Low carb diet for type 2 diabetes
A CSIRO study published in the American Journal of Clinical Nutrition earlier this year caused quite a stir in the nutrition and diabetes community. They did a RCT on a somewhat controversial topic – a low carb diet for type 2 diabetes- and found positive results. We ask lead researcher Grant Brinkworth about the study.
Associate Professor Grant Brinkworth is Principal Research Scientist at CSIRO. Grant leads strategic research teams and manages large-scale clinical research studies evaluating the effects of diet, nutrition and physical therapies for the prevention and treatment of lifestyle diseases, including obesity, diabetes, cardiovascular and metabolic disease in normal and clinical populations.
Design: A RCT over 52 weeks. N=115 people with obesity and type 2 diabetes
Intervention: Hypocaloric, energy matched Low carbohydrate (LC) vs High Carbohydrate (HC) diet.
Supervised aerobic and resistance exercise 60min x 3d/week.
LC: very low carbohydrate (14%E,HC: high carbohydrate (53%E) low protein (17%), low fat, low saturated fat diet (30%E,
Measures: Glycemic control: HbA1C, Fasting blood glucose, glycaemic variability via 48-h continuous glucose monitoring, diabetes medication use
CVD risk factors: weight, blood pressure, lipids (baseline, 24 and 52 weeks)
Results: Similar completion rates and reductions in weight. Compared to HC diet, the LC group achieved greater mean reduction in diabetes medication, glycaemic variability over 48 hours, and triglycerides, and increase in HDL cholesterol
Conclusions: both diets achieved substantial weight loss and reduced HbA1C and fasting glucose. The LC diet achieved greater improvements in lipid profile, blood glucose stability and diabetes medication requirements.
What made you investigate this dietary approach for type 2 diabetes?
There is a lot of research suggesting that restriction of carbohydrates and higher intakes of protein and unsaturated fats has positive effects on key health targets for people with type 2 diabetes, independent of weight loss, ie
Could you describe the very low-carb diet the subjects followed?
The low carbohydrate, higher protein, high unsaturated fat diet was a whole food dietary pattern that was nutritionally replete and included a variety of foods from across the five food groups. This included lots of low-carbohydrate vegetables, nuts, lean meats (or alternatives) and healthy (unsaturated) fats and oils, with some low-fat dairy and bread and cereals (lower carbohydrate, low GI varieties), starchy vegetables and/or fruit
You can see a sample daily meal plan here.
(In case you’re wondering, they did have a couple of vegetarians in the trial. It is still possible to follow this style of eating but because of the extra carbohydrate content of legumes they were encouraged to use tofu as a meat alternative as it has little or no carbohydrates).
How often did the participants see a dietitian over the 52 weeks of the study?
Participants were counselled by a research dietitian every 2 weeks for the first 12 weeks of the study and then monthly for the remainder of the study. In addition, all individuals participated in 60-minute, group-based exercise classes 3 times a week that were supervised by an exercise professional.
What challenges did the participants report in following a restricted carbohydrate diet?
The biggest challenge experienced by all participants whether they were following the LC diet or the HC diet was the need to reduce the number of discretionary ‘snack’ foods and limiting alcohol in their usual diet. The level of adherence and acceptance of the restricted carbohydrate diet plan was similar to that reported with the higher carbohydrate comparison diet.
For those participants following the carbohydrate restricted diet, the biggest challenge was to limiting their intake of carbohydrate rich foods such as breads and cereals like pasta and rice, starchy vegetables like potatoes and sweet potatoes, and tropical fruits. However, participants were able to incorporate some of these foods using an exchanged based system that enabled them to achieve dietary flexibility but to remain within their carbohydrate intake targets.
How much of the positive results do you think could be attributed to the exercise and calorie restriction rather than macronutrient composition of the diet?
It was clear that independent of the macronutrient composition the exercise and calorie restricted diet had substantial health benefits. Both groups had substantial reductions in body weight and several metabolic disease risk factors including blood pressure, blood cholesterol, insulin resistance, blood glucose and associated medication requirements. Both groups also experienced improvements in mood, quality of life and cognitive function.
However, the low carbohydrate, higher protein, high unsaturated fat diet was able to better magnify the effects of the lifestyle program by achieving even greater effects on lipid profile and glycemic control, specifically:
Do you think a moderate carbohydrate content (with a focus on quality such as wholegrain, low GI, high fibre) combined with a low saturated fat content could provide similar benefits? Is there are threshold: how high is too high?
The high carbohydrate, low fat control diet used in this study did focus on quality carbohydrates such as whole grain, low GI and high fibre, and showed substantial benefits as well. Compared to a high carbohydrate low fat diet, it could be expected that a diet with only moderate carbohydrate would have a lower glycemic load and could possibly improve the blood glucose profile in individuals with type 2 diabetes. However, our study didn’t specifically examine a moderate carbohydrate diet so I can’t say for certain it would produce similar benefits to a very low carbohydrate diet. There is a lack of well controlled dose response studies determining whether there is any specific carbohydrate restriction or intake threshold that should be maintained to achieve the benefits we observed in our study.