All athletes need to pay attention to hydration. And if you’re training in our tropical climate (where many of us break into a sweat just walking out to get lunch) you need to be especially careful about managing your your fluid intake. So what form of hydration do you need when exercising?

1. Staying Hydrated – The Fluid Balance

About 70% of the human body is made up of fluid, and just 5% loss of fluids can prevent our body from functioning normally. The right fluid balance is vital for maintaining optimal energy and alertness levels.

Even mild dehydration can lead to dizziness, moodiness and headaches; whilst severe dehydration can be life-threatening.

2. How Often to Drink

We need the right amount of fluid for our activities. Someone who spends much of the day working indoors on a computer would not need to drink as often as, say, a construction worker.

High-performance athletes need to go a step further, and calibrate their fluid intake according to activity levels during their race or workout. A common mistake is to do most of the drinking towards the end of the race, as we start to tire and slow down. In fact, this could mean that you have not drunk enough in the earlier part of the race, and are now drinking more than you need. The additional (and unnecessary) fluids won’t make you perform better; just make you feel sick as it slushes around your tummy whilst you try to sprint towards the finish line.

I usually advise my competitive triathletes to work out the amount of fluid replacement they need for the entire race, then figure out the intervals at which they need to be drinking, in order to perform at their best.

For example, if you need 500ml of water per hour when working at 80% effort, but do not replenish at the correct rate, you may find that your body begins to tire and you are only able to work at 70% effort or less (even though it will feel like 90% to you).

3. How Much to Drink

For athletes and any one else who needs to keep a close eye on the amount they are drinking, they might want to know how much fluid their body loses, so that they can ensure an equivalent amount is replaced.

It’s not easy to derive an exact figure for the volume of fluids lost in a workout, particularly since we lose water through a number of channels (eg. our exhalation breaths, sweat and urine). Having said that, you can derive a rough estimate by comparing your weight before and after an intense workout or race. All other things being equal, the difference in your weight would indicate the amount of fluid lost during the workout.

Another factor to consider (besides how much to replace) is how much liquid your body can absorb. Drinking much more than the absorbable volume will send you to the toilet more often, without increasing your hydration levels. In a healthy individual, the average rate of absorption is assumed to be about 1L per hour. http://www.healthline.com/health/overhydration#Prevention7

4. What to Drink – Is Water Enough?

When we sweat, the body loses both electrolytes and water. Electrolytes keep the cells in the body functioning normally, and during periods of prolonged physical activity, when a lot of fluid is lost, it is important to restore the balance of both electrolytes and water.

In mild cases of electrolyte depletion, your body may be unable to absorb any water you drink as it tries to protect itself from dilution of your electrolyte levels.

In more serious cases, overhydration can occur when electrolytes lost through sweat are not replaced, yet excessive amounts of water are consumed. Overhydration can lead to potentially dangerous imbalances of electrolytes, including hyponatremia, a serious condition in which the sodium level in the blood becomes too low. Hyponatremia can be a problem for athletes who experience excessive sodium loss through perspiration as part of prolonged exercise or heat exposure, such as running a marathon. The World Health Organization (WHO) has documented this condition in mountain climbers who used melted snow to prepare their beverages, without supplementing them with the necessary ions or electrolytes.

So, after a long workout or race, a sports drink (with its added electrolytes) is justified, and a better choice than plain water.

5. How Much Sports Drink Do We Need?

Each person’s rehydration needs are different, so make the effort to work out your own optimal level. For instance, if you find yourself sweating more, you need to replenish fluid. If your sweat tends to be very salty (telltale signs include stinging your eyes or leaving white streaks on your workout gear), then you may need to pay more attention to replacing lost electrolytes.

There are laboratory tests available for those who want to know the precise composition of water and electrolytes you lose in a workout, but they are costly and the results may vary as circumstances change. For sports enthusiasts and amateur sportsmen, the cost and effort involved in detailed testing may be hard to justify.

So here are a few basic pointers you can use to figure out your own hydration needs for your next race:

Weigh yourself (in kg) after you go to toilet and before you go for your workout.
Plan for a workout that mimics the conditions of the race that you would be doing. As a minimum, the time of the day, weather conditions and terrain should be as close as possible to that of the actual event.
You may not need to cover the entire race distance for this workout, but you do need to cover a significant distance – eg. if you are preparing for a full marathon, aim to run at least 10km. If you are not doing the full race distance, then I suggest that you time your workout to match your expected finishing time, in order to have sense of how things might feel towards the end of the race. So if you are doing a full marathon starting at 5:30am, and expect to complete it within 5 hours, then your 10km workout should start at 9am.
Measure the amount of you drink (in litres) during the workout – the easiest way to do this is to fill a marked bottle at the start of your workout, and drink from that.
After your workout, weigh yourself (without your sweaty workout gear).

You can now estimate the rate of fluid lost during activity using this formula:

Your weight before workout – (Your weight after workout + fluid ingested during workout)
Length of workout (in hrs)

This basic formula will help you compute the amount of liquid that you need to replace to avoid dehydration. For prolonged workouts, it is wise to keep an eye on electrolyte levels too. You can experiment with different proportions of water v sports drinks, to work out the right one for you.


I weigh 60kg before the workout. I ran 10km in 60mins, during which I drank a total of 1L of water. After the workout, I weighed 58kg.

Therefore, I am losing fluid at the rate of [60 – (58+1)] / 1 = 1
ie. 1L per hour of running at marathon intensity.

So for a full marathon expected to last 5 hours, I will need an estimated 5L of water with electrolytes. In a race like the Standard Chartered Marathon, where conditions are expected to be hot and humid (especially towards the end of the race), I will drink small amounts at each drink station (rather 1L at the end of each hour) to avoid slowing down due to dehydration.

How do I know how much to drink at each station? Since there are 16 drink stations and I need a total of 5L of fluid, I should be drinking 330ml at each station. Each cup holds about 100ml. Does this mean I need to ensure I drink exactly 3.33 cups of water at each stop? Probably not. Since I can drink after the race, I can probably do with 3 cups (total 300ml) at each station. As my sweat is not especially salty, I would drink 2 cups of water and 1 cup of isotonic drink.

That’s my hypothetical hydration plan for a full marathon like the Standard Chartered Marathon, what’s yours?