THE EFFECT OF A LOW-CARBOHYDRATE DIET ON PERFORMANCE, HORMONAL AND METABOLIC RESPONSES TO A 30-S BOUT OF SUPRAMAXIMAL EXERCISE

The aim of this study was to find our whether a low-carbohydrate diet (L-CHO) affects. (1) the capacity for all-out anaerobic exercise, and (2) hormonal and metabolic responses to this type of exercise. To this purpose. eight healthy subjects underwent a 30-s bicycle Wingate test preceded by either 3 days of a controlled mixed diet (130 kJ:kg of bo dy mass daily, 50% carbohydrate, 30% fat, 20% protein) or 3 days of an isoenergetic L-CHO diet (up to 5% carbohydrate. 50% fat, 45% protein) in a randomized order. Before and during 1 h after the exercise venou s blood samples were taken for measurement of blood lactate (LA). beta -hydroxybutyrate (beta-HB), glucose, adrenaline (A), noradrenaline (NA ) and insulin levels. Oxygen consumption(VO2) was also determined. It was found that the L-CHO diet diminished the mean power output during the 30-s exercise bout [533 (7) W vs 581 (7) W, P < 0.05] without chan ging the maximal power attained during the first or second 5-s interva l of the exercise. In comparison with the data obtained after the cons umption of a mixed diet, after the consumption of a L-CHO diet resting plasma concentrations of beta-HB [2.38 (0.18) vs 0.23 (0.01) mmol.l(- 1), P < 0.001] and NA [4.51 (0.68) vs 2.2 (0.31) nmol.l(-1), P < 0.05] were higher, while glucose (4.6 (0.1) vs 5.7 (0.2) mmol.l(-1), P < 0. 05] and insulin concentrations [11.9 (0.9) vs 21.8 (1.8) mU.l(-1)] wer e lower. The l-h post-exercise excess of VO2 [9.1 (0.25) vs 10.6 (0.25 ) 1. P < 0.05], and blood LA measured 3 min after the exercise [9.5 (0 .4) vs 10.6 (0.5) mmol.l(-1). P < 0.05] were lower following the L-CHO treatment, whilst plasma NA and A concentrations reached higher value s [2.24 (0.40) vs 1.21 (0.13) nmol.l(-1) and 14.30 (1.41) vs 8.20 (1.3 1) nmol.l(-1), P < 0.01. respectively], In subjects on the L-CHO diet, the plasma beta-HB concentration decreased quickly after exercise, at taining approximate to 30% of the pre-exercise value within 60 min, wh ile insulin and glucose levels were elevated. The main conclusions of this study are: (1) a L-CHO diet is detrimental to anaerobic work capa city, possibly because of a reduced muscle glycogens store and decreas ed rate of glycolysis; (2) reduced carbohydrate intake for 3 days enha nces activity of the sympathoadrenal system at rest and after exercise .