Hydrating in the Heat

In October we attended the Sports Dietitian Australia Conference in Melbourne. This year it was all about training in the heat, hydration and sodium. On the back of this (and in light of the weather heating up!), we have put an article together about taking care of your hydration in the heat.

What happens when you exercise in the heat?

Exercising in the heat puts a strain on the ‘thermoregulatory’ processes that allow your body to stay at its happy temperature, as well as cause physical strain on the body.

  1. Your blood travels to your skin and muscles to cool you down and fuel the work.
  2. Your aerobic capacity (oxygen using) is reduced and things just feel harder.
  3. We rely more on our anaerobic system (no oxygen) – but this uses more carbohydrate and we may fatigue more quickly.
  4. Your core temperature rises and you have a harder time cooling your body.
  5. Your sweat rate increases… putting you at risk of dehydration.

Dehydration – and why you want to avoid it!

Dehydration is one of the most common nutrition issues in sport. It can occur at any time, but the risk goes up when exercising in hot conditions due to an elevated sweat rate.

Dehydration can cause:

  • Impaired skill, concentration and decision making.
  • Increased perceived exhaustion = exercise feels harder.
  • Increased risk of heat illness – due to difficulty regulating your body temp.
  • Increased risk of GI symptoms, including nausea, vomiting and diarrhoea.

It is generally accepted that a 2-3% body weight loss (of fluid) will result in a measurable performance deficit via dehydration. This means if you are 70kg and lose 1.4kg of fluid while exercising, you are at risk of your performance taking a hit.

Your sweat rate.

Sweat rate (= how much you sweat) can vary dramatically based on factors such as the environment and fitness level. For vigorous activity in a hot environment 1-1.5L/sweat/hour is pretty standard, but some people can exceed well above 2.5L/sweat/hour, and well trained people sweat more.

Are you a heavy sweater? Find out…

  1. Weigh yourself in minimal clothing pre-exercise.
  2. Complete your activity or event (record any fluid you consume, as well as toilet trips).
  3. Weigh yourself post-exercise, minimal clothing and towel dried to remove any sweat.
  4. Take that final number – and deduct any fluid (L) consumed during exercise, and add on any toilet trips (assume 0.3L per trip if not measured).
  5. Find the difference between your starting weight and final number.
  6. Divide the difference by the number of hours in the session to find your hourly sweat rate.

Know that the rate found is only relevant to the environmental conditions at the time, and the intensity and duration of session – so testing in multiple conditions at different intensities is recommended to get a good idea of your sweat rate.

How to stay hydrated!

We have all heard we need to drink 8 glasses of water per day – but is this true? Not exactly – there is no solid evidence to confirm 8 glasses a day is the magic number for everyone. But.. it’s probably not far off for most and it’s pretty easy to remember!

When looking at fluid for sport and exercise it can get a little more complex, and there are 3 key areas think about – before, during and post event/activity/session.

Before: Start well hydrated by monitoring your urine – see our handy chart here. Aim to keep on top of your hydration in the days before an activity. Pre-event guidelines recommend anywhere from 5-10ml/kg the evening before and/or 2h pre-event. You may also like to ‘prime’ with a bolus of 300-400ml pre-event to boost fluid delivery from the stomach to the intestine for absorption.

Hyper-hydration (‘over-hydration’) strategies are worth discussing with a dietitian if you think you might benefit from these.

During: Drinking to thirst is a strategy that will work for most – but perhaps not in situations in which severe dehydration is expected. Aim for ~200-250ml every 15 minutes in moderate-intense exercise – but know that this is variable. You likely won’t replace 100% of your losses, but instead minimise the deficit (keeping it <2% body weight). You do not want to gain weight during exercise – this indicates you are over-drinking and should pull back.

Post: Rehydration is key to optimise recovery – especially if you have another session coming up. Your sweat rate comes in here… aim to drink 125-150% of what you lost over the next 2-6 hours.

Do I need a sports drink?

A sports drink contains a mix of fluid, carbohydrate and sodium. The reason it contains sodium is to drive thirst, promote fluid retention and potentially replace some sodium lost via sweat in heavy sweaters or prolonged sessions.

Generally, water is going to be the best option for most active people and recreational athletes – even for exercise up to ~2.5 hours – but there are some situations in which sports drink might be beneficial. If you have a high load, long session, hot session, are a heavy sweater or a short rehydration period until your next session (8h) a sports drink may help in terms of facilitating rehydration more quickly. They might also benefit people working long hours out in the heat. There’s also some great evidence for the re-hydrating properties of milk drinks – which contain a perfect mix of carbohydrate, protein, fluid and electrolytes!

In exercise more than 2.5 hours, sodium replacement is more critical given the risk of hyponatreamia – a condition in which blood sodium is diluted, causing unpleasant symptoms and, while uncommon, has resulted in the athlete deaths. Generally, this occurs a result of excessive drinking, not inadequate sodium – but sodium replacement may be worthwhile nonetheless. Sports drinks may play a role in replacing some of this lost sodium – but are usually lower in sodium than may be required so they actually taste good.

We rarely drink in isolation though – and meals & snacks consumed pre, during and post- exercise contribute to our sodium intake substantially. Sodium loading techniques and supplements might also be utilised when competing in a hot environment for a prolonged time.

We hope this gives you a few tips and tricks you can take on board to hydrate right! If you need more personalised advice, check in with a sports dietitian.