Isolating the acute metabolic effects of carbohydrate restriction on postprandial metabolism with or without energy restriction: a crossover study

Low-carbohydrate diets and intermittent energy restriction may offer metabolic advantages in fuel utilisation, that are independent of weight loss. The underlying mechanisms for these effects are unclear but may involve extensions of the catabolic phase and/or attenuation of insulin secretion. To address this gap, we aimed to investigate the independent acute metabolic effect of carbohydrate restriction at varying energy levels. Twelve, (six female) healthy overweight/obese participants (27.3 ± 1.8 years; 25.2 ± 1.6 kg/m²) completed this three-way study. Volunteers followed three diets for one day (36 h, covering the intervention day and overnight fasting), separated by 5-day washout: a normal carbohydrate, energy-balanced diet (nEB, 55% CHO), a low-carbohydrate, energy-balanced diet (LCEB, 50 g/day CHO), and a low-carbohydrate, energy-restricted diet (LC25, 50 g/day CHO with 75% energy restriction). Fasting and serial postprandial (360 min) measurements to a mixed test meal were collected the following morning. Additionally, subjective appetite responses and two-day subsequent ad libitum food intake was assessed. Both low-carbohydrate with and without energy restriction diets induced comparable decrease in triacylglycerol iAUC (p = 0.02, p = 0.04, respectively), and respiratory quotient (both p < 0.01) along with increase in non-esterified fatty acids (both p < 0.01) and 3-hydroxybutyrate (p = 0.001, p = 0.01, respectively) levels. Compared to a non-restricted carbohydrate, energy-balanced diet, postprandial glucose levels significantly increased in the LCEB arm (p = 0.024) and showed a rising trend in the LC25 arm (p = 0.07). Neither insulin responses nor resting, and diet-induced thermogenesis were significantly altered by variations in energy or carbohydrate content. These findings demonstrate that carbohydrate restriction, without altering energy intake, can elicit effects similar to those observed in short-term fasting. As such we propose a strategy of repeated carbohydrate restriction cycles alone may be an emerging alternative approach for the enhancement of cardiometabolic health, warranting further investigation.