Is There an Optimal Diet for Patients With Type 2 Diabetes?
Is There an Optimal Diet for Patients With Type 2 Diabetes?
Studies in those with diabetes are limited, with few over a duration longer than 12 weeks, but indicate potentially initial greater weight loss but no difference over the longer term. One non-randomised study comparing a ketogenic very low carbohydrate diet with a low calorie diet over 24 weeks did demonstrate greater weight loss, improvements in glycaemic control and lipid profile with the very low carbohydrate approach. However no long term or randomised controlled trials have demonstrated any superiority. It has been hypothesised that the generation of ketosis is central to facilitating weight loss with a low carbohydrate diet, however this is not supported by Boden or Krebs et al who showed that weight loss was directly related to the reduction of total energy intake achieved by removing one macronutrient from the diet without appreciable compensatory increase in actual fat or protein intake.
Significantly restricting carbohydrate in the diet has profound effects on glucose metabolism. Several small and very short studies ranging from seven days to five weeks have variably demonstrated reduced glucose and insulin concentrations and improved glycaemic control. In short-term studies people with T2DM require less antidiabetic agents to control glucose levels and achieve improvements in HbA1c. In some studies these improvements are greater with a very low carbohydrate diet compared with other approaches, but data are not consistent and there are no long term studies convincingly demonstrating any greater benefit. Davis et al demonstrated similar modest weight loss but no significant improvement in glycaemic control in those with type 2 diabetes on a low-fat or low-carbohydrate diet after one year. In the longest very low carbohydrate diet trial currently reported, a very low carbohydrate diet achieved a -0.9% reduction in HbA1c at 2 years compared with -0.5% in a Mediterranean diet and -0.4 % in a low fat diet, however the difference between groups was not significant.
One of the concerns raised about very low carbohydrate diets is the potential for increases in dietary saturated fat that may accompany the foods chosen as part of this diet, will adversely affect lipid profile. However this has not been generally observed when weight loss is achieved, though can be seen in isolated individuals who do not lose weight. Hussein et al demonstrate significant reduction in triglycerides, total and LDL cholesterol and an increase in HDL cholesterol over 24 weeks. This global benefit is not observed in all trials with increases in HDL cholesterol being the most consistent observation. Blood pressure has been shown to be reduced more with very low carbohydrate diets compared with alternatives over 3 months, but no difference after 12 months.
Once again the only data on long-term cardiovascular outcomes are from observational studies rather than randomised controlled trials. These are reciprocal analyses of the protein intake as described above. One study showed impairment of flow mediated dilatation, but improvements in other endothelial function markers, after 12 months on a very low carbohydrate diet in overweight and obese patients, but this was not specifically in those with diabetes.
Weight Loss
Studies in those with diabetes are limited, with few over a duration longer than 12 weeks, but indicate potentially initial greater weight loss but no difference over the longer term. One non-randomised study comparing a ketogenic very low carbohydrate diet with a low calorie diet over 24 weeks did demonstrate greater weight loss, improvements in glycaemic control and lipid profile with the very low carbohydrate approach. However no long term or randomised controlled trials have demonstrated any superiority. It has been hypothesised that the generation of ketosis is central to facilitating weight loss with a low carbohydrate diet, however this is not supported by Boden or Krebs et al who showed that weight loss was directly related to the reduction of total energy intake achieved by removing one macronutrient from the diet without appreciable compensatory increase in actual fat or protein intake.
Glycaemic Control
Significantly restricting carbohydrate in the diet has profound effects on glucose metabolism. Several small and very short studies ranging from seven days to five weeks have variably demonstrated reduced glucose and insulin concentrations and improved glycaemic control. In short-term studies people with T2DM require less antidiabetic agents to control glucose levels and achieve improvements in HbA1c. In some studies these improvements are greater with a very low carbohydrate diet compared with other approaches, but data are not consistent and there are no long term studies convincingly demonstrating any greater benefit. Davis et al demonstrated similar modest weight loss but no significant improvement in glycaemic control in those with type 2 diabetes on a low-fat or low-carbohydrate diet after one year. In the longest very low carbohydrate diet trial currently reported, a very low carbohydrate diet achieved a -0.9% reduction in HbA1c at 2 years compared with -0.5% in a Mediterranean diet and -0.4 % in a low fat diet, however the difference between groups was not significant.
CVD Risk Factors
One of the concerns raised about very low carbohydrate diets is the potential for increases in dietary saturated fat that may accompany the foods chosen as part of this diet, will adversely affect lipid profile. However this has not been generally observed when weight loss is achieved, though can be seen in isolated individuals who do not lose weight. Hussein et al demonstrate significant reduction in triglycerides, total and LDL cholesterol and an increase in HDL cholesterol over 24 weeks. This global benefit is not observed in all trials with increases in HDL cholesterol being the most consistent observation. Blood pressure has been shown to be reduced more with very low carbohydrate diets compared with alternatives over 3 months, but no difference after 12 months.
Once again the only data on long-term cardiovascular outcomes are from observational studies rather than randomised controlled trials. These are reciprocal analyses of the protein intake as described above. One study showed impairment of flow mediated dilatation, but improvements in other endothelial function markers, after 12 months on a very low carbohydrate diet in overweight and obese patients, but this was not specifically in those with diabetes.