Why fluids matter during hard exercise
Water makes up a large share of the body and is involved in temperature regulation, circulation and many basic functions. During intense exercise you produce more heat, and sweating is the main way the body cools itself. Sweat contains water and small amounts of salts, so fluid and electrolytes are both lost.
Being slightly low on fluid can make exercise feel harder, and larger losses can contribute to dizziness, cramps, poor concentration and, in serious cases, heat illness. The aim of good hydration habits is to start well, replace fluids sensibly, and avoid both too little and too much.
How much people need varies widely with body size, climate, sweat rate, clothing, fitness and the length and intensity of the session. There is no single number that fits everyone. If you have a medical condition, such as heart or kidney disease, or take medication that affects fluids, ask your doctor about the right approach for you.
Build the habit through the whole day
Hydration does not begin at the gym door. Drinking regularly throughout the day usually matters more than trying to catch up all at once beforehand.
- Keep a water bottle where you can see it.
- Drink with meals and when you are thirsty.
- Notice that foods such as soups, fruits and vegetables contribute fluid too.
- Remember that other everyday drinks count, though some contain added sugar. Caffeinated drinks can contribute fluid for most people, but caffeine affects each person differently.
A common, informal check is urine color. Pale yellow is generally associated with adequate hydration, while very dark urine can suggest you need more fluid. It is only a rough guide, and some foods, vitamins and medications change urine color.
Before a hard workout
In the hours before exercise, drink fluids with your meals and sip some water as you prepare. Avoid drinking a large volume right before a session, since it can leave you feeling sloshy and uncomfortable. If you will train in heat, humidity or at altitude, begin your day well hydrated.
Your food choices count too. A pre-workout meal that includes fluid-containing foods adds to your total, as described in pre-workout meals.
During exercise
The most reliable general advice is to drink according to thirst and to have fluids available. Thirst is a reasonable guide for many people in typical conditions.
- Short sessions (generally under an hour): water is usually enough.
- Longer or very demanding sessions, or hot conditions: fluids with some carbohydrate and sodium may be useful for some people. Needs are individual, so test on training days before relying on a drink for an event.
- Sip regularly rather than gulping large amounts at once.
- Adjust for heat. In hot or humid conditions, reduce intensity, take breaks in shade or cooler areas, and wear light clothing.
Understanding how hard you are working can also help you notice problems. Using perceived exertion to judge effort, you can notice when the same pace suddenly feels much harder than usual, which can be a sign of heat or fluid problems.
After exercise
Afterward, drink fluids and eat a normal meal. Foods with sodium, such as soups or salty snacks, can help the body retain the fluid you drink. Replacing fluids typically happens over several hours rather than all at once. If you lost a lot of sweat, such as after a long hot session, you may want to drink more steadily through the next few hours.
A weigh-in before and after training is sometimes used by athletes to estimate fluid loss, though this is not required for most people.
Signs to take seriously
Mild thirst is normal. These symptoms deserve attention:
- Dizziness or lightheadedness
- Headache
- Unusual fatigue or weakness
- Muscle cramps
- Nausea
- Very dark urine, or very little urine
- A rapid heartbeat that seems out of proportion to the effort
If you notice these, stop exercising, move to a cool place, rest and drink fluids slowly. Take stronger signs very seriously, including confusion, fainting, vomiting that does not stop, hot dry skin with no sweating, or a severe headache. These can indicate heat illness, which is a medical emergency. Seek urgent medical help.
Overhydration is possible too
Drinking far more than you need, especially over long events and without any sodium, can dilute the salt level in the blood, a condition called hyponatremia. It is uncommon in ordinary workouts but can occur in long endurance events. Symptoms can resemble those of dehydration, including headache, nausea and confusion. This is another reason to drink to thirst rather than forcing large amounts.
Practical tips
- Carry a bottle to workouts, and refill it before you leave home.
- Plan extra fluids in hot weather or when you start a new outdoor routine.
- Be careful with alcohol, which can add to dehydration.
- Track how you feel. Notice if energy dips, such as unusual energy dips in the afternoon, regularly coincide with low fluid intake.
- Don’t rely on sugary drinks as your main source of fluid. Added sugar adds up quickly.
- If you have frequent cramps or dizziness, discuss them with a doctor.
Summary of an approach
Drink steadily through the day, begin workouts feeling well hydrated, sip during exercise according to thirst and conditions, and replace fluids with your next meals. Watch for warning signs, and treat them seriously. Your needs may change with weather and fitness, so stay flexible and pay attention to how your body responds.