Type 2 diabetes is a common condition characterized by high levels of blood sugar, usually due to insulin resistance.
The treatment of type 2 diabetes involves medication, but lifestyle strategies to lower blood sugar levels are very important as well.
Although low-carb diets have become popular for managing type 2 diabetes, few high-quality studies have investigated their long-term effects on blood sugar control and risk factors for heart disease (2).
A team of Australian researchers set out to compare the long-term health effects of a low-carb diet and a high-carb diet, focusing on differences in blood sugar control and risk factors for heart disease.
Tay et al. Comparison of Low- and High-Carbohydrate Diets for Type 2 Diabetes Management: A Randomized Trial. American Journal of Clinical Nutrition 2015.
This was a randomized trial that spanned one year, or 52 weeks total.
A total of 115 obese and overweight adults with type 2 diabetes participated. Their age ranged from 35 to 68 years.
The participants were randomly assigned to one of two diets that contained an equal amount of calories:
- Low-carb diet (LC): Carbs, protein and fat comprised 14%, 28% and 58% of calories, respectively. The total carb content was under 50 grams per day.
- High-carb diet (HC): Carbs, protein and fat comprised 53%, 17% and 30% of calories, respectively.
Both diets restricted calories in order to produce weight loss. Calories were restricted by 30%, which amounted to 500–1000 calories, depending on the individual.
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The fat content of the diets was mainly unsaturated, with less than 10% of calories from saturated fats.
In addition, all participants had 60-minute, supervised exercise sessions three times per week. These sessions focused on moderate-intensity aerobic and resistance exercise.
Blood samples were taken at the beginning, in the middle (24 weeks), and at the end (52 weeks) of the study.
78 people completed the study, which is 68% of participants.
Due to regular exercise and the reduction of calories, participants in both groups lost weight and achieved significant health improvements.
This graph shows the weight changes in the two groups:
Changes in the following health markers were similar between groups:
- Weight loss: The LC group lost 9.8 kg (21.6 lbs) and the HC group lost 10.1 kg (22.3 lbs).
- HbA1c: Levels dropped by 1% in both groups.
- Fasting blood sugar: Decreased by 0.7 mmol/L (27.02 mg/dL) in the LC group and 1.5 mmol/L (57.9 mg/dL) in the HC group.
- LDL cholesterol: Levels dropped by 0.1 mmol/L (3.9 mg/dL) in the LC group and 0.2 mmol/L (7.7 mg/dL) in the HC group.
- Blood pressure: Fell by 7.1 mmHg in the LC group and 5.8 mmHg in the HC group.
- Inflammation: Estimated by the C-reactive protein, this decreased by 0.9 mg/L in the LC group and 1.2 mg/L in the HC group.
- Insulin resistance: Also decreased substantially.
However, the low-carb group experienced significantly greater improvements in several health markers.
These included improvements in the following:
- Medication: Greater reduction in the need for diabetes medication.
- Glycemic variability: At least twice the decrease in glycemic variability, indicating greater stability in blood sugar levels.
- Hyperglycemia: Less risk of experiencing high blood sugar levels.
- Triglycerides: 0.4 mmol/L (7.2 mg/dl) reduction in triglycerides, versus a 0.01 mmol/L (0.18 mg/dl) reduction in the HC group.
- HDL cholesterol: Increases of 0.1 mmol/L (1.8 mg/dl), versus 0.06 mmol/L (1.08 mg/dl) in the HC group.
For overweight or obese people with type 2 diabetes, calorie-restricted diets and regular exercise lead to weight loss, improved blood sugar control and reduced risk factors for heart disease.
This happens regardless of whether people follow low-carb or high-carb diets.
However, low-carb diets lead to greater improvements in blood sugar control and blood lipid profile than high-carb diets.
Additionally, low-carb diets decrease the need for diabetes medication and help stabilize blood sugar levels.
For this reason, low-carb diets can help effectively manage type 2 diabetes.
This study was well designed, but had a few limitations.
- It did not include a control group of participants who did not change their diet, which would have helped isolate the effects of the lifestyle modifications.
- The assignment to diets was not blind.
- The LC group’s protein intake was much higher. Protein can affect weight loss as well as glycemic control, so this may have impacted the results of the study.
- By the end of the study, the LC group had increased their carbohydrate intake to 74 g, and saturated fat accounted for 11% of calories. Calorie intake was increased throughout the study in both groups.
- Adherence to the diet was good but not perfect, which is a common limitation of these types of studies.
What Do Other Studies Say?
The main outcome variable of the current study was HbA1c.
HbA1c is also known as glycated hemoglobin. Measuring HbA1c gives clinicians and scientists an idea of the average blood sugar levels over the previous few months.
High HbA1c levels are associated with an increased risk of developing diabetes-related complications.
Some studies have found no significant differences in the effects of low- and high-carb diets on HbA1c (3, 4). Conversely, other studies have reported greater reductions in HbA1c with LC diets (5, 6, 7).
Studies that have compared LC diets with calorie-restricted HC diets among patients with type 2 diabetes have provided mixed results.
In the current study, both diets contained the same amount of calories, and weight loss was similar.
Supporting the present study, one 24-week randomized controlled trial in obese adults with type 2 diabetes showed that a LC diet caused greater improvements in blood sugar control and blood lipid profile than a HC diet (10).
In the current study, similar reductions in cholesterol were seen in both the LC and HC diets.
These differences may be explained by the type of fat replacing carbs in LC diets. In the present study, carbs were replaced with unsaturated fat, which may lower LDL-cholesterol.
Summary and Real-Life Application
In conclusion, reducing calories and increasing exercise can lead to weight loss and improved health — regardless of whether people are on a low-carb or high-carb diet.
However, for obese people who have type 2 diabetes, a low-carb diet seems to be more effective at improving blood sugar control and reducing the risk factors for heart disease.
This is supported by several studies showing that low-carb diets are effective for diabetics.