Alterations in carbohydrate metabolism in response to short-term dietary carbohydrate restriction
Dietary carbohydrate restriction (CR) presents a challenge to glucose homeostasis. Despite the popularity of CR diets, little is known regarding the metabolic effects of CR. The purpose of this study was to examine changes in whole body carbohydrate oxidation, glucose availability, endogenous glucose production, and peripheral glucose uptake after dietary CR, without the confounding influence of a negative energy balance. Postabsorptive rates of glucose appearance in plasma (Ra; i.e., endogenous glucose production) and disappearance from plasma (Rd; i.e., glucose uptake) were measured using isotope dilution methods after a conventional diet [60% carbohydrate (CHO), 30% fat, and 10% protein; kcals = 1.3 × resting energy expenditure (REE)] and after 2 days and 7 days of CR (5% CHO, 60% fat, and 35% protein; kcals = 1.3 × REE) in eight subjects (means ± SE; 29 ± 4 yr; BMI 24 ± 1 kg/m2) during a 9-day hospital visit. Postabsorptive plasma glucose concentration was reduced ( P = 0.01) after 2 days but returned to prediet levels the next day and remained at euglycemic levels throughout the diet (5.1 ± 0.2, 4.3 ± 0.3, and 4.8 ± 0.4 mmol/l for prediet, 2 days and 7 days, respectively). Glucose Ra and glucose Rd were reduced to below prediet levels (9.8 ± 0.6 μmol·kg−1·min−1) after 2 days of CR (7.9 ± 0.3 μmol·kg−1·min−1) and remained suppressed after 7 days (8.3 ± 0.4 μmol·kg−1·min−1; both P < 0.001). A greater suppression in carbohydrate oxidation, compared with the reduction in glucose Rd, led to an increased (all P ≤ 0.05) rate of nonoxidative glucose disposal at 7 days (5.2 ± 0.5 μmol·kg−1·min−1), compared with 2 days (2.7 ± 0.5 μmol·kg−1·min−1) and prediet (1.6 ± 0.8 μmol·kg−1·min−1). In response to eucaloric CR, a marked increase in nonoxidative glucose disposal may help maintain systemic glucose availability.