have been done with women with GDM. Practitioners reviewing postprandial glucose levels for women with GDM often observe that some foods cause higher glucose values, even when total carbohydrate level is controlled. The glycemic index can help explain these glucose spikes and guide nutrition recommendations. Because there is wide interindividual variability in the glycemic index, each woman needs to determine which foods to avoid or use in smaller portions at all meals or during specific times of the day, for the duration of her pregnancy. Practice guidelines have avoided labeling foods as “good” or “bad” based on the glycemic index (8).

High-fiber diets have also been studied for the management of glucose control in the nonpregnant state; however, no specific glucose benefit has been identified (43). One study compared low-fiber (20 g), moderate-fiber (40–60 g), and high-fiber (70–80 g) diets in non–insulin-requiring women with GDM. This pilot study demonstrated that high-fiber diets were not associated with a lowering of blood glucose levels (44).

Table 3 lists the 2001 Dietary Reference Intakes for pregnancy (20). There is no indication that women with GDM should not follow the same guidelines for nutrient intakes that are indicated for all pregnant women. Because calorie and carbohydrate restriction is often used to manage women with GDM, these values provide a minimum level of intake for maternal and fetal health. If a woman with GDM consumes less than an adequate intake to achieve glucose targets, then combining MNT with pharmacological therapy is indicated.

Physical activity has been shown, in nonpregnant individuals with diabetes, to improve blood glucose control, reduce insulin resistance, reduce cardiovascular risk factors, contribute to weight control, and improve well-being (45). Diabetes prevention trials using exercise and weight reduction have shown a 56% decrease in the incidence of diabetes in a population of people with impaired glucose tolerance (46,47). Therefore, it is reasonable to consider that regular exercise may prevent GDM. In one study, women who participated in any physical activity before and during pregnancy experienced a 69% reduced risk of GDM (48).

Exercise is an obvious adjunct therapy to MNT for women with GDM. One study of the acute effect of exercise on glucose levels showed a 23 mg/dl (1.3 mmol/l) drop in glucose values at 30 min (49). However, the safety of prescribed exercise for glucose management has been a concern. Women should monitor fetal activity and blood glucose levels before and after exercise and limit physical activity to 15–30 min. Women who have been physically active before becoming pregnant are encouraged to continue an active lifestyle.

MNT is the primary therapy for 30–90% of woman with GDM (5,6). Criteria for adding pharmacological therapy vary in research studies. In many studies, the criteria are one or more blood glucose values outside the target range within a designated time frame. Also, elevated fasting glucose values alone are criteria for insulin or glyburide, as nutrition therapy primarily targets postprandial glucose levels. Additional criteria to consider are weight loss, positive ketone levels, and inadequate nutrient intake; blood glucose values alone are not enough to judge the need for additional therapy.

Nutrition recommendations for women with GDM, including gestational weight gain, calorie intake, and macronutrient composition and distribution, are based on limited scientific evidence. Further research is needed in all areas (6). In particular, randomized, controlled trials comparing intensive dietary strategies would help to guide specific nutrition recommendations regarding the type, amount, and distribution of carbohydrate. Because many women with GDM are overweight or obese before becoming pregnant, questions regarding the level of energy restriction and safety of ketone levels remain unclear. Research is also needed on how to implement strategies to manage weight gain during pregnancy for these women. New awareness of the risks of postpartum weight retention and the fact that women with GDM are at risk for developing type 2 diabetes are additional reasons to figure out ways to safely control nutrient intake and increase physical activity in GDM pregnancies.

Regardless, MNT remains the cornerstone of treatment for GDM. The food plan should be designed to fulfill minimum nutrient requirements for pregnancy set by the Institute of Medicine and to achieve glycemic goals without inducing weight loss and ketonemia. MNT is best prescribed by a registered dietitian or a qualified individual with experience in the management of GDM. Food plans should be culturally appropriate and individualized to take into account the patient's body habitus, weight gain, and physical activity and be modified as needed throughout pregnancy to achieve treatment goals. Women with GDM are at risk for developing type 2 diabetes postpartum. Nutrition interventions for GDM should emphasize overall healthy food choices, portion control, and cooking practices that can be continued postpartum and may help prevent later diabetes, obesity, cardiovascular disease, and cancer.