Those who have resolved to start eating better in the new year may have considered a low-carb diet. Some evidence suggests that low-carbohydrate diets can improve metabolic health and help people lose weight more quickly than low-fat eating plans. However, there is little research on the effects of low-carbohydrate diets on long-term weight maintenance.

In a new study, researchers at the Harvard T.H. Chan School of Public Health compared five low-carbohydrate diets, each emphasizing different macronutrient composition and the quality of those macronutrients, to see which diet is better for long-term weight changes.

"Our study goes beyond the simple question of 'carbohydrates or not,'" said Binkai Liu, the study's first author. "It dissects low-carbohydrate diets and provides a nuanced look at how the components of these diets affect health over years, not just weeks or months."

Researchers used data from three large, ongoing prospective U.S. studies, the Nurses' Health Study (NHS), Nurses' Health Study II (NHHSII), and Health Professionals Follow-up Study (HPFS), to analyze the diet and weight of 123,332 healthy adults between 1986 and 2018. The average age of participants was 45 years old, and 83.8% were female. Each participant provided self-reports of diet and weight every four years.

Researchers scored participants' diets based on their adherence to five categories of low-carbohydrate diets (LCD): Total Low-Carbohydrate Diet (TLCD), which emphasizes an overall reduction in carbohydrate intake; Animal-based Low-Carbohydrate Diet (ALCD), which emphasizes animal protein and fat; Plant-based Low-Carbohydrate Diet (V LCD), which emphasizes plant-based proteins and fats; a Healthy Low-Carb Diet (HLCD), which emphasizes plant-based proteins, healthy fats, and fewer refined carbohydrates; and an Unhealthy Low-Carbohydrate Diet (ULCD), which emphasizes animal-based proteins, unhealthy fats, and carbohydrates from unhealthy sources such as processed breads and grains.

The researchers designed an LCD score ranging from 0 to 30, with higher scores indicating greater adherence to a specific diet. The primary outcome of the study was four-year score change, calculated by subtracting the score at the end of each four-year period from the score at the beginning of each four-year period. They did the same for participants' weight changes.

The researchers found that the results of the three groups of studies were basically consistent: increases in TLCD, ALCD, and ULCD scores were all associated with weight gain, while increases in HLCD scores were associated with less weight gain. These associations were strongest among participants who were under 55 years old, overweight or obese, and/or less physically active. The VLCD results of different groups are not consistent. In NHSII, greater adherence to VLCD was significantly associated with less weight gain, whereas in NHS and HPFS this association was less clear.

These findings suggest that the quality of a low-carbohydrate diet may play a key role in long-term weight change. Only diets that emphasized high-quality protein, fat, and carbohydrates from whole grains and other plant foods were associated with less weight gain.

"The key takeaway here is that not all low-carbohydrate diets are created equal when it comes to long-term weight control," said Sun Qi, one of the study's co-authors. "Our findings may upend our thinking about popular low-carbohydrate diets and suggest that public health initiatives should continue to promote eating patterns that emphasize healthy foods such as whole grains, fruits, vegetables, and low-fat dairy products."

The researchers noted several strengths of their study. That is, they created five versions of the low-carb diet based on the quality and quantity of macronutrients, which provided a broader scope for studying the association between eating plans and weight changes. At the same time, the large sample size and long-term follow-up allowed them to explore this association with relatively large statistical power. However, the self-reported nature of the data is prone to measurement error. And the study didn't measure body composition, so the researchers couldn't determine the effects of diet on lean body mass and body fat. There was a larger proportion of white participants, which may limit the generalizability of the study.

However, this study highlights the importance of a low-carbohydrate diet for weight management. The researchers suggest that future studies should validate these findings in more diverse populations and explore the underlying mechanisms of these associations.

The study was published in JAMA Network Open.