Hydration – Health and Performance
Hydration and the Human Body
50-70% of body weight is water. Approximately 60% of male and 50% of female body mass (BM) is water. Also, water makes up ~73% of muscle mass, but only ~10% of fat mass.
Water’s function is to dissolve and transport substances, it is a major component of blood plasma, protects and lubricates tissues, and is the first line of defence (immunity). Plasma contains 91% to 92% of water and 8% to 9% of solids.
Therefore, hydration is crucial for many reasons including but not limited to regulating body temperature, keep joints lubricated, prevent infections, delivering nutrients to cells, and keeping organs functioning properly. Staying hydrated increases energy, improves movement, recovery and agility, thermoregulation, and aids in mental clarity and activity – all of which can improve physical performance and reduce the risk of injuries.
Dehydration is the process of losing body water (e.g., during exercise). Dehydration can impact both health and performance quite dramatically. There is a link between dehydration and weight loss, especially not being able to lose weight if dehydrated. Dehydration causes a decrease in sweat rate during exercise, which results in a decreased ability for the body to cool through evaporation relating sweat rates specifically to performance.
In this Article
Topic of hydration is vast. In this article we cover the following subtopics while trying to keep the article as brief as possible without omitting the basics:
- Water in the human body
- Regulating fluid balance
- Hydration and weight loss
- Factors affecting sweat rates
- Estimating individual sweat rates
- Dehydration in health & performance
- Measuring hydration status
- Rehydration and hyponatremia
- Practical advice for maintaining hydration
- Maintaining Hydration
Water in the Human Body
Intracellular vs. extracellular:
Total water is basically within the different compartments of the human body, where water is stored as intracellular and extracellular fluid.
Total Body Water = intracellular + extracellular
- Intracellular fluid – water located inside human body cells. In healthy people, it makes up the other 2/3 of the water inside a human body.
- Extracellular fluid – water located outside cells, In blood vessels (intravascular space) & spaces between cells (interstitial space) The water in blood falls into this category. Roughly 1/3 of fluid is attributed to extracellular, and this water is found in interstitial fluid, transcellular fluid, and blood plasma.
If intravascular water is lost, the body can compensate by shifting water from cells into the blood vessels. This is a very short-term solution.
Male vs Female:
Huge percentage of our human body is made of water, about 50 to 70%, a little bit more in men versus females. That’s purely because there is more water in muscle and men tend to have more muscle mass. Water makes up about 73% of muscle and only about 10% of fat. You could have a female that was particularly muscly, and they would be more than 50% body water and we could have a male that was over fat with less than 60%.
Regulating Fluid Balance
Homeostasis is the self-regulating process by which an organism maintains internal stability while adjusting to changing external conditions. Fluid balance is an aspect of the homeostasis of organisms in which the amount of water in the organism needs to be controlled, via osmoregulation and behaviour.
Fluid balance is maintained with fluid motion, both fluid gain and fluid loss.
Human body loses fluid from respiration and sweat, and loses some fluids just from breathing that goes up during exercise. As you are breathing heavier, you will lose slightly more water through respiration, so that’s kind of fluid loss. An average sedentary person excretes 2.7 litres of water per day, though it is hugely variable. So again, like with 50 to 70% of total body water, how much someone excretes is very, very individualised.
One gains fluid with food, drink, and metabolism. A lot of foods contain a very large percentage of fluid, so it’s surprising how much fluid one gets from food.
One gains fluid from metabolism also. As the human body breaks down glycogen, it gets not only glucose but also gets 3 grams of water which is added to the body. Again, during exercise, you will gain more fluid, more water to the body because you are obviously breaking down carbohydrates for exercise for energy.
Here, it is important to know what osmolality and osmolarity are.
Osmolality and Osmolarity
Osmolality indicates the concentration of all the particles dissolved in body fluid. It is routinely measured in clinical laboratories for the differential diagnosis of disorders related to the hydrolytic balance regulation, renal function, and small-molecule poisonings. Osmolality tests measure the amount of certain substances in blood, urine, or stool. These include glucose (sugar), urea (a waste product made in the liver), and several electrolytes, such as sodium, potassium, and chloride.
Osmolarity refers to the concentration of a solution expressed as the total number of solute particles per litre.
Osmolarity refers to the number of solute particles per 1 L of solvent, whereas osmolality is the number of solute particles in 1 kg of solvent. These are measured in Osm/kg or Osm/L.
Key Hormones – Aldosterone & ADH
In time of fluid Regulation, there are two key hormones involved in regulating fluid balance, making sure that plasma osmolality or plasma volume doesn’t change too much outside of the norms.
Aldosterone: A steroid hormone made by the adrenal cortex (the outer layer of the adrenal gland). It helps control the balance of water and salts in the kidney by keeping sodium in and releasing potassium from the body.
ADH: A hormone that helps blood vessels constrict and helps the kidneys control the amount of water and salt in the body. This helps control blood pressure and the amount of urine that is made.
When the body loses fluid, there are osmoreceptors within the body that recognize plasma volume decrease, blood osmolarity increases. An osmoreceptor is a sensory receptor primarily found in the hypothalamus of most homeothermic organisms that detects changes in osmotic pressure.
Because there is less fluid that obviously means that possibilities of the concentration of dissolved particles within the blood increases. This stimulates what’s called renin and the tension system, which is basically what you can think of as fluid regulation. The stimulation of that increases the secretion of these two key hormones. They both act on the kidneys to essentially reabsorb sodium and water, which means kidneys retain fluid, urine output is reduced.
This results in plasma volume increase and blood osmolality reduction causing restoration of fluid balance (homeostasis).
Note: Renin is an enzyme that helps control your blood pressure and maintain healthy levels of sodium and potassium in your body. Made by special cells in your kidneys, renin is released into your bloodstream when your blood pressure drops too low.
It works exactly the opposite way round as well. If body gets a fluid gain, it’s literally reversed exactly like any other homeostatic system.
Hydration and Weight Loss
Drinking lots of water is commonly espoused in weight loss regimens and is regarded as healthy; however, research connecting weight loss with hydration is limited. Also, the research that does exist is done in animals whereby fluid intake isn’t even measured.
In a study of 14 healthy, normal-weight subjects (seven men and seven women), the effect of drinking 500 ml of water was assessed on energy expenditure. Drinking 500 ml of water increased metabolic rate by 30%. The increase occurred within 10 min and reached a maximum after 30–40 min. The total thermogenic response was about 100 kJ. Thus, it was concluded that drinking 2 litres of water per day would augment energy expenditure by approximately 400 kJ. Though no other published human study supports this, the thermogenic effect of water should be considered when estimating energy expenditure, particularly during weight loss programs.
How might hydration affect weight loss?
- Small suppression of appetite
- Feelings of lethargy
- Reduction in NEAT/physical activity increased energy (sugar) intake
- Go to the toilet more
If someone drinks 500 mils of fluid before eating breakfast, 500 millilitres before lunch, 500 millilitres before dinner, that surely affects appetite. It does seem people seem to eat less having done that. But all that is suppression of appetite. If you drink 500 millilitres of fluid. Yeah, your stomachs are a bit full and then you eat less. So, it’s not really a surprise.
Hydration helps weight loss, really there is no solid or scientific evidence to support this claim at all.
Sweating is the release of liquid from the body’s sweat glands. It is a bodily function that helps regulate body temperature, so obviously it is a natural method of getting rid of heat. Heat loss and sweat rates are kind of individualized, varies person to person. But you can have one individual that is the same gender, same body weight, same exercise intensity and yet have completely different sweat rates. Fluid intake, especially during exercise and post exercise, should reflect the fact that sweat rates are individualized.
Factors affecting sweat rates
Many factors affect sweat rate. Individual sweat rates vary and depend on factors such as ambient temperature, humidity, air movement, exercise intensity, insulating clothing or equipment, and body size. Some of the factors are:
- Gender (and menstrual cycle/menopause) – Males typically tend to sweat more than females. There’s also some interesting research on the females and the menstrual cycle. There are certain phases within the menstrual cycle where women will sweat more. Excessive sweating can happen before menstruation, especially at night. This is caused by fluctuations in hormone levels and a slight body temperature elevation in the luteal phase of the cycle.
- Bodyweight – The heavier one is the one who tends to sweat more, and that’s potentially just because you produce more metabolic heat from exercise, so there is more to get rid of. Therefore, your sweat and rate will be higher.
- Exercise intensity – Exercise intensity is an obvious factor, more intense exercise, the more sweat you would expect.
- Environmental Temperature – As the environment warms-up, the body tends to warm-up as well. The body’s internal “thermostat” maintains a constant inner body temperature by pumping more blood to the skin and by increasing sweat production. In this way, the body increases the rate of heat loss to balance the heat burden.
- Humidity – The real cooling mechanism behind sweating is sweat evaporating off the skin. So, if it’s really humid outside, there is less kind of opportunity for sweat to evaporate. And you must sweat more to get rid of the same amount of heat as compared to if it was the same temperature but in a dry heat. That’s also the reason why you shouldn’t really wipe sweat off with the towel, because the real cooling mechanism is evaporation. So just something to bear in mind if you are in the gym and forever wiping your sweat off with the towel. I know it’s a bit gross, but actually you might be better just sit it out and let it evaporate, and that might help you cool down a little bit better than wiping it.
- Fitness – sweating sensitivity – The researchers found that there is a relationship between fitness and sweating sensitivity. People who are very fit sweat more than their less-fit counterparts. But if fit people and less-fit people are performing the same task, the less-fit person will sweat more because they have to expend more energy to perform the same task.
- Health/illness – It is more related to body temperature. Obviously, if one has got a temperature, one is more likely to sweat because of trying to get rid of heat that way.
- Pre-exercise hydration status – For replacing the sweat you’ve lost, maybe starting exercise while hydrated is the way to go in terms of your sweat rate. If you start dehydrated, you are much less likely to sweat loads during the exercise session, it’s just your body retaining water.
Average Sweat Rates
|Waterpolo||0.55||0.30 to 0.80|
|Cycling||0.80||0.29 to 1.25|
|Cricket||0.87||0.50 to 1.40|
|Running||1.10||0.54 to 1.83|
|Basketball||1.11||0.70 to 1.60|
|Soccer||1.17||0.70 to 2.10|
|Rugby||2.06||1.60 to 2.60|
Estimating individual sweat rates
Is it necessary? No. Is it Beneficial? Yes, having an idea of how much someone might sweat, what their sweat rate per hour is, could be really useful for putting into place an individualised hydration and fuelling strategy.
Really simple, weigh before a training session and weigh drinks bottle or at least take a measure and take a note. Write down how much fluid you are starting with. Do your exercise session and weigh afterwards including drinks bottle or measure how much drink has been consumed in terms of weighing.
Obviously before you start your training session, strip off before you weigh, though it may be a bit difficult, but sweat can get trapped in clothes, so strip to shorts at least.
Once you’ve done that, all you need to do is use this equation.
- Sweat Loss (L) = Change in Body Mass (kg) + Fluid intake (L) – Urine losses (L)
- Sweat Rate (L/h) = (Sweat loss (L) / Exercise time (mins)) x 60
- Change in BM (kg) + Fluid intake (L) – Urine losses (L) = Sweat Loss (L)
1.2 kg + 0.4 L – 0 L = 1.6 L
- (Sweat loss (L) / Exercise time (mins)) x 60 = Sweat Rate (L/h)
(1.6 L / 90 mins ) x 60 = 1.06 L/h
Remember this calculation only really applies to exercise of this intensity/duration and in this environmental temperature.
Points to remember about Sweat
- Sweat isn’t just water. Electrolytes are also present in sweat
- Magnesium and calcium
- Sweat concentration is also highly individual
- Salty Sweaters – Know whether you’re particularly a salty sweater, for this you’ll taste your sweat. It will taste really, really salty, and some people even get like real salt crystals on their clothes. Just bear in mind that the salt in your sweat is affected by your dietary salt intake, so there is a regulation there. If you eat a lot of salt, you will tend to pee out a lot of salt and your sweat will also be a bit saltier too. Another thing is that improved fitness helps is your ability to retain salts and you get better at so, basically sodium reabsorption. Your sweat glands are to absorb salt and not basically to sweat it out, which is really important about things like hyponatremia.
- Sweat concentration is acutely affected by dietary sodium
- Heat acclimation, adaptation, and improved fitness increases the sodium ion reabsorption capacity of the human eccrine sweat gland.
- Can reduce the amount of salt lost in sweat
Dehydration in health & performance
Dehydration, why should we be avoiding it? There’s lots of physiological responses to dehydration, so the first one being reduced blood, blood, plasma volume, and blood pressure. Traditional view of dehydration is:
- Being dehydrated can affect physical performance and cognitive ability
- Even slight dehydration (2%) can affect physical performance by 20% – this is not enough to feel thirsty
- By the time you feel thirsty, you are already dehydrated
- Fluid intake should equal sweat losses
- “Stay ahead of thirst”
- “Drink as much as tolerable”
Physiological response to dehydration
- Reduced plasma volume & BP – When the body doesn’t have enough water, the amount of blood in the body (blood volume) decreases. This can cause blood pressure to drop.
- Also affected by salt intake – What happens when you eat salt is essentially draws fluid from the GI tract into the intravascular space and you get an increase in fluid within the intravascular space and therefore you get an acute increase in blood pressure. That’s why people say one shouldn’t eat lots of salt as it increases your blood pressure.
- Increased heart rate
- to maintain cardiac output despite a reduced stroke volume – When you are dehydrated your blood volume, or the amount of blood circulating through your body, decreases. The heart has to work harder to pump blood around. To compensate, your heart beats faster, increasing your heart rate.
- Increased glycogenolysis – The breakdown of glycogen to generate glucose is called glycogenolysis. Exercise is a condition, where muscular energy demand and consumption rise together. The energy demand is met with increased release of glucose. Dehydration may cause an increased rate of glycogen use. People find exercise loads harder when they are dehydrated.
- Increased RPE – Rating of perceived exertion (RPE) is a subjective estimation of the intensity or difficulty of a physical task, and is widely used by professionals in the field of exercise sciences, coaching and sports medicine to monitor or prescribe exercise intensity. Dehydration makes exercise effort feel harder.
- Reduced skin blood flow
- Increased core body temperature during exercise –
- Reduced cell volume
Dehydration & Performance
- Dehydration ≥2% has consistently shown to impair exercise performance
- Endurance exercise
- Resistance exercise
- Team sports
- Skill-based sports
- Is it just fluid restriction?
- When hydration status is blinded, 3% dehydration does not impact cycling TT performance
- Drinking to replace an absolute volume of fluid during exercise may have no performance advantage over drinking ad libitum
Symptoms of Dehydration
- Dry mouth, lips & eyes
- Feelings of lethargy/tiredness
- Decrease in cognitive function
- Concentration, short-term & long-term memory, motor
- coordination, reaction times
Tolerance to dehydration
Tolerance to dehydration is individualized as well, some people seem to cope better with being dehydrated. However, it can also be potentially ‘trained’ and Marathon World Record holders are testament to this! There are athletes that finished the marathon 10% dehydrated.
A research group decided to test whether you could train yourself to cope better with dehydration during exercise. What they basically did was 45-minute steady run. They did that in a dehydrated state. In Dehydrated (DH) state they performed 5.8% worse compared to the hydrated state.
They then did four habituation sessions, so they made the subjects exercise. This was a 60-minute steady state treadmill exercise. Four sessions in a dehydrated state to see if they could train them to get used to being dehydrated. And this is what they found. They did the same kind of protocol as pre intervention and this time the dehydration group was only 1.2% worse than the hydrated group.
So basically, proving that yes, you can train yourself to become dehydrated.
Alcohol and Caffeine
How alcohol and caffeine affect hydration?
Both alcohol and caffeine have diuretic effects, one of the main reasons we get such bad hangovers is due to dehydration. If you think about the symptoms of a hangover, these are dizziness, light headedness, headaches. They can all be attributed to alcohol dehydration. Caffeine is potentially overplayed, though it does have diuretic properties. Typically, if you take tea the amount of fluid that you get with a cup of tea or a mug of tea versus the amount of caffeine that you would get a net hydrating effect. If, on the other hand, you are drinking espressos all day long and the fluid content is relatively small, but the caffeine content is relatively high, then yes, that probably is dehydrating.
Pre workouts are worth considering simply because they tend to be much higher in caffeine. But again, you tend to have a fair bit of water with those, so just to kind of get you to think about the different concentrations, different amounts of fluid, whether they have a net net hydrating effect or not.
Measuring hydration status
Hydration can be measured both in the lab and in the field.
Hydration assessment techniques in the lab include:
- Total Body Water
- Tracer methods e.g. deuterium oxide
- Haematological Measures
- Plasma Osmolality
- Plasma Volume
- Urine Osmolality
- Concentration of a solution (mOsmol/kg)
- More concentrated = Greater osmolality
- Urine Specific Gravity
Hydration assessment techniques in the lab include:
- Bioelectrical impedance
- Body Weight
- Osmometer (potentially)
- Urine Colour
- Strongly correlated with Uosm and Usg
Rehydration and Hyponatremia
Rehydration is the replenishment of water and electrolytes lost through dehydration. It can be performed by mouth (oral rehydration) or by adding fluid and electrolytes directly into the bloodstream (intravenous rehydration) when they are suffering from dehydration (= a lack of water). The goal of oral rehydration therapy is to replenish the body’s fluid levels. It’s typically used to treat moderate dehydration due to diarrhoea, vomiting, or other conditions.
Water is your best bet for everyday hydration, since it is free of sugar, calories, and caffeine.
Hyponatremia is a condition where sodium levels in your blood are lower than normal. In many cases, too much water in your body dilutes sodium levels. It’s also possible to lose too much sodium. When this happens, body’s water levels rise, and cells begin to swell. This swelling can cause many health problems.
A normal blood sodium level is between 135 and 145 milliequivalents per litre (mEq/L). Hyponatremia occurs when the sodium in blood falls below 135 mEq/L.
Many possible conditions and lifestyle factors can lead to hyponatremia, including some medications, such as some water pills (diuretics), antidepressants and pain medications, can interfere with the normal hormonal and kidney processes that keep sodium concentrations within the healthy normal range.
Also, drinking excessive amounts of water can cause low sodium by overwhelming the kidneys’ ability to excrete water. Because you lose sodium through sweat, drinking too much water during endurance activities, such as marathons and triathlons, can also dilute the sodium content of your blood.
Below we list some of the practical advice for maintaining hydration.
Which drink should we be choosing?
- Consume drinks that “promote longer term fluid retention and maintenance of fluid balance for prolonged periods”. This isn’t relevant to most but could be useful to those who struggle to drink frequently.
- Glucose and sodium both stimulate the absorption of fluid across the membrane of the small intestine.
- Same osmolality as body fluids
- Lower osmolality than body fluids
- Higher osmolality than body fluids
- Carry a bottle at all times
- Choose palatable drinks
- If your clients don’t like water, suggest something else!
- Consider the BHI
- Hypotonic sports drinks
- Maintain clear urine colour
- Don’t forget about foods
- Fruit, veg/salad, soups, stews etc.
Some Basic Guidelines
Euhydration is the state of optimal total body water content as regulated by the brain. Intracellular and extracellular fluid volumes are maintained with minimal physiological adjustment. The body’s systems function most efficiently in this state.
- Hydration for health is simple
- Maintain clear urine colour
- Hydration for weight loss is a moot point
- Dehydration can impair exercise performance
- Less pronounced in temperate climates
- Focus should be on maintaining day-to-day hydration and therefore pre-exercise euhydration
- Drink to thirst during exercise in most situations
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Abhinav Malhotra is an award-winning personal trainer, coach and sports nutritionist in Dubai, UAE. He also offers online services to clients around the world. A personal trainer par excellence, Abhi has worked with the world’s leading fitness chains, supplement brands and founded his own fitness academy in India. He has achieved successes for many clients from all backgrounds and has trained the Indian Army Rugby Team. He is the first International Kettlebell Sport athlete from India.