While extremely low-carbohydrate diets may aid short term weight loss, they have mixed effects on health markers that can contribute to heart disease risk, according to new recommendations from the National Lipid Association.

Based on a review of existing research, the scientific statement emphasizes some advantages of a ketogenic, or very low-carb, diet including appetite suppression, lower lipid levels and lower blood sugar in people with diabetes. But a keto diet is also associated with spikes in the "bad" cholesterol that can build up in blood vessels and lead to clots, known as low-density lipoprotein cholesterol (LDL-C).

Over six months, people may lose more weight with popular low-carb diets like the Atkins, ketogenic, South Beach, and Zone diets, according to the recommendations published in the Journal of Clinical Lipidology.

But after a year, weight loss with these diets is similar to what people can achieve with diets that allow more carbohydrates, according to the recommendations. Extremely low-carb diets can also be harder to stick with over time, and may severely restrict nutrient-dense foods that offer cardiovascular benefits, the recommendations stress.

There doesn't appear to be a meaningful difference between low-carb and other types of diets for other markers of cardiometabolic health like blood pressure.

"While some patients prefer a low-carbohydrate eating pattern, which may be reasonable for short periods of time, long-term compliance is challenging, and long-term benefits and risks are not fully understood, especially with ketogenic diets," said Carol Kirkpatrick, lead author of the recommendations and a researcher at Idaho State University in Pocatello.

With ketogenic diets, people typically eat very few carbs and consume a lot of fat, putting the body into a metabolic state known as ketosis. This can make the body more efficient at burning fat for energy and trigger reductions in blood sugar, previous research has found.

Some people on ketogenic diets have lost two to three times more weight than individuals with different eating habits, but much of this is based on short-term results. "Weight loss with any dietary strategy is difficult and there are many factors that impact a person's ability to lose weight and maintain that weight loss," Kirkpatrick said by email. "Behavioral strategies, social support, and adequate physical activity have proven helpful for enhancing weight loss and helping with weight loss maintenance, no matter what type of weight loss diet is employed by a person needing to lose body fat."

People who might benefit from following an extremely low-carb diet for two to six months include individuals with diabetes and individuals with high levels of triglycerides in their blood, according to the recommendations.

Patients with a history of dangerously high cholesterol levels should avoid keto and extremely low-carb diets, the recommendations also note. More research over longer periods of time is still needed to determine whether keto or low-carb diets might be harmful or helpful to people trying to lose weight and improve their overall health, said Andrew Mente, a researcher at McMaster University in Hamilton, Canada, who wasn't involved in the study.

"Since there are no long-term trials of low-carb diets and health outcomes, this review is only able to report on intermediate risk markers rather than actual clinical outcome events like heart attacks, strokes, cardiovascular death, new diabetes events, and total mortality," Mente said by email.

While diets aren't one size fits all, most people should aim for a wide variety of healthy foods, said Dr. Sara Seidelmann, a cardiologist and nutritionist at Brigham and Women's Hospital and Harvard Medical School in Boston.

Ideally, adopt a diet full of plants and whole foods that provides overall cardio-protective qualities: rich in whole grains, vegetables, fruits, nuts, and legumes; eliminate processed foods and trans fats; limit refined grains, saturated fats, red meat and added sugars," Seidelmann, who wasn't involved in the study, said by email. "Adults that are interested in pursuing a very low or low carbohydrate diet for weight loss should consult with their clinician in order to weigh the risks and benefits of various diets and to share in the decision making process."