Fluid and electrolyte losses associated with working/exercising for prolonged periods in thermally stressful environments

When working or exercising for prolonged periods in hot and humid environmental conditions, high sweat rates can lead to dehydration and electrolyte imbalance. It is well established that these progressive conditions will impair work and exercise performance in both athletes and manual workers. Consequences of inadequate fluid and electrolyte replacement in both cases can range from uncomfortable heat cramps, to more serious cases of heat illness, which in extreme cases can be fatal. Fluid loss accompanying work or exercise in the heat has received extensive focus in the literature over the last 20 years, however, the sodium losses associated with large sweat losses have received far less attention. Due to the well-established link between high salt intake and hypertension, the World Health Organisation recommends that on a global scale individuals consume less than 5g salt per day. While these recommendations may be appropriate for the general population, the athlete or manual worker has substantially greater sodium losses and therefore is hypothesised to require a higher sodium intake. One of the principle aims of this thesis is to document sweat sodium losses and provide appropriate guidelines for fluid and sodium replacement in both manual workers and endurance athletes. Chronic hyponatremia is a condition previously reported to occur in military personnel where over several weeks sodium intake is inadequate to replace large sweat losses. To date no previous study has investigated the incidence of chronic hyponatremia in a group of manual workers. Maintaining adequate fluid and electrolyte balance is essential to this population who are often working for prolonged periods in hot and humid environments. To investigate the incidence of chronic hyponatremia in this population, plasma sodium levels were assessed at the end of both the summer and winter months. Despite the similar dietary sodium intake across the year, the incidence of chronic hyponatremia at the end of the summer period was significantly greater than at the end of the winter period. This was postulated to be due to the higher sweat losses as a result of the more harsh environmental conditions during the summer months. Dietary sodium intake was found to be inadequate to replace the estimated sodium losses due to the high proportion of rice and legumes consumed with very low sodium content. Increasing the sodium content of fluid and food provided to workers is warranted and may reduce the incidence of work related illness and accidents in this population. In the athlete population there has been a great deal of focus in the literature on the need to adequately replace fluid losses that occur as a result of heavy sweating. Far less attention has been on the need to replace the sweat sodium losses accompanying this, with few studies documenting sodium losses in an endurance athlete population. There is large inter-individual variation in both sweat rate and sweat sodium loss in athletes with both factors being influenced by exercise intensity and duration, environmental conditions, physical fitness, size and number of individual sweat glands as well as the acclimatisation state of an athlete. It is therefore difficult to establish general recommendations for sodium intake in this population. In some athletes sodium losses can be replaced by normal dietary intake, whereas in other cases increased dietary intake is essential to replace large sweat sodium losses. In order to accurately determine sweat electrolyte losses, comprehensive laboratory based assessment is required, however this is often not practical. Chapter 4 and 5 of this thesis determined the effect of exercise intensity, seasonal heat acclimatisation and physical training status on fluid and electrolyte losses in a group of endurance athletes. The results from both of these studies predicts that endurance athletes with high sweat rates are at risk of both sodium and potassium deficiency, which may impair performance, if dietary intake is not sufficient to replace losses. Simple measures such as pre and post exercise weighing may be a useful tool in determining individual fluid and sodium intake requirements when comprehensive laboratory assessment is not practical. Based on the findings from these three studies, fluid and electrolyte replacement recommendations have been made for both manual workers and athletes working and exercising in thermally stressful environments.