A new controlled clinical trial conducted during the pandemic compared the two diets by having 33 people with prediabetes or diabetes follow both diets back-to-back for three months. During the first four weeks of each diet, participants received a healthy supply of keto or Mediterranean food, then followed the meal plans on their own.
Researchers monitored participants’ weight, blood sugar (glucose) levels, cardiovascular risk factors, and dietary compliance. Which diet was still standing at the last bell?
“Both diets improved blood sugar control to a similar degree, and both groups lost a similar amount of weight,” said lead nutrition researcher Dr. Walter Willett, professor of epidemiology and nutrition at the Harvard TH Chan School of Public Health and professor of of Medicine at Harvard Medical School. He did not participate in the research.
However, when researchers examined the effects of the two diets on levels of blood fats that contribute to heart disease, the Mediterranean diet was the clear winner, according to the study published Friday in The American Journal of Clinical Nutrition.
The study looked at low-density lipoprotein, or LDL, known as “bad” cholesterol, and triglycerides, which are a different type of fat in the blood that also contribute to hardening of the arteries.
“The keto diet significantly raised LDL cholesterol by 10 percent, while the Mediterranean diet lowered LDL cholesterol by 5 percent,” said Dr. Frank Hu, chair of the department of nutrition at the Harvard TH Chan School of Public Health, who was not involved in the study.
“The difference between the two diets is quite large, and this may have long-term implications for cardiovascular disease,” Hu said.
While both diets lowered triglycerides, the keto diet did so more significantly, the study found. However, lowering triglycerides is not as important as raising bad cholesterol, Hu said.
“High LDL cholesterol is a much more powerful and important risk factor for cardiovascular disease than triglyceride levels,” he said. “So while both were quite effective in short-term glycemic control, I think the main issue is the potential long-term effects of keto on cardiovascular disease.”
“I tried to give each diet the best chance”
Keto achieves rapid weight loss success, proponents say, by putting people into ketosis, a state in which the body begins to burn stored fat for fuel. But to get into ketosis, carbs are drastically reduced to 20 to 50 grams per day. (A cup of cooked rice is about 50 grams.) Eating extra carbs takes you out of ketosis.
The typical American’s daily diet is 50 percent carbohydrates, Hu said, so reducing that intake to less than 50 grams is “a huge reduction. That’s hard for people to maintain.”
People often think of keto as a “meat” diet and fill their plates with full-fat dairy, sausage, bacon, and other meats with saturated fat, all of which can contribute to inflammation and chronic disease.
However, the study used a “well-formulated ketogenic diet” that limited high protein intake and emphasized non-starchy vegetables, said study author Christopher Gardner, a professor of medicine at the Stanford Prevention Research Center.
“I tried to give every diet the best shot. I didn’t try to make it crappy keto and good Mediterranean or crappy Mediterranean and good keto,” said Gardner, who is also director of clinical and translational research at the Stanford Diabetes Research Center.
The keto diet bans all grains, legumes, and fruits with the exception of a handful of berries. However, the Mediterranean diet emphasizes packing your plate with fruits, vegetables, beans, lentils, whole grains, nuts and seeds.
Both diets agree that “we’re eating too much added sugar and refined grains and not enough vegetables,” Gardner said. “So the whole study was designed to see if there’s an advantage to getting rid of fruit, whole grains, and legumes on keto — after doing the things everyone agrees on.”
In addition to an increase in bad cholesterol, people in the keto phase had “decreased intakes of thiamin, vitamins B6, C, D and E, and phosphorus,” as well as “incredibly low fiber,” said Shivam Joshi, Ph.D., a clinical asst. professor of medicine at New York University Grossman School of Medicine. He did not participate in the research.
“Whole grains and fruit have positive health benefits, and excluding them from the keto group raises some concern about long-term health effects,” Willett said. He also said, “Many people think that long-term adherence to a keto diet is difficult.”
In fact, the study found that most people gave up the keto diet after the study ended.
“They had keto delivered to their home. They had a health educator help them,” Gardner said. “Yet boom! Most people stopped following the keto diet almost immediately (when that part of the study ended), while many of those on the Mediterranean diet were still eating that way when the study ended.”
What is the main message of the study?
“The No. 1 take-home message for me is that severe restriction of certain healthy carbohydrates is not necessary to improve glycemic control and cardiac metabolic health,” Hu said.
“You can follow a healthy Mediterranean diet or a moderately low-carb diet or a very healthy vegetarian diet. There are different options for people with different dietary preferences.”
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