Researchers examine risks and benefits of popular diets

In a review of existing scientific studies on ketogenic and intermittent fasting diets, researchers at National Jewish Health concluded that these diets do seem to help people lose weight in the short-term, and modest evidence suggests they may contribute to cardiovascular health, according to a new study published in The American Journal of Medicine. However, these diets also allow consumption of foods that are known to increase cardiovascular risk and are unlikely to be as effective at preventing heart disease as well-established nutritional guidelines currently recommended by health experts.

The ketogenic diet is a very low carbohydrate dietary approach that sends the body into ketosis, a metabolic state in which it has reduced access to glucose and is instead mostly fueled by fat. While the limited study of the keto diet shows that those who follow it initially lose weight, it tends not to be sustainable according to 12-month data. It is also unclear whether the weight loss is caused by ketosis or simply by calorie restriction, the researchers said.

The researchers said they also have concerns about the type and amount of fat consumed by those following a ketogenic diet. While existing studies strictly controlled the type of fat and foods participants consumed, many who try ketogenic consume high amounts of unhealthy saturated fat, which is associated with an increased risk of heart disease and high lipid levels in the blood. There is also evidence that eating a keto diet for an extended period may lead to stiffening of the arteries, and several studies found that those who eat a ketogenic diet have a greater risk of death, according to the study.

A ketogenic diet does, however, show promise as a potential treatment for diabetes, with studies showing improved glucose levels as well as lower fasting glucose and insulin levels in mice fed a ketogenic diet. Further research is needed to confirm these benefits and assess risk before keto is clinically recommended, the researchers said.

In the study, the researchers said they are also optimistic about potential health benefits of intermittent fasting but are concerned about possible pitfalls. There are a wide range of practices being called intermittent fasting, with some fasting without food an entire day and others restricting meals to certain hours of the day. Experts also worry that the hunger induced by fasting causes many people to overeat when it is time for meals or make unhealthy choices that have adverse effects on their cardiovascular health.

Most of the current evidence regarding the potential benefits of intermittent fasting come from animal studies, which have shown increased longevity, weight loss, decreased blood pressure, improved glucose tolerance and controlled lipid levels, the researchers said.

While there is modest evidence regarding favorable effects of both dietary approaches, neither the ketogenic diet nor intermittent fasting is recommended for the treatment or prevention of any condition until large, long-term studies can more definitively examine their impact, the researchers said.

Instead, experts recommend diets that have been extensively studied and scientifically proven to prevent or even reverse cardiovascular issues, which include the Mediterranean diet, a whole food plant-based diet, and the National Institutes of Health's Dietary Approaches to Stop Hypertension (DASH). All of these share a common foundation that includes fruits, vegetables, legumes, nuts, and whole grains, they said.

"With diets like keto and intermittent fasting, social and popular media has been flooded with claims, promises and warnings that are at best unverified and at worst harmful to your health," said Andrew Freeman, MD, co-author of the study and director of cardiovascular prevention and wellness at National Jewish Health, in a statement. "Diets recommended by health experts, such as plant-based and Mediterranean diets, have been extensively studied for safety and efficacy, and demonstrated conclusively to improve cardiovascular health."