When the heat is on, rehydration is paramount.

But, what fluids are best to drink - and when? How much is enough?

Fluid loss and replacement in the human body involves complex mechanisms, which makes measuring fluid loss problematic.

Urine colour charts are a great way to give individuals an idea of how hydrated they are. However, they are a guide only and some vitamins and supplements can affect the colour of urine over the course of a few hours. No method is completely accurate and some are quite complex, but the use of urine specific gravity (SG) tests to assess hydration levels is now popular.

Water vs sports drinks

What should I drink to replace fluid lost through sweating?
Cool water.

The modern diet is already heavy on salt. For anyone acclimatised to working in heat, the human hormone aldosterone reduces the amount of electrolytes lost in our sweat. So, for light to moderate work, water is usually enough.

However, if you plan to work hard (or play intensive sport) in high temperatures, look for/use an electrolyte-replacement drink before you start, midway through, and afterwards. In between, drink cool water.

Another plus for these drinks is their taste: They encourage workers/players to consume enough fluid to maintain hydration levels. Rehydration icy poles are also now available, however be mindful of your calorie consumption of sports drinks and icy poles as they can contain added sugar.

The caffeine question

Does the caffeine in coffee, tea, some soft drinks, and energy drinks have a diuretic effect, accelerating fluid loss and dehydration, hindering rehydration?

Several studies have shown that a moderate caffeine intake neither alters fluid-electrolyte levels during exercise, nor compromises our ability to perform effectively in heat.

In fact, drinks containing moderate levels of caffeine can add to overall fluid uptake.

But, caffeine-containing drinks shouldn’t be used as the sole replacement fluid. And, too much caffeine can trigger nervousness, insomnia, gastro problems, tremors, and tachycardia (faster-than-normal resting heart rate).

For further reading on the right fluids at the right times visit www.thethermalenvironment.com/what-is-the-impact-of-caffeine-in-relation-to-heat-stress/

Heat and alcohol – a heady mix

Rather than providing hydration, alcohol dehydrates - rapidly. A 200ml glass of beer could result in you passing 300ml of fluid, as well as losing electrolytes.

Morning-after hangovers are due in part to dehydration. Being severely dehydrated before starting work dramatically increases the chances of a heat-related illness or accident on the job.

If you plan to drink at a social occasion:

• Hydrate well before your first alcoholic drink.
• Drink a glass or two of water during the event.
• Before bed, drink more water or an electrolyte-replacement drink (to help restore your electrolyte balance).

The take-home message is: Water is usually the best choice for rehydration. Drink until you’re no longer thirsty, then drink a little more – just to be sure.