The ketogenic diet may be more beneficial in the short term than other diets for people with obesity, according to a small clinical trial

It is a small but well-controlled and high-quality trial: participants were randomly assigned, received all meals, and all three groups lost approximately the same amount of weight. This allows for a fairly robust conclusion that, in people with obesity, prediabetes, and fatty liver, the ketogenic diet may reduce liver fat more effectively and improve some indicators of glucose control during weight loss. However, it was not superior in every respect: all three diets similarly improved muscle response to insulin, and there were no clear differences in blood pressure, LDL cholesterol, or apolipoprotein B. Therefore, claiming that the ketogenic diet is overall healthier or better than the Mediterranean diet exaggerates the results.

The main limitation is that only 42 people completed the study, 14 per diet, and the follow-up lasted about five months. Furthermore, receiving all meals free of charge, pre-prepared, and meeting weekly with a dietitian is far from typical conditions and does not allow us to know if the diet can be maintained long-term. Frozen meals and prepared snacks were used, some potentially ultra-processed, but insufficient information was provided on their ingredients and additives to classify them or to separate the effect of macronutrients from the effect of food quality and processing. Finally, the fact that a diet with 23% of calories from saturated fat did not worsen cholesterol levels during a few months of active weight loss is interesting, but it does not demonstrate that it is safe or beneficial over years. The Mediterranean diet continues to have much stronger evidence regarding long-term cardiovascular health.

The article describes a randomised clinical trial, so it is of high quality in terms of design; however, it should be noted that only 14 participants completed the trial in each group, meaning the sample size is very small, and they only underwent the dietary intervention for five months.

We were already aware that the ketogenic diet has interesting metabolic effects in the short term; my own research group has published several papers on this topic. It should also be noted that all three dietary intervention groups showed metabolic improvements, meaning that any dietary intervention that leads to weight loss improves the patient’s metabolism; in this study, the ketogenic diet group performed marginally better than the other two interventions. The major concern with the ketogenic diet is its safety and long-term effects; there are no reliable studies assessing its long-term effects, for example, on cardiovascular disease. The Mediterranean diet, on the other hand, has been shown to be safe in the long term and to significantly reduce the incidence of cardiovascular events. We must interpret these results with caution. The long-term effect of a diet so high in fat and so low in carbohydrates restricts the intake of certain foods that have been shown to provide significant cardiovascular benefits. At present, the evidence regarding the long-term health benefits of the Mediterranean diet is indisputable, and it should therefore continue to be the recommended diet for all individuals.

The sample size is very small and the inclusion criteria are very restrictive: only subjects with hepatic steatosis were included, and these results cannot be generalised to the general population or to those with other conditions, such as diabetes.

The only thing this study demonstrates, like others carried out previously, is that the ketogenic diet in the short term (less than six months’ intervention) improves metabolic health in obese individuals. However, it is incorrect to conclude that it is better than other diets and, above all, in the long term – which is how individuals suffering from obesity should be treated.

This study is significant because it uses a design that allows differences in the measured health markers to be attributed to the type of diet followed by the study participants. Although it is not stated whether the level of physical activity was similar across the groups of participants being compared – which could have affected the results had it differed between the comparison groups – the rest of the information regarding the study appears to be well-conducted. In the published study, it appears that all the diets compared included foods with a fairly healthy nutritional composition; therefore, the findings suggest that diets with a higher proportion of fat may be more beneficial for people who are overweight.

However, this study does not address the major issue with high-fat diets, which relates to their quality. It is not the same to follow a diet consisting mainly of foods high in unhealthy fats – such as those found in high-fat meats and processed foods – as it is to follow a diet comprising foods high in healthy fats, such as those found in olive oil and oily fish. Furthermore, high-carbohydrate diets may include healthy foods, such as whole grains, fruit and vegetables, and pulses, but they may also include foods that provide sugars – which are low-quality carbohydrates – such as sugary drinks or industrially produced pastries.

Our group recently published a study in which we found that a diet defined by its percentage of total fat or carbohydrates did not predict the risk of developing chronic diseases later in life. However, when this diet was defined by its quality – whether healthy or unhealthy, depending on the type of fats or carbohydrates consumed – people who followed unhealthy diets had a higher risk, of up to 11 per cent, of developing various diseases over the years.

The Spanish dietary guidelines recommend a diet that includes a moderate to high intake of healthy fats and a similarly moderate to high intake of healthy carbohydrates (adjusting the ratio depending on individual preferences or circumstances), alongside a moderate intake of animal protein and a high intake of plant-based protein. This combination appears to be the most balanced for preventing disease in the general population.