No doubt you have heard a million times how important it is to stay hydrated, especially during exercise. But why does it matter, and how much fluid do you really need? In this mini review I hope to provide these answers and more, so you can feel more comfortable choosing what, when, and how to drink. This information is geared towards the needs of athletes or anyone exercising regularly.
The average person is 60% water, with ranges from 45% to 75%. Fat free mass is 70-80% water, while adipose (fat tissue) is only around 10% water. Someone with greater muscle mass will therefore have a higher percentage of total body water (TBW). As the majority of what we are made of is water, it is quite important to maintain fluid balance, or hydration.
A good hydration plan starts with having enough fluids throughout each day. A general guideline for healthy people is to have about 1 ml fluid per 1 calorie (calories used should be calculated calorie needs). So, a 2000 calorie diet would be matched with 2 L fluid. (1 oz. = 30 ml) You could also use the guideline of 30-35 ml/kg per day fluid. This amount should be taken over the course of a day and should be mostly from water. We get fluids from all different kinds of drinks and foods, but having most fluids come from plain water is ideal, especially to limit calorie/sugar containing beverages. Tea and coffee do count towards your total fluid intake, but consuming more than 180 ml of caffeine may start to have a diuretic effect. Alcohol, especially in larger doses, can also increase fluid losses.
General hydration is pretty straightforward. Your body does a good job at letting you know when it needs more fluids. If you drink when you feel thirsty and keep in mind how much fluid you should aim for each day, you will likely achieve good hydration. With the addition of strenuous physical activity (yay!) to your day, you will likely need to increase your fluid intake on top of your daily intake to account for fluid lost during exercise.
Dehydration is the process of losing of body water. Hypohydration is the technical term for a deficit of body water, but to keep it simple, the term dehydration will be used for both. A person may be considered to be dehydrated when fluid losses result in a decrease of 2% or more of body weight. (Because of the water content differences between lean and fat mass, a person with a greater body fat mass may actually be more dehydrated than a person with greater muscle mass if they both have fluid losses of 2% body weight.) Checking urine color is another way to measure, although subjectively, hydration status. When well hydrated, urine should be pale-light yellow, and will be darker and browner in color the more dehydrated someone is. Dehydration causes greater physiologic strain on the body by increasing heart rate, core temperature, and perceived exertion performing activity. A hot environment increases stress on the body when dehydrated. Greater strain on the body generally can negatively affect athletic performance.
Individual responses to different degrees of dehydration exist and may also be influenced by environmental conditions and type of exercise being performed. Aerobic (running, low to moderate intensity activities, etc.) performance degrades starting around a fluid deficit of 2% or more body weight, especially in hot environments and with activity lasting longer than 90 minutes. A loss of 3% body weight can begin to affect performance of sport related skills, and this degree of fluid loss combined with heat stress can result in cognitive function decline.
Anaerobic activity, like weight lifting and sprinting, tend to be less impacted by moderate dehydration, even with fluid losses of 3-5% body weight.
Beyond decreasing your ability to perform at your best, dehydration has serious health risks. Loss of body water increases risks for heat exhaustion and stroke in hot environments. Rhabdomyolysis, which can be caused by excessive strain on muscles, is made worse by dehydration and could lead to acute kidney failure.
Drinking more fluids than your body needs results in hyperhydration. Hyperhydrating prior to an event may help some athletes to postpone fluid losses and dehydration by starting with greater water availability. However, too much fluid can lead to increased urine output, which is probably not ideal during competitions. Having a little extra water before activity may be a good idea when dehydration is of concern, like in very hot environments.
Electrolytes are minerals that conduct electricity and help your body perform processes like muscle contractions and maintaining fluid balance. Getting enough fluid and electrolytes is one important way to prevent muscle cramping. The main electrolytes lost through sweat are sodium, chloride, potassium, calcium, and magnesium. Sodium is likely the most well known electrolyte and generally is considered the most important. Sodium is lost through sweat in the greatest amount, and therefore should be the electrolyte with greatest emphasis on replacing with food or drinks.
Hyponatremia is the condition of having too little sodium in the body. It is seen when someone overhydrates with too much water or when too much sodium is lost through sweat. Although much less common than dehydration, it is the more dangerous of the two. A normal blood level for sodium is 135-145 mmol/L. Symptoms of hyponatremia, when sodium falls to < 130-125 mmol/L, present as swollen hands or feet, headache, vomiting, wheezy breathing, excessive fatigue, and disorientation. Sodium levels below 120 mmol/L can lead to seizure, fluid accumulation in the brain, coma, respiratory arrest, and even death.
Hyponatremia in events lasting less than 4 hours usually results from drinking too much water/non-sodium containing drinks around the event. In longer events, it is more likely to occur from too much sodium lost through sweat. Hydration is key in athletic performance, but it is also important to make sure that you aren’t over doing the water. Especially with longer events or when you are sweating a lot, replacing the sodium lost in sweat should be as much a priority as drinking enough water.
There is a seemingly infinite selection of sports drinks available today. What to look for when choosing one? Carbohydrates and electrolytes. Pretty much all sports drinks will have some sort of combination of the two. Electrolytes should be mainly in the form of sodium chloride (table salt) along with smaller amounts of potassium. This is because our body loses sodium and chloride at much higher rates than potassium (about 7x more).
Most regular sports drinks will also contain carbohydrates. Carbohydrates are not included for hydration purposes, but may help increase fluid intake because they give more interesting flavor to drinks. Carbs can help you keep up performance when exercising for ~ 1 hour or longer by providing quick burning fuel for energy. For exercise lasting~30-60 minutes, consuming a small amount of carbohydrate may boost performance not as fuel, but as a signal to the central nervous system. For more on carbs for athletic performance, read here.
Recently with the explosion of all things coconut, coconut water has been promoted as a good drink for rehydration instead of sports drinks. Coconut water does contain lots of potassium, but very little sodium and not enough carbohydrates. So, it really isn’t an appropriate replacement for sports drinks.
Do you need a sports drink though? If you are exercising for less than an hour, probably not. Good old water should be enough to keep you hydrated. But, heavy sweating or high heat during shorter activity may warrant sports drinks. If you are going to be active for longer than 60 minutes, a sports drink may be a good idea to keep your fluids and electrolytes balanced, and to give you a little energy to sustain performance.
Hydration Before Exercise
The goal of pre-exercise hydration is to come into the event optimally hydrated. Getting enough fluids throughout each day is the first step in being well hydrated for a game or event. If an athlete has multiple events, about 8-12 hours should be enough time to completely rehydrate in between. Around 4 hours before activity, it is recommended to drink ~ 5-7ml/kg fluids. If a person is still dehydrated, drinking another ~ 3-5 ml/kg fluid about 2 hours before the event is advised.
Having a little bit of salt before activity can stimulate thirst and help the body hold onto water. Pretzels are a great pre-event snack because they have salt as well as quickly digesting carbohydrates for energy. Being well hydrated going into a performance is important, but be careful to not drink too much. Excess fluid intake can require untimely bathroom breaks, could cause an upset stomach, or in rare cases with continued over-hydration, lead to hyponatremia.
Hydration During Exercise
The objective of hydration during physical activity is to replace the fluids and electrolytes lost through sweat, keeping any fluid losses to ideally < 1% body weight. I think we can all agree that sweating isn’t the most enjoyable thing. It can be irritating for us to deal with all the stinky soaked clothes after a good workout or game. But, all that sweat is important, as it’s our body’s way of staying cool enough to maintain performance. When you exercise at high intensities, for a long time, or a combination of both, your muscles generate lots of heat. To stop you from overheating, heat is carried from your core to your skin, where it is cooler, via blood. At the same time, sweat glands are releasing sweat, a water and electrolyte solution, through your skin. Evaporation of sweat makes your skin, and then blood cooler, effectively decreasing and regulating your temperature.
How much you sweat depends on many factors. Most influential is likely how hot the environment around you is. The hotter the air is the more heat stress your body will be under, and unsurprisingly, the more you will sweat to try to stay cool. Humidity exacerbates how hot you feel. Other factors include if you are genetically predisposed to sweat more or less, how big of a person you are, how much clothing or equipment you are wearing, if you are in direct sunlight, and type and duration of exercise. The amount that you sweat will determine how much you should be drinking during exercise. Balancing intake and losses keeps your body hydrated and able to give your best physically and mentally.
Sweat rates can vary from around 0.3 – 2.4 L per hour. This makes it difficult to set general hydration recommendations to apply to all types of athletes/people and situations. So how can know how much you need to drink then? There are multiple methods to determine hydration status, plasma osmolality measurements being most accurate, but these are unpractical for the vast majority of people. The best, most feasible way for most people to determine hydration status, sweat rate, and fluid needs is with body weight.
How to: First determine your baseline weight. Weigh yourself before your exercise session/event. Keep track of how much fluid you drink. If you can, avoid using the bathroom during the session, or else you will have to measure urine output to be accurate in the calculations. Weigh yourself after exercise. Before and after weights are ideally taken with no clothing, as sweat is absorbed by your clothes and will result in an incorrect weight. Subtract your after exercise weight from your before exercise weight to find the difference/weight loss. Every 1 gram lost is considered 1 ml fluid lost. To this difference, add any fluid that you drank and subtract any urine produced during the exercise session. Now you have the amount of fluid you lost during exercise. To find your sweat rate, divide this amount by the time spent exercising.
Here is an example to explain a bit better:
An athlete weighs 80 kg (176lbs.) before a training session.
He drinks 1.5 L water over the 2 hour session and does not go to the bathroom.
After training, he weighs 79 kg.
80 kg before weight – 79 kg after weight = 1 kg weight difference
1 kg weight difference + 1.5 L fluid intake – 0 ml urine produced = 2.5 kg total weight lost
2.5 kg total weight lost / 2 hours session duration = 1.25 kg lost per hour
1.25 kg lost per hour = 1.25 L per hour sweat rate
Your sweat rate becomes your target fluid intake rate to maintain hydration. This goal intake can, and likely will, fluctuate though as factors such as environmental heat and exercise intensity and duration change. For an athlete trying to establish the amount of fluid needed to maintain performance in a game/event/match, it would make the most sense to assess their fluid losses during a game with similar conditions rather than practice. Although practice can mimic competition, it can differ in time, equipment worn, duration of physical activity, and type of movements performed. Sweat rate during practice can therefore be quite different than that during competition. Assessing sweat rate over a couple events can provide a useful average if desired.
To assess hydration status daily, you can compare your weight to your baseline weight. The best way to find your baseline is by weighing yourself after you wake up and use the bathroom. Because the body experiences small normal weight fluctuations (especially in women), take this measurement a few times to get a good average. To avoid inaccurate hydration calculations caused by food and fluid intake, compare to the baseline a weight taken also in the morning after going to the bathroom. This method is not really appropriate for people trying to lose or gain weight, as their baseline will hopefully be changing according to their goals.
Hydration After Exercise
The purpose of hydration after exercise is to replace fluid and electrolytes lost during activity. Following hydration guidelines while exercising can be hard for many athletes to actually achieve due to barriers like lack of opportunities, inadequate thirst or desire to drink, etc. Even if you were hydrating well during an event, it is important to continue doing so after it has ended, and especially if you have another event in the coming hours. If you do need to rehydrate quickly, it is recommended to drink 1.5 L/kg body weight lost during activity. Intake of more than 100% of fluid losses is advised to account for your body likely getting rid of (pee) some of such a large influx of fluid.
Like with before activity, having a salty snack can help increase thirst and retain water. If you don’t need rapid rehydration, drink to match your thirst and then continue having normal meals and drinks throughout the day to adequately restore hydration. This slower, more natural way of rehydrating is preferred over rehydrating with lots of water very fast. Adding salt to food, as you probably already do, is an easy way to make sure you are replacing any sodium deficit. However, a typical American diet tends to be quite high in sodium already, so it is possible you don’t need to add much or even any extra salt to adequately restore sodium levels. Because everyone loses sodium at different rates through sweat, it is hard to tell how much someone may need without testing blood. You may be a heavy sodium sweater though if dried sweat leaves white stains/crystals on your clothes.
Your personal hydration plan may or may not need to follow all these guidelines. Each person experiences differences in sweat rate, environment of activity, performance goals, and more. If you are planning for a half or full marathon, you might want to think about just how you are going to balance your fluid and electrolytes along to course to stay hydrated to get your best time. If you go to the gym a couple times a week for a hour or so, a hydration plan is likely not as necessary for you. Whatever your situation may be, understanding how and why hydration influences your athletic performance is valuable knowledge that can only be to your benefit.
Thanks for reading all this way, and I hope I was able to present you with useful information on one of the core sports nutrition concepts. The resources I used as references are below. They are fantastic, researched-based papers that I would definitely recommend further inquiry into if you have more interest in hydration.
ACSM Position Stand: Exercise and Fluid Replacement
ACSM Position Stand: Nutrition and Athletic Performance
Kenefick & Cheuvront 2012, Hydration for recreational sport and physical activity.
Shirreffs & Sawka 2011, Fluid and electrolyte needs for training, competition, and recovery.