Intermittent energy restriction (IER) diets have been found to be successful in improving metabolic disease markers(1) and affect postprandial glucose-lipid metabolism in the short term(2). Given that carbohydrate plays a key role in glucose homeostasis and substrate handling, and the restricted periods of IER are inherently low in carbohydrates, it is of interest to determine whether the metabolic alterations of energy restriction are elicited simply due to the reduced availability of carbohydrate. Therefore, this study aimed to investigate the short-term effects of carbohydrate restriction in comparison to an isocaloric energy balanced diet.
Five (one male) healthy (27.2±9.68y; 24.88±1.96kg/m2) subjects were recruited into this acute, two-way cross-over study. Subjects consumed two pre-prepared intervention diets for one day (36hr) separated by a 5-days washout period: normal carbohydrate (55% of energy) energy balanced diet (nEB), and low-carbohydrate (50g/day) energy balanced diet (LCEB). Glucose levels were measured during a 3-day trial period via fitted continuous glucose monitors (CGM). Following both 36 hours of LCEB and nEB blood metabolites were measured in the morning fasted state and serially across 360 minutes postprandially. At these same points, substrate utilisation (RQ) and energy expenditure were assessed by indirect calorimetry. Appetite responses were examined with ad libitum lunch on the study day and over a 3-day period. Data were analysed using repeated-measures ANOVA and Wilcoxon signed-ranks and presented as mean ± SEM.
CGM measurement during the day of intervention showed significantly lower levels of glucose during the LCEB leg (p<0.01). However, following 36 hours of a low-carbohydrate (LCEB) diet postprandial glucose response appeared impaired in LCEB, with a trend towards greater glycaemia (p= 0.064) and a non-significant delay in the glucose peak time (p=0.188). Fasting glucose levels were similar between the two diets (p>0.05). In contrast to those of glucose, postprandial triacylglycerol was lowered by 20% following the LCEB leg (p=0.006). Similarly, the LCEB led to reduced fasting triacylglycerol levels with a statistical trend (p=0.063). As anticipated hepatic production of postprandial 3-β-hydroxybutyrate (3-βOHB) was elevated in the LCEB leg (P=0.043), but fasting results showed no effect of diet on 3-βOHB (p>0.05). No significant differences in resting energy expenditure or fasted RQ were observed between legs, although carbohydrate restriction significantly lowered postprandial RQ (P=0.040).
This preliminary data suggests that carbohydrate restriction without energy restriction may mimic the acute metabolic response observed in intermittent energy restriction. Further studies are planned to improve the sample size and compare the acute effects of carbohydrate restriction with and without energy restriction.

Investigation to isolate the acute effects of carbohydrate restriction in energy matched diets on postprandial substrate metabolism: preliminary data from a cross-over dietary intervention.

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