Which Diet for Type 2 Diabetes?

Prescribed diets can be trying for both patients and providers; patients often struggle to adhere to them, and providers must determine which plan is suitable for which patient. The optimal diet for patients with diabetes—and whether it is sustainable—remains controversial.

A plant-based diet high in polyunsaturated and monounsaturated fats, with limited saturated fat and avoidance of trans-fatty acids, is supported by the American Association of Clinical Endocrinologists. Caloric restriction is recommended when weight loss is appropriate.¹ The American Diabetes Association (ADA) recommends a Mediterranean-style diet rich in monounsaturated fats with carbohydrates from whole grains, vegetables, fruits, legumes, and dairy products, and an emphasis on foods higher in fiber and lower in glycemic load.²

Additionally, the ADA, the American Association of Diabetes Educators, and the Academy of Nutrition and Dietetics advise that all individuals with diabetes receive individualized Medical Nutrition Therapy (MNT), preferably with a registered dietitian nutritionist (RDN) knowledgeable and skilled in providing diabetes-specific nutrition education. MNT delivered by an RDN has been shown to reduce A1C levels by up to 2% in people with type 2 diabetes (T2DM).³

This flexibility in recommendations creates uncertainty about the correct dietary choice. Several diet plans are endorsed for the management of diabetes, including Mediterranean, low carbohydrate, Paleolithic, vegan, high fiber, and glycemic index (GI). Which should your patients adhere to? Several randomized controlled trials (RCTs), meta-analyses, and literature reviews have examined and compared the benefits of these eating habits for management of diabetes.

MEDITERRANEAN

The Mediterranean diet incorporates plant foods such as greens, tomatoes, onions, garlic, herbs, whole grains, legumes, nuts, and olive oil as the primary source of fat. A crossover trial of adults with T2DM demonstrated a statistically significant A1C reduction (from 7.1% to 6.8%) after 12 weeks on the Mediterranean diet.⁴

In a systematic review of 20 RCTs, Ajala et al analyzed data for nearly 3,500 patients with T2DM who adhered to either a low-carbohydrate, vegetarian, vegan, low-GI, high-fiber, Mediterranean, or high-protein diet for at least six months. The researchers found that Mediterranean, low-carbohydrate, low-GI, and high-protein diets all led to A1C reductions—but the largest reduction was observed with patients on the Mediterranean diet. Low-carbohydrate and Mediterranean diets resulted in the most weight loss.⁵

LOW-CARBOHYDRATE

Low-carbohydrate diets have decreased in popularity due to concerns about their effects on renal function, possible lack of nutrients, and speculation that their macronutrient composition may have effects on weight beyond those explained by caloric deficit. A meta-analysis of 13 studies of adults with T2DM following a low-carbohydrate diet (≤ 45% of calories from carbohydrates) demonstrated beneficial effects on fasting glucose, A1C, and triglyceride levels. Nine of the studies evaluated glycemic control and found A1C reduction with lower carbohydrate diets; the greatest reductions in A1C and triglycerides were correlated with the lowest carbohydrate intakes. No significant effects were seen for total, HDL, or LDL cholesterol.⁶

In the literature review by Ajala et al, low-carbohydrate, low-GI, and Mediterranean diets all improved lipid profiles. HDL cholesterol increased the most with a low-carbohydrate diet.⁵

A two-week study of 10 adults with T2DM found that just one week on a low-carbohydrate diet decreased the average 24-h plasma glucose from 135 mg/dL to 113 mg/dL. Over the two-week study period, triglycerides decreased by 35%, cholesterol by 10%, and A1C by 0.5%. Patients were allowed to consume as much protein and fat as desired. Food sources included beef and ground turkey patties, chicken breasts, turkey, ham, steamed vegetables, butter, diet gelatin, and a limited amount of cheese. Mean calorie intake decreased from 3,111 to 2,164 calories/d. Carbohydrate intake decreased from 300 to 20 g/d. Weight loss was entirely explained by the mean energy deficit.⁶ Patients experienced no difference in hunger, satisfaction, or energy level with a low-carb diet compared to their usual diet.⁷

A literature review of six studies examined the effects of low-carb diets (between 20-95 g/d) on body weight and A1C in patients with T2DM. Three of the studies restricted carbohydrate intake to less than 50 g/d. All reported reductions in body weight and A1C. In two studies, the majority of the weight loss was explained by a decrease in body fat, not loss of water weight. No deleterious effects on cardiovascular disease risk, renal function, or nutritional intake were seen. The researchers concluded that low-carb diets are safe and effective over the short term for people with T2DM.⁸

PALEOLITHIC

The Paleolithic diet (also referred to as the caveman diet, Stone Age diet, and hunter-gatherer diet) involves eating foods believed to have been available to humans before agriculture—this period began about 2.5 million years ago and ended about 100,000 years ago. Food sources include wild animal meat (lean meat and fish) and uncultivated plant foods (vegetables, fruits, roots, eggs, and nuts). It excludes grains, legumes, dairy products, salt, refined sugar, and processed oils.