How much should you drink during exercise?

How much do you really need to drink to perform well, and when does it become too much? We explore the role of fluids in the body, how exercise and heat affect fluid needs, and how you can think about water, sports drinks, and electrolytes. The main question is how much fluid loss is acceptable before performance is negatively impacted. The conclusion is that you don't benefit from overdrinking, but too much weight loss during exercise increases heart rate, body temperature, and fatigue. Adjust your intake based on body weight, intensity, and climate—using thirst as a guide, but with a well-thought-out plan.

Water is essential, meaning vital, because it is used in our metabolism. This means we need a continuous supply of water. As our activity increases, so does metabolism, and therefore the need for hydration.

Over a 24-hour period, at normal room temperature and without exercise, fluid balance varies no more than about 0.2 percent of body weight. For example, if you weigh 80 kg, your fluid balance doesn't vary more than a drinking glass (1.5 dl) throughout the day. The body is brilliant at balancing fluid levels, and thirst is often the best indicator when at rest.

About 70-80% of our daily fluid intake comes through beverages (water, juice, coffee, etc.), and the remaining 20-30% through food. Practically, this corresponds to roughly 1.5 liters from beverages and 0.5 liters from food. We also produce about 2.5-3.5 dl of water ourselves by breaking down glucose, fat, and protein (albumin). These hydrogen atoms, combined with the oxygen we breathe in, are converted into water. If you keep track of the math, you'll see that you should intake about 2 liters of water a day - with at least 1.5 liters coming from beverages.

But what happens if we add physical activity into the equation?

When body temperature rises, we release heat by sweating. Each gram of sweat takes about 2.4 kilojoules (kJ) of energy (heat) from the body through evaporation. If you sweat out 1 liter, you've transported away approximately 580 kcal (about a light lunch) (source). The evaporation of sweat is a very effective way to get rid of excess heat, but it requires significant amounts of fluid when the temperature rises, and we can sweat as much as 1-2 liters per hour. In high humidity, sweat cannot evaporate as the air is already saturated, and the sweat runs off the body instead. This is a much less efficient way to cool down and means that we need even more fluids.

The physiological temperature regulation is an entirely autonomous system that includes the control of metabolism, sweating, and vasomotor tone. You can't manually control your temperature regulation. The body sweats when necessary, and blood flow is increased or restricted and distributed as needed. When regulating blood flow isn't enough to maintain body temperature, the body shifts to the second gear: sweating, to dissipate heat and maintain body temperature. The autonomic response to heat stress is greater with an elevated core temperature compared to an elevated skin temperature. A 1°C change in core, deep body temperature elicits a nine times greater thermoregulatory response (such as sweating) compared to a 1°C change in skin temperature (source). You might have noticed that you don't sweat as much when lying on the beach in 40-degree sun and your skin gets hot, but when you're running hard and your core temperature rises by one degree, you sweat much more.

For every liter of water we lose, our body temperature increases by approximately 0.3°C. The blood volume the heart pumps out each minute decreases, leading to an average increase in heart rate by 8 beats per minute at a given exertion (Ref: Gisolfi, C.V, Fluid Balance for Optimal Performance, 1996, non-digital format). An increase of just a few heartbeats over time causes the feeling of fatigue to come earlier than expected. Therefore, adequate fluid intake is crucial for performance, especially if you train for longer than two hours and/or are out when the temperature sneaks past the 25-degree mark.

Drinking more than necessary won't automatically improve your performance, but drinking too little can negatively impact it. But what does "too little" mean?

Early research on fluid intake and dehydration in athletes showed performance declines with just a 1% drop in body weight. This doesn't mean you've lost exactly 1% in pure fluid (you're also depleting some glycogen), but it indicates that performance starts to be affected. There are, however, conflicts of interest: some studies leading to guidelines against exceeding a 2% weight loss were sponsored by companies like Gatorade, which makes sports drinks.

More recent research is done in more realistic settings. Previously, participants used exercise bikes and treadmills in controlled lab environments. Useful, but without wind resistance, the results don't entirely represent outdoor training. In studies that more closely mirror reality, it is often stated that losing more than 3% of body weight may begin to negatively affect performance. Up to a 4% weight loss might even be acceptable (source). For an 80 kg person, this could mean a weight loss of up to 3 kg.

The best way to decide how you should hydrate is to weigh yourself before and after a workout to gauge your fluid loss. Exercising for 60 minutes at moderate to high intensity can result in about 0.5 kg of weight loss just from glycogen depletion, plus the water bound to glycogen (2.4 g of water per gram of glycogen). Your performance is impacted by glycogen depletion, so carbohydrates should be replenished, though fluid loss isn't the main issue early in a session. For example, in the Göteborgsvarvet, you should expect to lose at least 1 kg, which is about 2% of body weight. Passing this threshold increases the risk of unnecessary temperature rise and pulse elevation.

As stated above, most of us can sweat out 1-2 liters per hour, and really well-trained individuals can have even higher sweat rates. Alberto Salazar recorded sweating 3.71 liters per hour during his peak in a 1986 study. If you're training hard for an hour on a sunny day, you could easily lose 1.5 kg of body fluid without drinking anything. This equates to about a 2% weight loss for an 80 kg person and can lead to noticeable increased fatigue, especially for runners without wind resistance. Add to that the fatigue from glycogen depletion and muscle wear—you'll be worn out.

Currently, fluid recommendations based on body weight are given to elite athletes; these are around 0.2 ml of fluid per kg of body weight per minute (source). Tailoring recommendations based on body weight makes them more individualized.

There's no need to "hydrate up" by drinking large amounts before training. Research shows that it doesn't affect body temperature; it just makes you heavier and increases the need to urinate. Thirst is a reliable indicator for fluid intake for most. The downside of relying solely on thirst is that it can disrupt your energy plan since fluid and carbohydrate intake often come in the same bottle via sports drinks or gels. But if you're on top of your energy intake, thirst is a great cue. There are also risks with drinking more than what thirst dictates; stomach issues and the "sloshing" feel in the gut are real downsides.

Both water and sports drinks with 6-8% carbohydrates (60–80 g/liter) move through the stomach at about the same rate. Even factors like osmolarity or isotonicity seem to impact gastric emptying only slightly (around a 3.5 percent difference). A 3% dehydration doesn't significantly affect gastric emptying either, so this part of the process remains stable, handling over 2 liters per hour comfortably (Ref: Gisolfi, C.V, Fluid Balance for Optimal Performance, 1996, not in digital format).

The fluid is then absorbed in the small intestine, which can take up just over 2 liters per hour (36 ml/min) if the water is mixed with 6% carbohydrates and supplied continuously. Otherwise, the limit is about 600-800 ml/hour. Sodium (salt) supplementation in doses of 25-50 mmol/L (a common amount in sports drinks) does not affect fluid absorption if the drink already contains carbohydrates (glucose). The intestinal membrane can release sodium; just 10 cm of the intestine can release enough sodium to increase the sodium content in 1 liter of pure water from 0 to 60 mmol/L – an amount often higher than in many sports drinks.

Nevertheless, electrolytes should be included in a good sports drink, but for reasons beyond just fluid absorption. Primarily, they compensate for the electrolyte losses that occur through sweat and reduce the risk of hyponatremia, which is a depletion of salts that can happen if you drink a lot of plain water while sweating heavily (source). The more trained you are, the better the body is at conserving salt, and the more "pure" fluid sweated out. But even if you don't train much generally, you can still lose a significant amount of salt during a challenging training day. 

If you want to optimize your fluid balance during a hot summer and enjoy staying at home in the kitchen, the classic variant recommended by health services is: 1 liter of water, 6 teaspoons of sugar, and half a teaspoon of salt. Bring to a boil, let cool, then flavor with cordial or juice concentrate and drink.

Sports drink is also a good option, for example, U Sport which contains both carbohydrates and electrolytes. Or Resorb, which is widely available. Make sure it’s a resorb/fluid replacement with carbohydrates in it and not just salts, especially if you want to optimize fluid intake and balance during physical activity. Our recommendation is to buy ready-made, tailored sports drinks if you’re not comfortable in the kitchen or if your homemade version isn’t the tastiest. The flavor experience matters; no one drinks something that tastes awful. Especially not frequently and in small sips.

• Aim to consume 0.5 liters of liquid per hour if you’re out for more than an hour and the intensity is moderate to high.
• Preferably with 60–90 g of carbohydrates and about 2 g of salt per hour.
• Spread the intake of drinks and energy over 3–4 doses per hour.

If you weigh the same at the finish as when you started, you've drunk too much. A weight loss of 2% is normal. But avoid a weight loss of over 3%. We also want to note that the recommendations mainly apply to runners who generate more heat at a given intensity because there's no wind compared to cyclists. Looking at a meta-analysis for cyclists, a reduction in body weight of up to 4% can be acceptable.

Good luck!