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Both the ketogenic and Mediterranean diets successfully cut blood sugar levels in patients with prediabetes or type 2 diabetes, according to a small randomized crossover trial.

Among 33 adults, the well-formulated ketogenic diet (WFKD) resulted in a 9% drop in HbA1c values after 12 weeks, while the Mediterranean-plus diet (Med-Plus) resulted in a 7% drop, reported Christopher Gardner, PhD, of Stanford University in California, and colleagues.

Both diet types also led to decreases in weight (8% vs 7%, respectively), without any significant differences between the groups, they noted in the .

Both diets are considered "low carb" and are based on the foundation of limiting added sugars and refined grains, and consuming more non-starchy vegetables. However, WFKD avoids legumes, fruits, and whole intact grains that are otherwise included in the Med-Plus diet.

"The one thing everybody did pretty well was limiting added sugar and refined grains. That was the main message for both diets," Gardner explained in a statement

"The keto diet was more polarizing," he added. "What we were hearing is that some people just couldn't do the keto because it was too restrictive."

As for cardiometabolic parameters, the WFKD dieters saw a greater reduction in triglycerides -- a 16% drop from baseline versus only a 5% drop for Med-Plus dieters (P=0.02). LDL cholesterol increased by 10% with the WFKD diet and decreased by 5% with Med-Plus, while HDL cholesterol increased by 11% with WFKD and 7% with Med-Plus.

However, the WFKD diet fell flat for nutrients, as those who stuck to this diet had lower intake of fiber; thiamine; vitamins B6, C, D, and E; and phosphorus compared with the Med-Plus dieters. Vitamin B12 levels were higher with WFKD.

"The lower in carbs you go, the more you're wiping out entire food groups that are considered very nutrient dense and healthy," Gardner said. "What is it about this diet that would be so compelling that you would give up some of those central tenets of health and nutrition?"

"Restricting added sugars and refined grains and emphasizing the inclusion of vegetables should be the focus," he added. "There's no reason to restrict heart-healthy, quality carbohydrate foods above and beyond."

The San Francisco-based study included adults with prediabetes (HbA1c 5.7% to 6.4% or fasting glucose 100 to 125 mg/dL) or type 2 diabetes (HbA1c 6.5% or higher or fasting glucose 126 mg/dL or higher). The 33 participants were randomized evenly to each diet type for 12 weeks.

The researchers didn't necessarily discourage weight loss, but there wasn't a particular calorie restriction put on the participants. On average, participants consumed about 250 to 300 fewer calories per day compared with baseline.

Participants on the WFKD diet were instructed to sustain nutritional ketosis by limiting carbohydrates to 20 to 50 g per day and proteins to around 1.5 g/kg ideal body weight per day, with the remaining calories coming from fats. They were also told to consume more than 3 servings per day of non-starchy vegetables and maintain adequate mineral and fluid intake for the ketogenic state, with about 3 to 5 g of sodium and 3 to 4 g of potassium per day.

Med-Plus dieters were told to simply avoid added sugars and refined grains, while adhering to a chiefly plant-based diet.

A total of four adverse events were reported, with just one -- an elevated alanine transaminase level on WFKD -- considered to be likely related to the study.

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