What You Need To Know About Hydration

Everything you need to quench your thirst for knowledge.

If there is one thing that all the health and fitness professionals seem to agree upon, it is hydration. Usually you hear statments like “drinking water is important” and seemingly random recommendations like “drink 8-10 glasses of water a day”, but nobody explains why it is important and what happens if you don’t drink enough, what you should drink and if guidelines like “8-10 glasses of water” actually make sense for everybody. I wanted to know more, I wanted to jump into the murky waters of hydration and this is what I found. Enjoy!

the basics

Water is essential for life. Without water, humans can survive only for days. Water comprises from 75% body weight in infants to 55% in elderly and is essential for cellular homeostasis and life.

When we speak of water we mean all types of water, be they soft or hard, spring or well, carbonated or distilled water. Although not strictly water, beverages (caloric and non-caloric) consist mainly of water and provide us with hydration.

Furthermore we get water not only directly as a beverage but from food and to a very small extent also from oxidation of macronutrients (metabolic water). The proportion of water that comes from beverages and food varies with the proportion of fruits and vegetables in the diet.

The Water Content Range for Selected Foods. Popkin et al. 2010

As you can see, unprocessed foods generally are quite high in water content, helping not only with hydration, but also with satiety. As water has no calories, these foods increase volume, which is an important signal to stop eating. Just another reason why it is a good idea to limit processed foods in your diet.

Why we drink

Although everyone experiences thirst from time to time, and drinking to quench our thirst is it’s primary function, it plays little day-to-day role in healthy people living in temperate climates. We generally consume fluids not to quench our thirst, but as components of everyday foods (e.g. soup, milk), as beverages used as mild stimulants (tea, coffee) and for pure pleasure (that’s a glass of proper craft beer for me).

Drinks are also consumed for their energy content, as in soft drinks and milk, and are used in warm weather for cooling and in cold weather for warming. Drinking seems also to be mediated through the taste buds, which communicate with the brain in a kind of “reward system” , the mechanisms of which are just beginning to be understood.

Drinking fluids other than water can contribute to an energy surplus, or in alcohol consumption may cause addiction. Our drinking habits have changed over the years, for example, total fluid intake increased from 79 fluid ounces in 1989 to 100 fluid ounces in 2002 among US adults, mostly from caloric beverages. You don’t have to be a dietician to see the impact caloric beverages have on the obesity pandemic we are currently facing.

Effects of aging on drinking

Most of us already know that elderly people just drink less. Buy why is that so?

Following water deprivation older persons are less thirsty and drink less fluid compared to younger persons. Defects in various receptors appear to exist as well as changes in regulatory mechanisms mediated by opioid receptors. Also older individuals have impaired renal fluid conservation mechanisms (so they excrete more water) and have impaired responses to heat and cold stress.

Because of their low water reserves, it may be a good idea for the elderly to learn to drink regularly when not thirsty and to moderately increase their salt intake when they sweat. Given that many drugs heavily tax the kidneys, and kidneys need lots of water to work properly, it may be of even greater importance for the elderly to drink a lot, as they often take a larger amount of drugs.


Ahh, finally we get closer to what really interests us. What actually happens if we start getting dehydrated? And is this really as dramatic as mainstream media wants us to believe?

  • Physical performance: Under relatively mild levels of dehydration, individuals engaging in physical activity will experience decrements in performance related to reduced endurance, increased fatigue, altered thermoregulatory capability, reduced motivation, and increased perceived effort. Hypohydration (“under-hydration”) appears to have a more significant impact on high-intensity and endurance activity such as tennis and long-distance running than on anaerobic activities such as weight lifting or on shorter-duration activities, such as rowing.
  • Cognitive performance: Mild levels of dehydration can produce disruptions in mood and cognitive functioning. Mild dehydration produces alterations in a number of important aspects of cognitive function such as concentration, alertness and short-term memory (although the heat stress of this study may be an important confounding factor). Relatively little is known about the mechanism of mild dehydration’s effects on mental performance. It has been proposed that mild dehydration acts as a physiological stressor which competes with and draws attention from cognitive processes. However, research on this hypothesis is limited and merits further exploration.
  • Dehydration and delirium: Dehydration is a risk factor for delirium and delirium presenting as dementia in the elderly and in the very ill. Recent work shows that dehydration is one of several predisposing factors in observed confusion in long-term care residents.
  • Gastrointestinal function: Inadequate fluid consumption is touted as a common culprit in constipation, and increasing fluid intake is a frequently recommended treatment. Evidence suggests, however, that increasing fluids is only of usefulness in individuals in a hypohydrated state, and is of little utility in normally hydrated individuals. In older individuals, low fluid intake is a predictor for increased levels of acute constipation.
  • Kidney function: The kidney is crucial in regulating water balance and blood pressure as well as removing waste from the body. The ability to both concentrate and dilute urine decreases with age. How much water the kidneys really need to function properly is difficult to calculate though.
  • Heart function: Blood volume, blood pressure, and heart rate are closely linked. Blood volume is normally tightly regulated by matching water intake and water output. In healthy individuals, slight changes in heart rate and vasoconstriction act to balance the effect of normal fluctuations in blood volume on blood pressure. Water ingestion is also beneficial in preventing vasovagal reaction with syncope in blood donors at high risk for post-donation syncope. Overall there is little literature on effects of hydration on heart function.
  • Headache: Water deprivation and dehydration can lead to the development of headache. some observational studies indicate that water deprivation, in addition to impairing concentration and increasing irritability, can serve as a trigger for migraine and also prolong migraine. In those with water deprivation-induced headache, ingestion of water provided relief from headache in most individuals within 30 min to 3 h. However it remains unclear if water consumption per se has the ability to stave of headaches.
  • Skin: One of the more common myths regarding water intake is the improvement of the skin or it’s complexion. Numerous lay sources such as beauty and health magazines as well as “the Internetz” suggest that drinking 8–10 glasses of water a day will “flush toxins from the skin” and “give a glowing complexion” despite a general lack of evidence to support these proposals. However, water intake, particularly in individuals with low initial water intake, can improve skin thickness and density and can improve skin hydration. Adequate skin hydration, however, is not sufficient to prevent wrinkles or other signs of aging, which are related to genetics, smoking (stop it!), sun exposure and environmental damage.
  • Chronic diseases: Many chronic diseases have multifactorial origins. In particular, differences in lifestyle and the impact of environment are known to be involved and constitute risk factors that are still being evaluated. There is strong evidence showing that good hydration reduces the risk of urinary stone formation. Less strong evidence links good hydration with reduced incidence of constipation, exercise asthma, hypertonic dehydration in the infant, and hyperglycemia in diabetic ketoacidosis. Good hydration is associated with a reduction in urinary tract infections, hypertension, fatal coronary heart disease, venous thromboembolism, and cerebral infarct but all these effects need to be confirmed by clinical trials (more detail here).

In general, provision of water is beneficial in those with a water deficit, but little research supports the notion that additional water in adequately hydrated individuals confers any benefit. As often, more is not always more.

effects of water consumption on overall energy intake

There is an extensive literature that focuses on the impact of sugar-sweetened beverages on weight and risk of obesity, diabetes and heart disease (spoiler: it’s bad); however the perspective of providing more water and its impact on health has not been examined yet.

We know though that in general drinking water, when replacing sugar-sweetened beverages, juice and milk is linked with reduced energy intake. Or vice versa, consuming caloric beverages increases total energy intake. At first, this finding seems little surprising, but it shows that our body is quite bad at registering the calories we consume by drinking beverages, making it incredibly easy to overconsume calories.

Differences in total energy intake when (sugar) sweetened beverages displace water. Note the almost 8% increase in calories consumed in adults. Source: Daniels et al. 2010.

Yes, this also works as an intervention for weight loss, as shown by a German study that promoted drinking water to school children, reducing their risk of overweight by 31%. Eliminating caloric beverages from your life is an important part in managing your weight!

How much SHOULD YOU drink?

Despite its critical importance in health and nutrition, the array of available research that serves as a basis for determining requirements for water or fluid intake, is limited compared to most other nutrients.

Given the extreme variability in water needs that are not solely based on differences in metabolism, but also on environmental conditions and activities, there is not a single level of water intake that would assure adequate hydration and optimum health for everyone. Hydration is individual!

Water intake needs to match needs imposed by not only metabolism and environmental conditions, but also body size, gender, age and physical activity. Those are well studied factors which allow a rather precise calculation of energy intake requirements, suggesting the link between water intake and calorie intake as it is already used in clinical settings.

Adequate Intake (AI) of water for different populations. Popkin et al. 2010

We have no acceptable biomarkers of hydration status at the population level. There is no equivalent to a BMI for hydration. This represents a topic understudied though certainly scholars are focused on attempting to create biomarkers for measurement of hydration status.

All of that being said, the average person will typically take care of their hydration needs by letting thirst be their guide and paying attention to urine color. If urine is darker and/or you’re incredibly thirsty, chances are you need to drink some more water. If you’re not very thirsty at all and your urine is transparent, then it’s safe to say that you’re well hydrated at that point in time.

What to drink?

We now know pretty much everything about drinking, but what quenches our thirst best and keeps us hydrated the longest? In lay media we can find plenty of claims about drinks and potions that claim to do wonders to our hydration, but what actually works best? Luckily we have a great study that shines some light on this.

Source: mysportscience.com

As you can see, there is little difference in tap water, soft drinks and even sports drinks. Even lager, coffee and tea are as hydrating as good, old water, despite what you might have heard. On the other hand, orange juice (and I suppose other fruit and vegetable juices) and milk are the true stars of hydration, although their relatively high energy content warrant caution in their consumption.


I think the whole “hydration is soooo important”-thing is massively overblown by media. Yes, proper hydration is important, but usually just drinking ad libitum (as you like) to quench your thirst is enough. Drinking more just for the sake of it, offers no benefits.

However, if you are an endurance athlete or a little older, you should pay more attention to hydration. Matching your water intake to your calories might be a better idea than guidelines like “8 glasses of water” that you usually find, as it considers activity and metabolism.

Water should be the go-to beverage to quench your thirst. If you really want to make sure you stay hydrated longer (endurance sports), fruit juices and milk come in handy. Make sure you gradually introduce them to your training, as not everybody can stomach caloric beverages during exertion and you may have to experiment a little until you find out what works for you and what doesn’t.

Switching from caloric beverages to water or calorie free beverages is a low hanging fruit for everybody who wants to drop some weight. It is easily done and helps tremendously in weight control. It may be a little weird to “only” drink water if you aren’t used to it, but it is just a matter of a little time. Trust me, in my “fat days” as a kid I only drank lemonade and now I wonder how I managed to even drink that sticky, sweet stuff by the litre. On the other hand, if you struggle with gaining weight, caloric beverages make it incredibly easy to get lots of macros in.

I hope I’ve now answered all of your questions regarding hydration. If you are still uncertain, don’t be shy, leave a comment with your question and I’ll be happy to answer it as soon as possible! Until then, stay strong and eat well!

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