Carbohydrate in the mouth immediately facilitates motor output

Abstract

The presence of carbohydrate in the mouth can immediately improve physical performance. How this occurs is not well understood. Here we used transcranial magnetic stimulation of primary motor cortex (M1) to investigate the effects of non-sweet carbohydrate on corticomotor excitability and voluntary force production. In Experiment 1, 16 participants performed a fatiguing isometric elbow flexion exercise for 30 min, and Motor evoked potentials (MEPs) were recorded from the biceps brachii during maximal voluntary force (MVF) produced every 2 min. After 11 min participants drank a carbohydrate solution (CHO) or an energy-free placebo solution (PLA), in a double-blind, cross-over protocol. MEP amplitude increased by 30%, and MVF increased by 2%, immediately after carbohydrate ingestion. There was no relationship between the facilitation of MEP amplitude and plasma glucose or magnitude of fatigue. In a control experiment, 17 participants alternately mouth-rinsed CHO and PLA, in a randomized, double-blind protocol. MEPs were recorded from right first dorsal interosseous at rest or during isometric contraction. MEP amplitude increased by 9% with CHO, when the muscle was voluntarily activated. In both experiments, there were no effects on silent period duration, indicating that MEP facilitation was not due to reduced inhibition within M1. This is the first demonstration that carbohydrate in the mouth immediately increases the excitability of the corticomotor pathway, prior to its ingestion. Afference from oral receptors is integrated with descending motor output, perhaps via nuclei in the brainstem. This novel form of sensorimotor integration facilitates corticomotor output to both fatigued and fresh muscle.