Despite a large body of evidence supporting carbohydrate-restricted diets as an effective method for short-term weight loss, the optimal carbohydrate intake for the most efficacious weight-loss intervention in adults who are overweight and obese remains unclear.
To evaluate the potential dose-dependent effects of carbohydrate restriction on body weight, researchers from Iran performed a systematic review and meta-analysis of 110 randomised controlled trials (RCTs).
The RCTs were published between 1981 and 2021, and conducted exclusively with overweight or obese participants with a BMI ranging from 25 to 44.6 kg/m2. Their mean age varied between 20 and 67.8 years.
A carbohydrate-restricted diet is defined as a diet with carbohydrate intake comprising 45% or less of total energy intake.
In the linear dose-response meta-analysis, each 10% decrease in carbohydrate intake reduced body weight by 0.64kg at the six-month follow-up, and by 1.15 kg at the 12-month follow-up.
Non-linear dose-response meta-analyses showed a monotonic reduction in body weight with the decrease in carbohydrate intake, with the greatest reduction at 5% at the six-month follow-up and 10% at the 12-month follow-up.
At follow-ups longer than 12 months, carbohydrate intake between 30% and 40% of calorie intake resulted in a significant reduction in body weight.
Notably, carbohydrate intake of lower than 30% and higher than 40% did not significantly reduce body weight at follow-ups beyond 12 months.
Although the effect of carbohydrate-restricted diets on weight loss did not surpass the threshold (5% or 4.39kg) set as minimal clinically important difference (MCID) in the main analysis, subgroup analyses found that the threshold was exceeded when the intervention was coupled with calorie-restriction and exercise programmes.
“Our findings indicated a proportional reduction in body weight with the decrease in carbohydrate intake in the short- and moderate-term. This suggests that very-low-carbohydrate (ketogenic) diets are the most effective among different types of carbohydrate-restricted diets for weight loss.
“This effect may be enhanced when carbohydrate restriction is accompanied by calorie restriction or an exercise programme,” the authors wrote.
Long-term effect still a challenge
The energy intake-to-expenditure ratio has a major impact on body weight and the subsequent risk of adiposity. Lifestyle modifications, such as dietary control, are often the first-choice option for obesity management.
Dietary strategies to lose weight have predominantly focused on calorie restriction by limiting the consumption of carbohydrates or fats.
Carbohydrate-restricted diets can aid in weight loss by reducing appetite via the production of ketone bodies, increasing energy expenditure and insulin sensitivity, and stimulating lipolysis.
Three of the most common carbohydrate-restricted diets are ketogenic diet (less than or 10% of total calorie, or 20g to 50g per day), low-carbohydrate diet (10% to 26% of total calorie, or 50g to 130g per day), and moderate-carbohydrate diet (26% to 45% of total calorie, or 130 to 230g per day).
“In line with our findings, most previous studies agree that in the short term, low-carbohydrate diets are more effective in losing weight, while in the long term, there is not much difference between weight-loss diets.
“The weakened effect of low-carbohydrate diets in the long run is mainly associated with reduced adherence. The craving for sweet treats, diminished pleasure of eating, and limited dietary choices make carbohydrate-restricted diets difficult to maintain,” said the authors.
To achieve weight loss in the short term — for example, if a patient needs to lose weight for surgery or if obesity is associated with serious complications — a diet with 5% to 10% carbohydrate intake could produce a greater effect.
It is also recommended to keep protein consumption at 20% to 25% of calorie intake.
“For long-term goals, optimum carbohydrate consumption could be between 30% and 40% of total calorie intake. Although there are concerns regarding low-carbohydrate diets that may be high in protein or fat, such as dysbiosis, gastrointestinal issues, hyperuricemia, renal impairment, osteoporosis, and increased blood cholesterol, there is no firm evidence indicating an elevated risk of nutrient deficiency.
“Based on our study, the adverse effects of low-carbohydrate diets were generally mild to moderate, including hair loss and muscle cramps.”
It should be noted that there are some limitations in this study, including large heterogeneity in the data, and the lack of examination on the effect of carbohydrate restriction on obesity grade and other anthropometric measurements.
“The dietary sources of both macronutrients and micronutrients may affect body weight, which was not considered in most of the included studies.
“Further trials with a longer duration of intervention are needed to assess the effect of dietary composition and eating behaviours, as well as the efficacy of low-carbohydrate diets, on body weight and other health outcomes.”
“Effect of carbohydrate restriction on body weight in overweight and obese adults: a systematic review and dose–response meta-analysis of 110 randomized controlled trials”
Authors: Sepideh Soltani, et al