Keto or Mediterranean diet for diabetes: Do we have a clear winner?

The ketogenic (keto) diet appears as good as the Mediterranean diet in reducing blood sugar in individuals with prediabetes, or type 2 diabetes (T2D), according to a small, randomized, crossover trial.

HbA_1c values similarly improved from baseline with both diets. There was a 9-percent drop in HbA_1c values with the keto diet relative to a 7-percent drop with the Mediterranean diet.

Additionally, there was a more significant reduction in triglyceride levels with the keto diet. However, this benefit came with potential untoward risks from elevated LDL-cholesterol (LDL-C) and lower nutrient intake due to the avoidance of legumes, fruits, and whole, intact grains with this diet. Additionally, the keto diet was less sustainable.

Weight loss was 8 percent and 7 percent with keto and Mediterranean diets, respectively. “The one thing everybody did pretty well was limiting added sugar and refined grains. That was the main message for both diets,” said study author Dr Christopher Gardner from Stanford University in Stanford, California, US.

Both diets are based on the low carbohydrate concept endorsed by diabetes societies which restrict added sugar and limit refined grains but include nonstarchy vegetables. The one difference was that the keto diet avoids most fruits, whole grains, and legumes, which are allowed in the Mediterranean diet.

“I should say the keto diet was more polarizing,” opined Gardner. “Some people just couldn’t do the keto for long as it was too restrictive.”

Only 12 weeks

Participants (n=33) were adults with prediabetes (HbA_1c 5.7 –6.4 percent or fasting glucose 100–125 mg/dL) or T2D (HbA_1c ≥6.5 percent or fasting glucose ≥126 mg/dL) who were randomly assigned to the keto or Mediterranean diet for 12 weeks. [Am J Clin Nutr 2022;doi:10.1093/ajcn/nqac154]

The primary outcome was the percentage change in HbA_1c from baseline after 12 weeks on each diet; secondary outcomes included fasting insulin and blood lipid values. There was no calorie restriction imposed.

Those on the keto diet were instructed to sustain nutritional ketosis – a metabolic state in which the body primarily uses fat for energy instead of sugar – by limiting carbohydrates to 20–50 g/day and proteins to 1.5 g/kg ideal body weight/day, with the calories coming from fats. They were also told to consume >3 servings/day of nonstarchy vegetables and maintain adequate mineral and fluid intake for the ketogenic state, with about 3–5 g of sodium and 3–4 g of potassium daily.

Those on the Mediterranean diet were advised to avoid added sugars and refined grains, while adhering to a chiefly plant-based diet that included legumes, fruits, whole intact grains, nuts, and seeds, with fish as the primary animal protein and olive oil as the primary fat.

In both diets, whole foods were promoted. All processed foods and added sugars were strongly discouraged. On average, participants consumed 250–300 fewer calories daily compared with baseline.

Taking into account the cardiometabolic parameters, participants on the keto diet had a 16-percent reduction from baseline in triglycerides vs only 5 percent for those on the Mediterranean diet (p=0.02). LDL-C was up by 10 percent with keto and down by 5 percent with the Mediterranean diet. HDL-cholesterol  (HDL-C) increased by 11 percent and 7 percent, respectively.  

However, those on the keto diet missed out on nutrients, given their lower intake of fibre, thiamine, vitamins B6, C, D, E, and phosphorus. However, vitamin B12 levels were higher with the keto diet.

Include veggies!

“The lower you go with carbs, the more you’re wiping out nutrient-dense and healthy food groups,” Gardner pointed out. “Restricting added sugars and refined grains and inclusion of vegetables should be the focus,” he added.

Importantly, not one diet should be promoted as the best approach, he pointed out. “Rather, patients should be supported in choosing a diet that fits their needs and preferences. We should allow them to make an informed choice,” Gardner said in closing.