Low-carb or low-fat? Study pinpoints better diet

plate with low-carb foods (fish, vegetables)
In one of the most rigorous clinical trials to date, low-carb diets were better than low-fat diets at reducing cardio-metabolic risk factors. (Image: Adobe Stock. Illustration: Sebastian Stankiewicz, Boston Children's)

Despite the popularity of low-carbohydrate diets, health experts have pushed low-fat diets for the past 50 years, citing saturated fat as the main health risk. As a result, grocery shelves are stocked with low-fat and fat-free foods, many of which are high in processed carbs.

Drs. David Ludwig and Cara Ebbeling, who co-direct the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital, decided to test low-carb and low-fat diets head to head, launching a clinical trial that is one of the largest, most rigorous to date. The results were clear: low-carb diets were better at improving cardiovascular and metabolic profiles.

Carbohydrates, insulin resistance, and disease

Traditionally, experts have viewed high LDL cholesterol or “bad” cholesterol as the main risk factor for heart disease. But recent research has pointed to a group of other risk factors: high triglycerides, low HDL (“good”) cholesterol, high blood pressure, high blood sugar, chronic inflammation, a tendency toward blood clotting, and fatty liver.

These risk factors comprise what’s known as metabolic syndrome, also called insulin resistance syndrome because the body becomes less responsive to signals from insulin to remove sugars from the blood. And mounting evidence suggests that increased carbohydrate consumption — especially highly processed carbs like refined grains and added sugars — is one reason why metabolic syndrome is rising.

Comparing low-carb and low-fat diets

In partnership with Framingham State University, Drs. Ebbeling and Ludwig enrolled 164 adults with high body weight who had lost 10 to 14 percent of their weight on a reduced-calorie diet.

The participants then followed one of three weight maintenance diets for five months, assigned at random:

  • low-carb diet (20 percent carbs, 60 percent fat, 20 percent protein)
  • moderate-carb diet (40 percent carbs, 40 percent fat, 20 percent protein)
  • high-carb diet (60 percent carbs, 20 percent fat, 20 percent protein)

In all three diets, 35 percent of the fat consumed was saturated fat. This meant that the low-carb diet had three times the saturated fat of the high-carb diet (21 vs. 7 percent), well above the range of current recommendations.

All participants received fully prepared, customized meals that they could eat in cafeterias or take to go. This protocol ensured that they stuck to the diets. In contrast, many other studies give people nutritional guidance to follow on their own.

“We had a commercial food service provide more than 100,000 meals, tailored to each participant’s caloric needs,” says Dr. Ebbeling. “The meals were designed to keep participants at the same weight throughout the five months, so that everything we saw in this study would be independent of weight loss.”

Of note, the low-carb diet wasn’t a ketogenic diet — a restrictive very-low-carb, very high-fat diet in which carbs typically constitute less than 10 percent of calories.  

“The low-carb diet we studied is less restrictive,” Dr. Ludwig says. “There’s still room for whole fruits, all the non-starchy vegetables you want, beans, and small amounts of grains.”


Although the clinical trial was done in adults, low-carb diets can be appropriate for children, says Dr. Ludwig, and pediatric cardiologists are starting to embrace them. Children coming to the Optimal Wellness for Life (OWL) clinic at Boston Children’s may be prescribed diets with reduced proportions of carbohydrates depending on their needs.

“The roots of heart disease are in childhood,” Dr. Ludwig says. “A moderate change now could lead to a huge benefit when kids reach middle age, and if you establish healthy habits early, they’re more likely to stick. We’re trying to shift the lifetime trajectory of heart disease.”


* refined carbohydrates — like white bread, white rice, most breakfast cereals, and highly processed snack foods
* foods with added sugars, such as cereals, yogurt, bread, and more — check the nutrition label
* sugary beverages, including sports drinks and juices

Seek out:

* whole foods with slow-digesting carbohydrates, such as beans, fruits, vegetables, and whole-kernel grains
* healthy high-fat foods, like avocados, nuts, olive oil, and full-fat yogurt
* high-protein foods, which can be plant- or animal-based
* unsweetened beverages like iced tea, seltzer, and water
* snack combinations that pair slow-digesting carbs with protein and fat (examples: celery and peanut butter or apples and cheese)
* new foods, new recipes, and new flavors — be adventurous

See Boston Children’s fit kit for more tips.

The benefits of restricting carbs

As compared with low-fat, higher-carb diets, the low-carb diet improved the profiles of a range of blood lipids (fats) related to insulin resistance and cardiovascular disease. The low-carb diet also increased adiponectin, a hormone that promotes sensitivity to insulin and protects against atherosclerosis (the formation of fatty plaques in the arteries).

The low-carb diet also reduced lipoprotein(a), a risk factor for cardiovascular disease that previously wasn’t thought to be influenced by diet, by nearly 15 percent on average. By comparison, there was a 2 percent reduction with the moderate-carb diet and a slight increase of 0.2 percent with the high-carb diet.

“Surprisingly, the low-carb diet did not adversely affect LDL cholesterol, even though its saturated fat levels were far in excess of current recommendations,” notes Dr. Ludwig.

In a related, recently published commentary, Drs. Ludwig, Ebbeling, and their colleagues specifically implicate refined carbohydrates — rather than just excess calories — in fueling the obesity epidemic. Foods like white bread, white rice, most breakfast cereals, and highly processed snack foods cause spikes in blood sugar and insulin that increase hunger, may slow metabolism, and set the stage for weight gain. 

This study was supported by the Nutrition Science Initiative, New Balance Foundation, Many Voices Foundation, Blue Cross Blue Shield, and the National Institute of Diabetes and Digestive and Kidney Diseases.

Visit the New Balance Foundation Obesity Prevention Center

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