Low-carbohydrate vs. balanced-carbohydrate diets for reducing weight and cardiovascular risk Original paper

This meta-analysis found little to no difference between low-carbohydrate and balanced-carbohydrate diets for weight loss in the short term (3–8.5 months) and long term (1–2 years). The results did not differ between overweight or obese individuals with and without type 2 diabetes.

This Study Summary was published on March 2, 2022.

Background

The 2017 Global Nutrition Report estimated that two billion adults worldwide are overweight or obese. For the first time in history, more people are dying from the consequences of overeating than starvation and malnutrition.[1] Interventions to treat obesity include pharmacotherapy, bariatric surgery, and counseling that target diet, physical activity, and behavior change, the latter of which is the first-line treatment.

There is no “one-size-fits-all” weight-reducing diet; different diets work for different people based on preferences and ease of adherence. Nevertheless, debate continues over whether a low-carbohydrate diet or high-carbohydrate tends to be more effective for weight loss, on average. It’s suggested that certain macronutrients may have a metabolic advantage over others — more specifically, a lower carbohydrate intake is more effective for weight loss, independent of energy intake.[2] The supposed reason is that a low-carbohydrate diet reduces insulin secretion and causes a greater release of free fatty acids from adipose tissue and increased energy expenditure.[3]

The study

This meta-analysis of 61 randomized controlled trials published between 1978 and 2021 compared the effects of low-carbohydrate weight-reducing diets with balanced-carbohydrate (45–65% of total energy intake from carbohydrates) weight-reducing diets on body weight and cardiovascular risk factors in 6,925 participants with overweight or obesity. Studies with an active weight-reducing intervention phase (i.e., diets implemented for the primary purpose of reducing body weight, with or without explicit advice to restrict total energy intake) of at least 12 weeks were included. Most trials investigated low-carbohydrate diets (51–150 grams of carbohydrates per day or < 45% of total energy intake from carbohydrates; 42 trials) and very-low carbohydrate diets (≤ 50 grams of carbs per day or < 10% of total energy intake from carbohydrates; 14 trials).

The primary outcome was the short-term (3 months to < 12 months) and long-term (1 to 2 years) changes in body weight (in kilograms; kg) from baseline. The secondary outcomes were the changes in diastolic blood pressure, systolic blood pressure, HbA1c, serum LDL-C, serum HDL-C, serum non-HDL cholesterol, serum total cholesterol, and serum triglycerides in the long term (≥ 12 months), as well as participant-reported adverse effects (e.g., weakness, headaches, constipation, diarrhea, mood disturbances).

The analyses were stratified into studies conducted in participants without type 2 diabetes (T2D; 45 trials) and studies conducted in participants with T2D (14 trials). The remaining 2 trials included participants with and without T2D.

The results

In participants without T2D, compared with the balanced-carbohydrate diet group, there was greater weight loss (−1.07 kg) in the low-carbohydrate diet group over 3 to 8.5 months. Similarly, there was greater weight loss (−0.93 kg) in the low-carbohydrate diet group over 1 to 2 years. For the secondary outcomes, there was a meager increase in HDL-C (+0.06 mmol/L) and a decrease in triglycerides (−0.11 mmol/L) in the low-carbohydrate diet group compared with the balanced-carbohydrate diet group. In the subgroup analysis, weight loss was greater with a very-low carbohydrate diet (−2.29 kg) compared with a balanced-carbohydrate diet over 3 to <12 months.

In participants with T2D, compared with the balanced-carbohydrate diet group, there was greater weight loss (−1.26 kg) in the low-carbohydrate group over 3 to 6 months. However, at 1 to 2 years, there was no difference in weight loss between groups. With respect to secondary outcomes, there was a slight decrease in total cholesterol in the balanced-carbohydrate diet group (−0.21 mmol/L) compared with the low-carbohydrate diet group.

Note

Most weight reduction trials are constrained by small samples, a lack of blinding, and a large loss to follow-up, which were observed across the included trials. None of the included trials were judged as having a low risk of bias overall.

When interpreting the results for weight loss, one should consider that the reported differences between groups were similar to the range typically reported for biological weight fluctuations, which are influenced by activity level, season, hydration status, and medications. Moreover, it's important to consider the total body water loss (2–3 kg) that typically follows dietary carbohydrate restriction due to diet-induced diuresis from glycogen depletion.[4][5]

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This Study Summary was published on March 2, 2022.

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