Article by Gill Cummings-Bell BA (hons) M.Sc MBA – Drummond Clinic/Education 

With this exceptional spell of hot weather, which we hope will last another couple of months, and with the holiday season coming up, it is important to consider training and recovery in the heat.

It is an everyday experience that exercise feels harder when the temperature is high and there is a corresponding reduction in exercise performance. This observation has been confirmed many times by well-controlled studies.

Given the high rates of heat production sustained whilst exercising in the heat, it is common to experience resulting declines in performance. As soon as the environmental temperature exceeds the skin temperature, it results in a more rapid rise in core temperature. This means that there can be a faster rate of sweat evaporation to increase the rate of heat loss. In turn, a faster sweat evaporation rate requires a faster sweat secretion rate. This requires in turn a high skin blood flow, diverting blood from the working muscles and/or increasing cardiac output that must be achieved.  This can lead to dehydration and also cause cardiac drift, suddenly increasing the heart rate by up to 20 bpm. This is often referred to as heat stress.

Performance of both physical and mental tasks is significantly reduced by heat and dehydration. The cardiovascular system, central nervous system (CNS) and thermoregulatory systems, are particularly stressed if exercising in excess of two hours in temperatures higher than the body temperature.  Provision of fluids can prevent dehydration and greatly reduce the adverse effects of heat stress however the high rate of loss can sometimes mean that no matter how much you replace your fluid its still not enough.  Its like trying to fill a bucket with hole in it. There is growing evidence that the effects of high ambient temperature and dehydration on exercise performance may be mediated by effects on the CNS. Hyperthermia (not Hypo) results from dehydration and can cause changes in the brain’s electrical activity, this in turn can cause a marked reduction in the ability to maintain voluntary contractions, and an increase in perceived exertion.

If you’re an athlete training in the heat, be that a recreational athlete, club or an elite recovery between your training sessions is important.  Many methods can be used in the heat including cold water immersion, stretching, compression and massage.  These are all components that can assist you, but what you eat and drink before, during and following exercise will have a significant effect on how you will perform in the next training session. For the purpose of this article we will focus on re-hydration after exercise or training.

There are three areas to consider that impact onto post training re-hydration:

  • Refuelling of muscle glycogen stores
  • Replacement of fluid and electrolytes lost in sweat
  • Regeneration and repair of tissue catabolism

Bottom line in the heat is replacing the fluid loss as this will be greater during the heat.

Refuelling Muscle Glycogen Without Affecting Re-hydration

The highest rate of replenishment of muscle glycogen stores occurs during the first hour after exercise.  If you don’t consume carbohydrate in this immediate phase of post-exercise recovery it will lead to low rates of glycogen restoration for the next training session. A lot of athletes to do this ingest high carbohydrate drinks immediately post training ‘hypertonic’ in nature. The challenge here is that this can often add to symptoms of dehydration as the body will have to extract water from the blood into the stomach to dilute a hypertonic drink in order for it to pass through the stomach wall.

Through the heat it would be better to focus on hydration fluid rather than carbohydrate replenishment fluid and consider ingesting the carbohydrate in this window in a solid form ensuring that your drink is hypo-tonic in nature (less than 6% carbohydrate) and contains electrolyte replacements for fast hydration.

Post Exercise Rehydration

Under normal circumstances fluid loss and maintenance of balance are well regulated by thirst and urine losses.  However under conditions of stress thirst is not a sufficient stimulus. Typically even on a normal training day athletes commonly under re-hydrate, only 30-70 percent of their sweat losses are replaced.  This can be much more serious in the hot weather where sweat and electrolyte loss is much great.

To fully replace fluid lost during a training session you need to consume 150 percent of the fluid loss.  If this loss is only replaced with water (which is low in sodium) and not a higher level electrolyte choice, it can increase urine loss adding to dehydration.

Sodium is the important factor.  This is the principle electrolyte lost in sweat, particularly during long bouts of heavy sweating.

Maughan and Leiper (1995) dehydrated subjects by 2 per cent body mass via exercise in a hot environment, then observed them for six hours of recovery after they had consumed 150 per cent of their fluid losses with test drinks providing varying levels of sodium. The research showed greater urine losses when subjects consumed the low sodium drinks (2 mmol/L and 26 mmol/L sodium) compared with higher sodium drinks (52 and 100 mmol/L sodium).

The athletes in this study were found to be in fluid balance by the end of the recovery period when they consumed the two higher sodium beverages, but were still in net negative fluid balance on the lower sodium trials, despite an intake of fluid that was 1.5 times their estimated sweat losses. Overall retention of the ingested fluid was related to the sodium content, however there was no difference in net fluid balance between the two higher sodium fluids.

Common sports drinks typically contain 10–25 mmol/L, therefore based on recent studies additional sodium may need be consumed in order to optimise post-exercise fluid retention. Don’t attempt to do this by just adding salt to the fluid, think of the available brands such as Nuun, High five, SIS or Dioralyte to name a few that will assist with a calculated replacement strategy.  Sports drinks typically have been balanced to taste and absorption so if this is altered you may interfere with its absorption rate.

Sodium-containing foods post-exercise can be a more practical solution. Studies by Maughan et al. (1996) and Ray et al. (1998) have shown that the intake of salt via everyday food choices enhances the retention of fluid consumed to rehydrate after exercise-induced dehydration.

A high rate of fluid consumption during the first two hours of post-exercise is key. Individuals looking to achieve rapid and complete recovery from dehydration should drink 1.5L of fluid for each kilogram of body weight loss (Sawka & Burke, 2007).

Post-exercise hydration should aim to correct any fluid loss accumulated during the practice or event. Rehydration should include water, which contains electrolytes to restore hydration status and only a small amount of carbohydrates to help replenish glycogen stores. The primary goal is the immediate return of physiologic function, especially if you are training again the next day.

If you would like to know more on this subject or about how the Drummond Clinic can help you or someone you know, please feel free to give us a call, jump on our website or drop us an email.

Wishing you all happy, enjoyable and safe training.