Effects of fluid ingestion on cognitive function after heat stress or exercise-induced dehydration

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Abstract

This study investigated the effects of heat exposure, exercise-induced dehydration and fluid ingestion on cognitive performance. Seven healthy men, unacclimatized to heat, were kept euhydrated or were dehydrated by controlled passive exposure to heat (H, two sessions) or by treadmill exercise (E, two sessions) up to a weight loss of 2.8%. On completion of a 1-h recovery period, the subjects drank a solution containing 50 g l−1 glucose and 1.34 g l−1 NaCl in a volume of water corresponding to 100% of his body weight loss induced by dehydration. (H1 and E1) or levels of fluid deficit were maintained (H0, E0). In the E0, H0 and control conditions, the subject drank a solution containing the same quantity of glucose diluted in 100 ml of water. Psychological tests were administered 30 min after the dehydration phase and 2 h after fluid ingestion. Both dehydration conditions impaired cognitive abilities (i.e. perceptive discrimination, short-term memory), as well as subjective estimates of fatigue, without any relevant differences between the methods. By 3.5 h after fluid deficit, dehydration (H0 and E0) no longer had any adverse effect, although the subjects felt increasingly tired. Thus, there was no beneficial effect of fluid ingestion (H1 and E1) on the cognitive variables. However, long-term memory retrieval was impaired in both control and dehydration situations, whereas there was no decrement in performance in the fluid ingestion condition (H1, E1).

Introduction

Studies have shown that the response to dehydration is a drop in performance levels for various fundamental cognitive abilities involving short-term memory, working memory or visuo-motor abilities (Sharma et al., 1986, Gopinathan et al., 1988, Cian et al., 2000). The reduction in performance is proportionate to the degree of dehydration (Gopinathan et al., 1988) and becomes significant with a 2% body weight loss (Sharma et al., 1986, Gopinathan et al., 1988). Moreover, dehydration via heat stress or via exercise have a similar detrimental effect on cognitive performance (Cian et al., 2000). Some of the effects observed can be rapidly reversed, although the subjects feel increasingly tired (2 h after dehydration). At the same time, decrement in long-term memory is greater in the exercise-induced than in the euhydration or heat stress dehydration conditions (Cian et al., 2000). In these former reports, however, performance was measured following an additional physical exercise, so that it is not clear whether decrements in performance are the consequence of the dehydration method or of differences in sensitivity to physical workload. To address this question, the present experiment was undertaken to replicate the effect of dehydration method — heat stress vs. exercise — on cognitive function effect 30 min and 3 h after dehydration. The effects of fluid replacement were also addressed.

Section snippets

Method

Seven healthy men, unacclimatized to heat, participated in the study. They gave their written informed consent in accordance with the ethics standards of the 1964 Declaration of Helsinki. The study was approved by the Committee on Human Protection in Biomedical Research in Grenoble, France. All the participants were endurance-trained (usual training: four 1-h running sessions per week). Their mean (S.D.) age was 25 (3.68) years, body mass 74 (4.3) kg, and height 1.78 (0.04) cm. Physical fitness

Levels of hydration and physiological measures

The dehydration levels reached are shown in Table 1. There was no variation in the weight loss attained by the subjects between the dehydration trials [F(3,18)=2.2, P=0.12]. The same degree of body mass loss occurred over the four trials of dehydration. Thus, there were no differences among rehydration trials (E1 and H1) in the volume of fluid ingested (1801±267 ml). From 2 h after the fluid intake period, body mass did not return to the baseline level (pre-dehydration body mass) in H1 and E1

Discussion

This experiment was designed to study the effect of heat stress and exercise-induced dehydration and fluid ingestion on mental performance. Consistent with other studies, heat stress and exercise had a detrimental effect on cognition, with no noticeable difference between the two dehydration methods. Moreover, it is important to emphasize the homogeneity of the measurements, as, following dehydration, there was no repeated measure effect. Reproducible responses between identical trials were a

Acknowledgements

This study was supported in part by the S.A. des Eaux Minérales d'Evian. We would like to thank the ‘Unité de Thermophysiologie’ (G. Savourey) and the ‘Service de laboratoire médical’ (S. Martin) of the CRSSA, N. Koulmann, C. Jimenez and X.A. Bigard for their assistance.

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