Eating a low-carb diet based on meat? You may gain weight later on, study finds

By Sandee LaMotte, CNN

(CNN) — If you want your low-carb diet to help you lose weight and keep it off, consider the quality of the food you eat, according to new research that compared five types of low-carb diets.

People who ate an unhealthy, meat-based low-carb diet gained weight over time compared with those who followed a heathier, plant-based version, the study found.

“When people consume diets that emphasize carbohydrates from whole grains, healthy non-tropical vegetable oils, and plant proteins, they have a better chance of keeping excess weight gain at bay,” said the study’s senior author Dr. Qi Sun, associate professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health in Boston.

The study compared an overall low-carbohydrate diet to one that primarily used animal protein and fat; a second diet that focused on vegetable-sourced protein and fat; a healthy low-carb diet that focused on eating less refined carbohydrates, more plant protein, and healthy fats such as olive oil; and finally, an unhealthy meal plan defined as one that included unhealthy fats, more animal protein and refined grains.

“To my knowledge, examining the effects on lasting weight loss of different low-carb variants is novel,” said Dr. David Katz, a specialist in preventive and lifestyle medicine who founded the nonprofit True Health Initiative, a global coalition of experts dedicated to evidence-based lifestyle medicine. He was not involved in the research.

All of the diets reduced carbohydrates to about 38% to 40% of daily calorie intake. However, people who ate an unhealthy carb diet full of animal protein and fat gained weight long-term compared to people who focused on higher intakes of fruit, whole grains and non-starchy vegetables and lower intakes of dairy, red and processed meat, sugar-sweetened beverages, sweets and desserts.

Those who adopted unhealthy low-carb diets as a primary strategy gained, on average, roughly 2.3 kg, or 5.1 lbs., over 4 years,” said first author Binkai Liu,  a research assistant in the department of nutrition at Harvard T.H. Chan School of Public Health.

“Those who adopted healthy low-carb diets as a primary strategy lost, on average, roughly 2.2 kg, or 4.9 lbs., for a mean, net difference between the two of 10 lbs.,” she said via email.

Large, high-quality studies

The study, published Wednesday in the journal JAMA Network Open, examined data on over 67,000 people who participated in three well-established longitudinal studies: the Nurses’ Health Study conducted between 1986 and 2010, the Nurses’ Health Study II, conducted between 1991 and 2015, and the Health Professionals Follow-up Study, conducted between 1986 and 2018.

“These are, of course, observational studies, and not designed to establish cause-and-effect; rather, they reveal associations,” Katz said. “However, when observed associations are potent, dose-responsive, hard to explain away and tethered to plausible mechanisms — cause-and-effect may at times be inferred.”

All participants in the three studies were healthy, under the age of 65 and had no preexisting chronic conditions. Weight loss or gain was self-reported at four-year intervals.

Low-carb diets that emphasized “high-quality macronutrients from healthy plant-based foods” were associated with less weight gain, the study noted, while low-carb diets that “emphasized animal-sourced proteins and fats or refined carbohydrates were associated with more weight gain.” The associations were more apparent among people who were younger, heavier and less active, the study found.

“The bottom line was that over a span of 4 years, simply adopting a ‘low carb’ diet in general was NOT associated with lasting weight loss among those attempting to lose weight, WHEREAS adopting a plant-based and/or high-quality low-carb diet WAS associated with sustained weight loss,” Katz said in an email.

While the study focused on low-carb eating, the importance of food quality is key in any diet, said Sun, who is also director of the Nutritional Biomarker Laboratory at Harvard T.H. Chan School of Public Health.

“It is always wise to choose a diet (that) emphasizes fresh fruits and non-starchy vegetables, whole grains, nuts, legumes, olive oil and other vegetable oils, coffee, tea or just water, modest red wine if drinking, low sodium, and other healthy ingredients,” Sun said.

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