The American Journal of Clinical Nutrition has published the results of a new trial pitting the keto diet against a lower-carb Mediterranean diet for individuals with type 2 diabetes and prediabetes.
Each diet restricted carbs, but the Mediterranean diet included generous amounts of fruit, legumes, and whole grains. Which diet had a more powerful effect on health, including blood sugar control and cardiovascular risk factors? Read on …
Although the trial was not especially large, with just 33 participants, it had a very clever design. Volunteers were asked to eat either a well-formulated keto diet or a lower-carb Mediterranean diet designed to share three key similarities. Each diet:
Restricted added sugars
Restricted refined grains
Emphasized nonstarchy vegetables
These three guidelines — each of which is endorsed by practically every nutrition authority out there — ensured that the two diets shared a nutritious foundation. Just as important, the substantial overlap between the two diets allowed the researchers to isolate the nutrition factors that they really wanted to study.
That brings us to the three key differences between the two diets. The keto diet eliminated the following three foods, and the Mediterranean diet emphasized them:
Most fruits (The keto diet allowed limited amounts of berries.)
Whole, intact grains
The image below summarizes these differences (“WFKD” stands for “well-formulated ketogenic diet,” and “Med-Plus” is the lower-carb Mediterranean diet).
These three differences were the real focus of the study. Restrictions against fruit, legumes, and whole grains arguably represent the most significant ways that keto and very low-carb diets violate nutrition orthodoxy — and help explain why diabetes authorities have appeared nervous about recommending them.
The primary question as, the study itself puts it, was:
Do people with an impaired glucose metabolism experience greater metabolic benefits or harms when restricting legumes, fruits, and whole, intact grains in addition to avoiding added sugars and refined grains?
Fruits, legumes, and whole grains are widely hailed as some of the healthiest ingredients you can put in your body, but low-carb advocates try to avoid or eliminate them because they are high-carb. Some argue that even the healthiest carbohydrates, those from minimally processed whole foods with plenty of fiber and protein, will still stress the metabolism, especially for those of us with diabetes.
The keto diet in this study, which followed the influential guidelines set forth by two of the diet’s most prominent advocates, aimed to limit carbs to 20 to 50 grams per day.
It’s important to reiterate that the Mediterranean diet used in this experiment may not match the Mediterranean diet you’re imagining. Because they were avoiding refined grains, study participants ate very little pasta or bread. The researchers clarified this by calling their diet “Mediterranean-plus.” It was essentially a lower-carb Mediterranean diet. The primary sources of protein and fats were seafood and olive oil.
Study participants, all of whom had prediabetes or type 2 diabetes, were randomly assigned to try one diet for 12 weeks, and then to switch and try the other diet for an additional 12 weeks. They were told to eat their fill — which means that any weight loss was incidental, a natural consequence of the diet and not the result of any specific attempt to lose weight.
There’s the design of a study, and then there’s the reality of it. You can tell volunteers that they’re not able to eat sugar for 24 weeks, but unless you have them under lock and key they’re bound to slip up. For some of the study period, volunteers were provided with carefully measured meals. But for the majority of it, they did their own shopping. During those sessions, unsurprisingly, they just about doubled their intake of added sugar and refined grains (although they remained far below their established baseline).
Arguably, this makes the findings of the study even more useful. In the real world, most people are going to cheat a little bit. A study that examines perfect adherence to an extreme diet may be of limited relevance to the average person who doesn’t want to say goodbye to cookies and pizza for the rest of their lives.
Before the study began, our participants averaged 746 calories per day from carbohydrates, much of it coming from a much larger amount of added sugar and starch. On the keto diet, participants took in about 250 calories per day from carbohydrates, and about 16 grams of fiber. On the Mediterranean diet, participants took in an average of 556 calories per day from carbohydrates, and 24 grams of fiber.
It seems like the minor straying didn’t matter much. During every part of the experiment, volunteers ate a lot fewer calories, 250 to 300 fewer than they were accustomed to eating. This total did not vary between diets, nor did it change when participants switched from premade meals to self-catered. Remember, the diners were allowed to eat as much as they wanted within diet restrictions, so this suggests that both diets drove effortless calorie cutting.
Both diets significantly improved glucose control, just about equally. Study participants improved their A1C and their fasting blood glucose.
Continuous glucose monitor (CGM) measurements suggested that the keto diet lowered blood sugar more effectively than the Mediterranean diet. And a variety of sensitivity analyses attempting to control for the effects of medication use, COVID-19 lockdowns, and other factors suggested that the keto diet may have conferred statistically significant glycemic improvements over the Mediterranean diet.
The bottom line? Both diets had big blood sugar benefits, but the keto diet may have narrowly outperformed the Mediterranean diet.
The cholesterol results were somewhat mixed.
Both diets increased HDL, so-called “good cholesterol,” and both diets decreased triglycerides; each change represents a major metabolic improvement. The keto diet fared somewhat better here. It conferred a larger decrease in triglycerides, and, according to some sensitivity analyses, may also have had a better impact on HDL levels.
But there was one big difference between the two diets: They had opposite effects on LDL (“bad”) cholesterol. The Mediterranean significantly reduced LDL cholesterol levels, but the keto diet significantly increased them.
Much study has linked high LDL cholesterol with bad cardiovascular outcomes, including early death from heart attack and stroke, which has made it a primary target of diabetes medical interventions. But many prominent low-carb advocates dispute the connection between LDL and cardiovascular disease, especially for those on a ketogenic diet.
Despite not actively attempting to shed weight, participants lost a good deal of weight on both diets. Those on the keto diet lost a bit more.
After phase 1, volunteers eating keto lost a very impressive average of 6.9 kilograms (kg) (about 15 pounds). Those eating the Mediterranean diet lost a still-notable 4.9 kg.
The Mediterranean diet was more nutritionally replete. Participants in the Med-Plus phase consumed more folate, vitamin C, and magnesium than those in the keto phase, although it’s not clear if these deficiencies rise to a concerning level. The Mediterranean diet also included about 50 percent more fiber.
Volunteers appeared to enjoy each diet equally. There was no significant difference between the two diets in quality of life measures or in adherence rates.
There was one subtle point in favor of the Mediterranean diet. Weeks after the formal completion of the study, researchers followed up with the volunteers. During this time they were free to eat whatever the heck they wanted to. It turned out that they seemed to stick closer to the Mediterranean eating pattern than the keto one, suggesting that they preferred the Med-Plus approach and that it may be more sustainable.
The most important takeaway is the fact that both diets had excellent results. All study participants lost weight without having to count calories, and enjoyed improvements in blood sugar, HDL cholesterol, and triglycerides.
That’s not a shock. By now it is a solid expert consensus that we should be eating less sugar and simple starches, and more nonstarchy vegetables. It would have been surprising if these two diets didn’t result in metabolic improvements for patients with diabetes.
So, how about those legumes, fruits, and whole grains? It appears that including those more-healthful carbohydrates in the diet of a person with prediabetes or diabetes may possibly result in slightly less weight loss, slightly higher blood sugar, and lesser improvements in HDL and triglycerides. But avoiding those three ingredients for the high-fat ketogenic diet comes with one potentially major downside: an increase in LDL cholesterol.
The authors of the paper conclude that in the context of an otherwise low-carb diet, the benefits of fruit, legumes, and whole grains outweigh the downsides. They reason that the modest improvements in glycemic control and weight conferred by the keto diet are not significant enough to overcome the rise in LDL, the lack of fiber, and the “greater potential for nutrient deficiencies.” So if you’ve chosen a low-carb diet to prioritize glycemic control, the answer may be yes: Maybe you should swap some of those low-carb ingredients for some healthful minimally processed carbohydrates.
But there’s little doubt that low-carb advocates would dispute the conclusion. The study was not a true test of a hardcore ketogenic diet, which may confer even greater metabolic benefits, and the meaning of rising LDL cholesterol remains controversial.
Either way, this study isn’t enough to definitively recommend one diet over the other. The authors aver that “patients should be supported in choosing a dietary pattern that fits their needs and preferences.” The American Diabetes Association has endorsed both low-carb and Mediterranean diets as viable eating patterns for people with type 2 diabetes.
Carbohydrate restriction can cause a rapid drop in blood glucose levels; diabetes patients that take medication with known risks of hypoglycemia, such as insulin or sulfonylureas, should be prepared to quickly or proactively change their dosage. This should be done in consultation with a doctor.
None of the participants in this study was using insulin or other powerful glucose-lowering drugs, such as GLP-1 receptor agonists. The results of this study are therefore not necessarily generalizable to patients that require insulin before every meal, including those with type 1 diabetes.