The study, which is a systematic review, was published in a recent issue of the American Journal of Clinical Nutrition. A systematic review is an unbiased survey of all the scientific evidence available on a given question. In this case, Ajala and coworkers from Plymouth UK addressed the efficacy of different diets to induce weight loss and improve glycemic control and lipid profile among people with type 2 diabetes.
Before we go further, let me touch on few basic issues, just for the sake of clarification. Type 2 diabetes is a chronic condition that affects the way our body metabolizes sugar (glucose). Glucose is the body’s main source of fuel. With type 2 diabetes, the body either resists the effects of insulin — a hormone that regulates the movement of sugar into our cells — or doesn’t produce enough insulin to maintain a normal glucose level.
Glucose comes from two major sources, food and the liver. Our liver produces and stores glucose. When glucose enters the blood stream, insulin is released from the pancreas. Insulin enables sugar to enter the cells of the body where it is used for energy production. Without insulin, or when insulin resistance is present, glucose builds up in the blood stream. This is what happens in diabetes. Elevated levels of blood glucose may cause a number of symptoms, and increase the risk of developing heart disease and stroke.
It is not fully understood why people develop type 2 diabetes, but certain factors increase the risk. Overweight is a risk factor. However, many overweight and obese individuals never have diabetes, and many people with type 2 diabetes are not overweight or obese. If our body stores fat primarily in the abdomen, the risk of diabetes is higher than if the body stores fat elsewhere such as on our hips and thighs. Family history and physical inactivity also increases the risk of diabetes. The risk of type 2 diabetes increases with age. Lately, type 2 diabetes has become increasingly common in children, adolescents and young adults. The main goals of therapy are to improve glucose metabolism and thereby reduce blood sugar levels, and to reduce the risk factors for the main complications of diabetes, heart disease and stroke. Overweight often contributes significantly to the development of type 2 diabetes, therefore weight loss often plays a large role for the treatment of the disorder.
Prediabetes is a condition in which blood sugar level is higher than normal, but not high enough to be classified as diabetes. Left untreated, prediabetes often progresses to type 2 diabetes. There is good evidence showing that dietary modification can prevent the progression from prediabetes to type 2 diabetes. Howvever, the optimal dietary approach to type 2 diabetes is less clear. Before I discuss the recent study by Ajala and coworkers, let´s have a look at the background stage.
For some years, experts and scientists have debated what dietary approach is best to control and treat type 2 diabetes. Most regulatory authorities, like the British Diabetic Association, European Association for the Study of Diabetes (EASD), American Diabetes Association (ADA), Canadian Diabetes Association and many more usually recommend a carbohydrate intake of 50-60% of total energy intake, total fat intake less than 30% of energy, with restriction of saturated and trans fat intake. Some experts believe there is insufficient evidence to justify these recommendations.
Almost five years ago, an international group of scientists and experts on diabetes, nutrition and carbohydrate metabolism published an overview paper suggesting a critical appraisal of the role of carbohydrate restriction in type 2 diabetes mellitus and the metabolic syndrome.The authors pointed out that current nutritional approaches often emphasized a reduction in dietary fat. They believed that such approaches often were ineffective, leading to more reliance on drug therapy. They argued that carbohydrate restricted diets were at least as effective for weight-loss as low-fat diets, and that the substitution of fat for carbohydrate was generally beneficial for the risk of cardiovascular disease. Furthermore, they provided evidence suggesting that carbohydrate restriction improved control of blood glucose and reduced insulin fluctuations. They believed the recommendation to reduce the intake of saturated fat in type 2 diabetes lacked scientific evidence. The authors concluded: “Finally, while no systematic study of clinical practice has been done, anecdotal evidence suggests that carbohydrate restriction is a common clinical recommendation for diabetes. We believe that there is a need to codify these recommendations in light of current evidence.”
In the recent systematic review by Ajala and coworkers, data was collected from all studies published up to July 2011 that compared low carbohydrate, vegetarian, vegan, low glycemic index (GI), high fiber, Mediterranean, and high protein diets with control diets including low-fat, high-GI, ADA diet, EASD diet, and low-protein diets. Only randomized controlled trials with an intervention lasting longer than six months were selected. A total of 20 studies including 3073 individuals fulfilled the defined criteria, and were selected for the metaanalysis. Measured outcomes were HbA1c (which reflects glucose control), difference in weight loss, and changes in HDL-cholesterol (“good cholesterol”), LDL-cholesterol (“bad cholesterol”) and triglycerides.
The low carbohydrate, low-GI, Mediterranean, and high protein diets all led to a greater improvement in blood glucose control (HbA1c) compared with their respective control diets, with the largest effect seen with the Mediterranean diet. Low-carbohydrate and Mediterranean diets led to a greater weight loss compared with their control diets. Low carbohydrate, low-GI, and Mediterranean diets all led to an elevation of HDL-cholesterol. Only the Mediterranean diet led to a significant reduction in triglycerides. High protein diets had no effects on markers of lipid profile. One study from their review compared the effects of a vegan diet to the low fat ADA diet. The vegan arm had a significantly lower levels of total cholesterol, LDL-cholesterol and HbA1c, indicating better glycemic control by the vegan diet. Similar results were obtained in one study that compared the effect of a vegan diet with the EASD diet, with more weight loss on the vegan diet.
Ajala and coworkers conclude that their review provides evidence that modifying the amount of macronutrients can improve glycemic control, weight and lipids in type 2 diabetes. In their analysis, low carbohydrate diets appeared to provide superior weight loss, better control of blood glucose, and better lipid profile, compared with low fat diets. The authors also conclude that vegan and vegetarian diet may improve glucose control and promote weight loss in type 2 diabetes.
The Mediterranean diet is rich in olive oil, legumes, unrefined cereals, fruit and vegetables, low in meat and meat products, and with moderate contents of dairy products (mostly cheese and yogurt), fish and wine. The total fat in this diet is typically 25-35% of calories, with saturated fat less than 8% of calories. The metaanalysis indicates that a Mediterranean diet provides better control of blood glucose, greater weight loss and a more favorable lipid profile compared with a conventional diet and ADA diet.
A metaanalysis of such widely different studies may be problematic. For example the control diets differed significantly between studies and the duration of the studies ranged between six months and four years. The definition of a low carbohydrate diet varies between studies. While the authors acknowledge the limitations of their study, they believe that low carbohydrate, low-GI, Mediterranean and high-protein diets should be considered in the overall strategy of diabetes management.
What is the best diet for type 2 diabetes? Although, there is probably not a simple answer, the question reflects one of the main challenges of modern medicine. It is likely that our dietary recommendations will have to be tailored to the needs of the individual. A one-size-fits-all approach is unrealistic. Although not providing any definitive answers, the study by Ajala and coworkers is an important contribution to our understanding of this highly important issue.