The participants ate a low-carbohydrate diet for 3 to 24 months, which provided 50 to 130 grams of carbohydrates per day or 10% to 40% of the total energy intake. The control groups followed either their usual diet or another diet that was not low in carbohydrates.

Compared to control diets, low-carbohydrate diets improved the following markers of metabolic syndrome:

BMI decreased by 0.43.
Waist circumference decreased by 0.77 centimeters (0.3 inches).
Systolic and diastolic blood pressure decreased by 1.2mmHg and 1.5 mmHg, respectively.
HbA1c decreased by 0.62%.
Triglycerides decreased by 0.24 mmol/L.
HDL-C increased by 0.06 mmol/L.

The included studies were mostly of high quality; 25 trials had a low risk of bias, 5 had some concerns, and 11 had a high risk of bias.

Can low-carbohydrate diets improve metabolic syndrome? Original paper