Gestational diabetes mellitus (GDM), defined as glucose intolerance first recognized during pregnancy, affects approximately 7% of pregnancies, with rates varying from 1% to 14% depending on the population and diagnostic criteria. In Spain, GDM prevalence is estimated at 8.8%. Medical nutrition therapy (MNT), focusing on controlling carbohydrate (CHO) amount and distribution, is the primary GDM treatment, aiming for optimal glycemic control without ketosis, alongside appropriate gestational weight gain. For the general population, CHO should constitute 45-65% of total daily calories, with pregnant women advised to consume over 175 g of CHO daily, distributed across three meals and two to four snacks. However, other methods are crucial in managing GDM, with clinical guidelines in Spain suggesting insulin treatment if MNT fails to control blood glucose levels.
The Role of Medical Nutrition Therapy in GDM Management
MNT serves to lower postprandial blood glucose values by modifying CHO distribution or components of the glycemic load (GL). In recent years, randomized controlled trials (RCTs) have explored the effect of low-GI diets in GDM. One study demonstrated a significantly lower proportion of women needing insulin treatment with a low-GI diet, without significant differences in obstetric and fetal outcomes. However, in some settings, MNT for GDM is based primarily on controlling the amount and distribution of CHO rather than focusing on the glycemic index (GI).
Examining the Impact of Low-CHO Diets in GDM
A randomized controlled trial (RCT) involving 152 women with GDM compared a low-CHO diet (40% of total diet energy) with a control diet (55% of total diet energy). The study found that a low-CHO diet did not significantly reduce the number of women requiring insulin, with similar pregnancy outcomes observed between the groups. Daily food records confirmed a difference in CHO consumption between the groups.
Study Design and Methodology
The open, parallel, randomized controlled trial included women aged 18-45 years diagnosed with GDM, with singleton pregnancies and a gestational age ≤35 weeks. GDM diagnosis followed the 2006 National Diabetes and Pregnancy Clinical Guidelines, with screening occurring at 24-28 weeks of gestation (or earlier if GDM risk factors were present). Participants received routine care based on institutional, local, and national guidelines, including self-monitoring of blood glucose (SMBG) with a glucose meter.
Dietary Interventions
The energy content of the diet for each patient was calculated based on pregestational weight, with a minimum of 1,800 kcal/day. The two diets had similar protein content (20% of total daily calories) but differed in CHO (40% in the low-CHO diet and 55% in the control diet) and fat (40% in the low-CHO diet and 25% in the control diet) content. Diets were divided into three main meals and three snacks, with a prespecified number of CHO servings. CHO intake was evaluated using the estimated food record method over three nonconsecutive days.
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Outcomes and Results
The primary outcome was the rate of women requiring insulin treatment. Secondary outcomes included pregnancy complications, ultrasound follow-up data, and pregnancy outcomes such as newborn weight and length, newborn hypoglycemia, and type of delivery. The intention-to-treat analysis showed that the cumulative rate of insulin treatment was 54.7% in both the control and low-CHO diet groups (P = 1). There were no significant differences in insulin dose or time to insulin treatment between the two groups. Daily CHO consumption differed between the groups, with lower total CHO and starch intake in the low-CHO group. Maternal weight gain from study allocation until delivery was higher in the control group, but this difference disappeared after correction for time to follow-up.
Interpretation of Findings
The RCT aimed to determine if a low-CHO diet could prevent insulin treatment in women with GDM without adverse pregnancy outcomes. However, the low-CHO diet did not significantly lower the rate of women requiring insulin treatment. This finding contrasts with some smaller, nonrandomized studies suggesting that lower CHO diets may reduce the need for insulin.
The Broader Context: Low-Carb Diets and GDM
While the study above found no significant difference in insulin needs, other research explores the broader impact of low-carb diets on GDM. Some studies suggest potential benefits in decreasing glycemia, but overall, low-carb diets do not consistently demonstrate clear advantages over more flexible carbohydrate approaches.
Arguments for Low-Carb Diets in GDM
Proponents of low-carb diets argue that reducing overall blood sugars improves perinatal outcomes. The recommended dietary allowance of carbohydrates for pregnant individuals is at least 175 g per day, with glucose being crucial for healthy fetal development. Historically, dietary approaches for GDM have focused on low carbohydrates to decrease perinatal mortality. Landmark studies have shown that gestational diabetes treatment, including low-carbohydrate diets, can decrease pregnancy complications like preeclampsia and large for gestational age infants.
Concerns and Considerations
Lowering carbohydrates raises concerns about consuming lower nutrient-dense foods and producing ketones, which may have negative effects on the developing baby. Some studies show no difference in ketonuria between pregnant women with GDM following a 55% carbohydrate diet or a 40% carbohydrate diet.
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The Case for Carbohydrate Flexibility
A flexible carbohydrate meal plan, which reduces simple sugars while increasing overall diet quality and complex carbohydrate intake, can be a viable alternative to strict low-carb diets. Some meta-analyses and systematic reviews have found no significant differences in outcomes between different dietary approaches for pregnant women with gestational diabetes.
The Impact of Pre-Pregnancy Low-Carbohydrate Diets on GDM
Recent research has also investigated the relationship between pre-pregnancy low-carbohydrate (LC) diets and maternal oral glucose tolerance test (OGTT) levels during pregnancy. One case-control study compared women with GDM who adhered to an LC diet (carbohydrate intake < 130 g/d) before conception with women with GDM on a conventional diet and healthy pregnant women on a conventional diet.
Study Design and Results
The study found that women with GDM who followed a pre-pregnancy LC diet had significantly higher OGTT-1 h and OGTT-2 h values compared to those on a conventional diet. Additionally, a higher percentage of women in the LC/GDM group had more than one abnormal OGTT value. These findings suggest a potential link between pre-pregnancy LC diets and more detrimental OGTT values in patients with GDM.
Dietary Intake and Biomarkers
The LC/GDM group had the lowest intake of carbohydrates before pregnancy, with an average of 95.3 g/d. This group also showed increased carbohydrate intake during the second trimester, although it remained lower than the other groups. Fasting blood glucose levels in the LC/GDM group were comparable to the Con/GDM group, but OGTT-1 h and OGTT-2 h values were significantly higher.
Practical Dietary Recommendations for Gestational Diabetes
For women with gestational diabetes who do not take insulin, a balanced, healthy diet is essential. This involves eating a variety of healthy foods, with attention to food labels to make informed choices.
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General Dietary Guidelines
In general, the diet should include:
- Plenty of whole fruits and vegetables
- Moderate amounts of lean proteins and healthy fats
- Moderate amounts of whole grains and starchy vegetables
- Fewer foods high in sugar
It is recommended to eat three small- to moderate-sized meals and one or more snacks each day, without skipping meals or snacks. Consistency in the amount and types of food consumed daily is crucial for maintaining stable blood sugar levels.
Carbohydrate Management
Carbohydrates should make up less than half of the total calories consumed. Prioritize high-fiber, whole-grain carbohydrates (complex carbohydrates) over simple carbohydrates, which can cause rapid blood sugar spikes. Vegetables are highly recommended for their health benefits and minimal impact on blood sugar.
Specific Food Group Recommendations
- Grains, Beans, and Starchy Vegetables: Aim for 6 or more servings a day, choosing whole-grain options.
- Vegetables: Consume 3 to 5 servings a day.
- Fruits: Opt for whole fruits over juices, with citrus fruits being a good choice.
- Milk and Dairy: Include 4 servings of low-fat or nonfat dairy products daily.
- Protein: Eat 2 to 3 servings a day, choosing lean meats, poultry, fish, beans, eggs, and nuts.
Other Lifestyle Considerations
Alongside dietary changes, a safe exercise plan, such as walking or low-impact exercises, can help manage blood sugar levels.
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