Exercise hydration

What and how much should you drink when exercising?
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01.The science of hydration

Hydration during exercise

With sales of over 50 million litres per year and a market value of $150m, sports drinks are continuing to grow in popularity year by year, with more sophisticated market segmentation and targeted marketing behind the growth.

Meanwhile the simple action of drinking when thirsty has been unnecessarily complicated by science and pseudoscience, to the point where people no longer trust their bodies to tell them when to drink.

In this article we take a look at the science of hydration and exercise, and how overhydration can lead to serious consequences.

About sports drinks

Food Standards Australia New Zealand (FSANZ) has created a standard for "electrolyte drinks" . Per litre, they must contain at least 230mg of sodium and 50-100g of sugars (no more than 50g of fructose).

Sports drinks have three basic ingredients

Strenuous exercise in hot conditions can lead to water loss of up to 2 litres per hour through sweat. If you sweat more than two per cent of your body weight your heart is placed under stress, your body temperature goes up, and your physical and mental performance declines, so it's important to replace lost water during exercise.

The sugar and salt in sports drinks are at the right concentrations to maximise the speed with which water moves from your gut into your bloodstream. But plain water is absorbed almost as quickly and is perfectly adequate for moderate exercise or if you’re exercising for less than about an hour.

The sugars in sports drinks can also help replenish the fuel you use during exercise. The body stores fuel as carbohydrates in the muscles and the liver and releases sugar into the bloodstream for instant energy. But after prolonged exercise - about an hour of intense exercise, like a fast run or hard cycling - the carbohydrate stores are depleted and blood-sugar levels can drop. A steady source of sugar during strenuous exercise for an extended period can help fight off fatigue and enhance performance. With lower intensity exercise, such as jogging, it may take two hours or more before energy needs replacing. If you’re simply an active person who plays social tennis, swims or goes to the gym a couple of times a week, you don’t need sports drinks.

The salts in sports drinks help replenish the body’s electrolytes lost in sweat. Sodium helps regulate the body’s fluid balance and plays a role in muscle contraction, and potassium is also involved in muscle contraction. But for the average exerciser this is less important than the ads would have you believe: most of us already get too much salt in our regular diet so your usual food intake should be more than enough to replace any electrolyte losses. A 600 mL bottle of some sports drinks gives you more than 10% of the maximum recommended daily intake of sodium.

However, for endurance athletes, sodium losses are a more serious issue: low sodium levels often coupled with excessive water consumption have caused more than a few deaths among athletes, hikers, military personnel and people undertaking strenuous activities over long periods of time in hot conditions - see Too much water?

Hydration and tonicity

Some sports drinks claim to be isotonic, which means they have the same concentration of solutes (substances dissolved in water or another solution) as the blood and cells. The Standard for electrolyte drinks specifies criteria for the osmolality (concentration of solutes, being salt and sugar in this case) of drinks that claim to be isotonic.

Drinks like fruit juice or soft drink are hypertonic, meaning they have a higher concentration of solutes than the blood and cell fluids. They are released from the stomach more slowly than weaker drinks, so don’t rehydrate as quickly, and also draw water from body tissues into the gut, potentially increasing dehydration.

Mizone Formulated Sports Water promises rapid hydration and is marketed for sports. While it meets the Standard for electrolyte drinks (or "sports drink") in terms of sodium, it has less sugar (3.7%) than other sports drinks (5-10%), which should be taken into account if energy is an important consideration – as for endurance events. There is some clinical evidence that hypotonic drinks like this are absorbed faster than water and isotonic or hypertonic drinks, and drinking them may feel more "refreshing".

Verdict: do you need a sports drink when exercising?

The bottom line is that for most people sports drinks are an unnecessary expense and provide unnecessary salt and kilojoules. However, for long periods (an hour or more) of strenuous exercise they may be beneficial.

Kids and sports drinks

Sports drinks are promoted as better than water for young kids because the kids tend to drink more than if they were only offered water, preventing dehydration. But sports drinks are acidic and can erode dental enamel, and the salt in sports drinks is even more undesirable for children than it is for adults because their maximum recommended intake is less (depending on age).

Experts argue that sports drinks are unnecessary for children and adolescents, and their consumption is part of the growing childhood obesity problem. If kids are thirsty, they’ll drink water if water is provided.

Homemade sports drink

If you regularly engage in strenuous activity of an hour or more and feel you could benefit from a ‘sports drink’, you don’t need to fork out for commercial sports drinks - here’s a recipe from the US Olympic Committee website:
1/4 cup sugar
1/4 teaspoon salt
1/4 cup hot water
1/4 cup orange juice
2 tablespoons lemon juice
1/2 cups cold water

Dissolve the sugar and salt in hot water, then add juices and cold water.

A simpler recipe is to combine 500mL of unsweetened fruit juice, 500mL of water and a quarter of a teaspoon of salt. Experiment a little with the proportions – less juice and more water, and half the salt will make it hypotonic.



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