When it comes to getting hydrated, what is your go-to drink? Plain water, coffee, or milk? Sports or energy drinks? What we think we should drink, and how much, are filled with half-truths, much of it from advertising. So, what does science tell us?
Spoiler Alert- Pure water remains the best source of hydration for nearly everyone.
But, many people find plain water “boring” or they think there should be more to it. A myriad of other types of fluids are now available, from sports and energy drinks to coconut water. Aside from taste, are any of these a suitable substitute, in terms of both your health and hydration? How important is it to think about your fluid intake?
We all know that humans are mostly water making up around half of our adult body mass. The body’s balance of water intake and output is tightly regulated to keep the concentration of salts and minerals, or electrolytes, in our blood at a precise level. (see: https://insight.jci.org/articles/view/130949). To prevent dehydration, hormonal and neural mechanisms are activated, stimulating thirst to encourage water intake and increasing water reabsorption by the kidneys to decrease water output.
How much water should you drink?
The question of whether we are drinking enough fluids for optimal health has been a matter of contention for decades. You may have heard that we should drink eight glasses of water per day, but this figure turns out to have no scientific basis. See https://www.newscientist.com/article/mg23331160-600-good-hydrations-water/
The first scientific recommendation on how much water we need to drink came from a rough calculation added to a set of dietary guidelines from the US National Academy of Sciences in 1945. (https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1753-6405.2012.00866.x). The total was reached by estimating that the average male diet involved consuming about 2500 kilocalories of food per day and might require 1 milliliter of water per kilocalorie for digestive purposes, equating to a daily water requirement of 2.5 liters. One problem with this figure is that it arguably overestimates how much water we need to drink because it does not take into account the fluid we ingest from our food, which makes up around 20 to 30 percent of our daily intake.
Nevertheless, these estimates have stuck.
To fully understand how much water is needed by the body each day, last year an international consortium tracked water input and loss in more than 5,500 people by giving them water to drink labeled with an isotope of hydrogen. John Speakman at the University of Aberdeen, UK found a typical man in his mid-20s, in the US or Europe, will require around 50 to 61 ounces per day, and a typical woman will require around 44 to 47 ounces. “These are just averages, so if you’re active and out a lot in hot weather, you will need more.” https://www.science.org/doi/10.1126/science.abm8668
Drinking more water than you need, above thirst, usually isn’t a problem, because our kidneys quickly adjust to produce more urine and expel the excess water. While it is possible to drink such an excess to cause hyponatremia, where the kidneys are unable to cope and the sodium content of the blood becomes dangerously diluted, Speakman says that, in practice, this is rare.
Instead, a more pressing problem is the consequences of repeatedly not meeting our daily water needs. The long-term effects of chronic underhydration are not well studied, but new data is starting to emerge. For example, a 2019 study in mice at the US National Institutes of Health showed that long-term suboptimal hydration caused inflammation, as well as accelerated age-related degeneration of the heart, kidneys, and central nervous system. It also shortened the animals’ lives by around six months, equivalent to 15 years in humans. https://insight.jci.org/articles/view/130949
Chronic underhydration
A recent study investigated the impacts in humans of long-term dehydration, in which the body’s water conservation mechanisms are activated to reduce urine output and blood sodium levels are slightly elevated but still within the normal range. https://www.thelancet.com/journals/ebiom/article/PIIS2352-3964(22)00586-2/fulltext
Using data gathered from 11,255 adults over a 30-year period, the researchers found that those with slightly elevated sodium levels displayed greater signs of biological aging based on how well their heart, lungs, kidneys, and immune systems were functioning.
The detrimental effects of chronic underhydration on long-term health outcomes is a relatively new concept. The study’s author concludes that it seems that insufficient hydration could promote accelerated aging and therefore increase the risk of developing chronic diseases. Other research backs up this view. One 2020 study, for example, found that underhydration in people aged over 50 – as measured by elevated blood sodium levels and/or concentration of salts in urine – is associated with obesity and chronic diseases. https://pubmed.ncbi.nlm.nih.gov/32224908/
Signs of Dehydration
How do you know if you are getting enough fluids? For most people, the first sign that you need fluids is a feeling Unfortunately, the so-called thirst reflex wanes as we get older. Other signs include a dry mouth, tiredness, darker-colored urine, and feeling lightheaded.
There are two main types of dehydration. One occurs when you lose fluid and electrolytes by sweating, for example during exercise, or by vomiting or experiencing diarrhea. The other, known as low-intake dehydration, occurs through not drinking enough. “In this case, your electrolyte levels stay the same, but there’s less fluid, so the concentration of your electrolytes rises across cells,” says Lee Hooper at the University of East Anglia in the UK. “It means that cells shrink a bit and change the environment inside them.”
With So Many Different Drinks Available, Pure Water Remains the Best Solution
Throughout most of human history, people have replenished fluids by drinking plain water. Today, we have an ever-increasing array of different options. Earlier this year, actor Florence Pugh spoke for many when she claimed that water is “too boring to drink”, preferring elderflower-pressed, orange juice, and tea. Scientists agree that while pure water is the best, some of the other options are fine. “Our message is that any [non-alcoholic] fluid is good fluid and if you want to drink tea then that’s adding fluid into your body,” says Jennie Wilson at the University of West London. “The same is the case for coffee. There is scant evidence for it acting as a diuretic and even if it does, that would require consumption of very large amounts.”
But a visit to the supermarket reveals a vast array of drink options that claim to give you a boost in other ways, not just hydration. Known in the industry as “functional beverages”, two main types take up much of the room on store shelves: energy drinks and sports drinks.
Sports drinks are formulated to replace fluids and electrolytes, such as sodium, potassium, and calcium, in the body during and after exercise. Originally developed for athletes, many are now marketed to the general public, though it is questionable whether these drinks offer any notable benefits over water for the average person.
Water is more than sufficient for the casual jogger or someone who goes to the gym a couple of times a week, says David Rowlands at Massey University, New Zealand. “Most of these sports drinks were designed for high performance,” he says. “For most people, exercising half an hour to an hour, they might lose 1 liter of fluid at the most, which is easily, and far more cheaply, replaced with four cups of pure water over a couple of hours following exercise.”
Many sports drinks are isotonic, which means they contain similar concentrations of sugars and salts in blood. However, they are still less effective than water at hydrating the body because the sugars they contain tend to be simple sugars like fructose, glucose, and sucrose, which are rapidly digested as soon as they hit the small intestine, creating a higher concentration of sugars in the gut, says Rowlands. “The problem is that the gut can only absorb so much sugar at a time,” he says. “The maximum rate seems to be around 1 gram per minute. If there’s more sugar than that coming in, the body holds water back, which limits the amount of fluid getting into the body.”
For serious amateur or professional athletes, some studies show that hypotonic drinks – drinks that contain some carbohydrates and salts, but at a lower concentration than in blood –are the best hydration agents. https://link.springer.com/article/10.1007/s40279-021-01558-y. This is because of a phenomenon called solvent drag, where the steady absorption of carbohydrates and electrolytes through the gut helps draw more water into the bloodstream.
Rowlands suggests that coconut water could be an alternative to pure water since it contains low concentrations of carbohydrates and electrolytes, similar to the formulation of hypotonic sports drinks.
Another drink touted for its ability to boost sports performance is beetroot juice, but this hasn’t to do with hydration. “This is more something that serious athletes take because it has a high nitrate content, which improves blood flow to the muscles,” says Rowlands. “There’s some suggestion it can improve performance in high-level amateurs.”
What About Energy Drinks?
Energy drinks are another fast-growing category with nearly $50 billion in sales. Particularly popular among younger people, these are formulated to give an instant energy boost through stimulants such as caffeine, guarana, and taurine. Many also contain a high dose of sugar. Red Bull, for example, contains 27 grams of sugar per 250 milliliter can, almost the entire 30-gram daily recommended maximum sugar intake, as well as 80 milligrams of caffeine, which is roughly the same as that in a cup of coffee.
While energy drinks may keep you alert in the short term, they are associated with poorer sleep quality and are problematic for anyone with a heart condition, due to the increased risk of palpitations or irregular heart rhythms – which is why these drinks carry warnings for people sensitive to caffeine and some are restricted to those over the age of 18. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8083152/
Evidence also suggests that sugary energy drinks may have a dehydrating effect as they are hypertonic, meaning they contain a higher concentration of salt and sugar than in the blood. “Hypertonic drinks actually draw fluid out of the body to aid in digestion,” says Rowlands.
Another current trend in the beverage industry is the addition of ingredients such as ginseng, green tea, turmeric, vitamins, and even protein to drinks. But Hew-Butler is skeptical about the health benefits. “A lot of it is just marketing,” she says. “If you’re eating a good diet, they’re things that you’ll be getting from your food anyway.”
What Are the Risks of Dehydration?
The people who are most likely to experience dehydration probably aren’t the ones rushing out to buy the latest hydration drink to hit the shelves: older adults. One study from earlier this year showed that a quarter of 65 and over people have low-intake dehydration because they do not consume a sufficient amount of fluid every day. https://pubmed.ncbi.nlm.nih.gov/37330324/. This is linked to a range of common health problems, from impaired cognition to urinary tract infections.
One of the reasons why older people are more vulnerable to dehydration is linked to the thirst reflex, which deteriorates with age. https://pubmed.ncbi.nlm.nih.gov/11528342/. In addition, some medications can encourage fluid loss, and many older adults actively choose to drink less due to worries about accessing a toilet or incontinence.
Older people also have smaller body water reserves, as body water is found in muscle which declines with age. They also concentrate urine less well, so they lose additional fluid when they go to the toilet, even when they haven’t drunk enough.
One common recommendation has been to check the color of your urine, as the darker the color, the more dehydrated you are. However, given our kidneys are less able to concentrate urine as we age, this test may mistakenly suggest that you are adequately hydrated, says Hooper.
So, Where Does This Leave Us? Pure Water Remains the Best Choice
The one consistent message from all of the studies and from all of the doctors is that pure water is the best source of hydration. Pure water does not contain any of the risks of things like added sugars and caffeine. Pure water allows your body to naturally process the water without interference from added ingredients. The only exception is the elite athletes who need hypotonic drinks – drinks that contain some carbohydrates and salts, but at a lower concentration than in blood- to maximize hydration.
