Do we need 8 glasses of water a day?

Photo by Jana Sabeth on Unsplash

‘Drink at least 8 glasses of water a day’ — it seems you will be healthier if you’re waterlogged. However, the science has never existed for this slogan, and still doesn’t. Furthermore, how much water a person may ‘need’ depends on age, gender, environment, physical activity, health and a host of other factors unique to the individual. It’s akin to recommending how much air we should breathe.

While the slogan is pervasive, its origins are obscure. Humans have usually seen water as restorative (e.g. ‘the fountain of youth’, spas, springs, ‘taking the waters’) and so we were probably primed for the advice. The first formal system, hydropathy, was developed in the early 1800s by an Austrian farmer (Vincent Priessnitz) who treated his broken ribs by wrapping them tightly in wet cloth. When his ribs healed he assumed it was because of the water and proposed that water could cure pretty much anything (from general malaise to cholera and typhoid).

His system involved cold showers, being doused with buckets of cold water, wrapping in cold wet sheets and then blankets, drinking heroic quantities of water and going on bracing walks. His farm became the first of many health retreats throughout Europe and North America, usually atop mountains, that attracted the rich and presumably revitalised the local funeral industry.

The first mention of a daily water allowance is thought to be in the 1945 dietary guidelines of the Food and Nutrition Board (FNB) of the US National Academy of Sciences, as described by Heinz Valtin:

A suitable allowance of water for adults is 2.5 liters daily in most instances. An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods.

The calculation is based on a daily intake of 2,500 calories. No reference to the ml/calorie ‘standard’ was provided and no clinical study was cited to support the recommendation. Make note of the last sentence though — its message has gone astray somewhere in the intervening years.

The first mention of a specific 8-glass rule has been attributed to the influential nutritionist Frederick Stare (chairman of the Department of Nutrition at the Harvard School of Public Health), also tracked down by Valtin:

How much water each day? This is usually well regulated by various physiological mechanisms, but for the average adult, somewhere around 6 to 8 glasses per 24 hours and this can be in the form of coffee, tea, milk, soft drinks, beer, etc. Fruits and vegetables are also good sources of water.

Again, no reference backing up this assertion was provided. The paragraph appears to be an afterthought at the end of a long book on other aspects of nutrition published in 1974. He is not usually cited by later works supporting the 8-glass rule, so whether this was the forerunner is not certain. In fact, no reference is cited for the rule by its advocates.

However, Stare makes two points that don’t get on the radar these days: Our water levels are ‘usually well regulated by various physiological mechanisms’ and; coffee, tea, alcohol and food are legitimate sources of water (in agreement with the FNB).

By the way, in case you think Stare might have been a visionary ahead of his time, he also urged people to consume more sugar and was an advocate for the nutritional benefits of processed white bread.

That’s the dubious background to this ubiquitous health slogan. A myth of uncertain origin that has become a meme.

In that case, how much water should we drink? Well, your body will tell you because it will signal thirst. When you are no longer thirsty, there is no need to drink more. This applies to most people most of the time, even endurance athletes competing in hot conditions. ‘Drink to thirst’ is the sensible recommendation proposed by Tim Noakes:

Sustained high rates of fluid ingestion […] either at rest or during exercise are neither sustainable by elite athletes nor safe for recreational athletes since they produce symptoms of intestinal distress including nausea, vomiting and on occasion, diarrhea, or progressive fluid overload leading ultimately to death from hyponatremia as has now been reported in at least 12 athletes. In contrast, this author has been unable to track a single published report in the past 20 years of exercise-related death or serious illness due solely to ‘dehydration’.

It was Noakes who described the first case of exercise-associated hyponatremia (from the Latin for sodium, natrium, also giving sodium its symbol Na). It was in a 1981 marathon race. This condition develops because the body cannot rid itself of ingested water quickly enough and sodium concentration falls to a deadly level. The brain is also vulnerable because cells in the body swell, but the brain is encased in its skull and has nowhere to expand to. The 12 deaths became 13 in 2002. All of these deaths were avoidable. Countless more have been made ill or hospitalised over the years.

The athletes were following recommendations from elite sporting bodies to drink the maximum tolerable amount of water before a race and to continue during the race. The advice was first proposed by the American College of Sports Medicine (ACSM). It was later revealed that members of the committee making that recommendation may have been receiving funds from the bottled water/sports drink industry.

As modern distance running increased in popularity, and as amateur athletes ran for longer times (and thus consumed more water), the incidence of hyponatremia increased. The signs at the finishing line (confusion, dizzyness etc) were often taken as signs of dehydration or heat stroke, and more water was administered orally or intravenously, exacerbating the problem. After decades of research and advocacy by Noakes and other scientists, sporting bodies are now changing their advice to ‘drink to thirst’. It took about 20 years to start making headway. The International Marathon Medical Directors Association (IMMDA) and the USA Track&Field (USATF), now both recommend drinking as needed with little or no consideration given to electrolyte replacement.

However, the bottled water/sports drink industry remains influential, and while hyponatremia is not a concern for most of us, industry is likely to be the main factor that keeps the 8-glass message alive and in the public consciousness. It’s a multi-billion dollar industry. Lets do a little calculation:

The 8-glass rule translates to about 2 litres a day. An online price-check for two national (Australian) supermarkets both showed Evian water advertised at $2.20 per litre (so much for competition). Thus, the total yearly cost of adhering to the 8-glass rule by drinking Evian would be $1,606 (AUD). If the rule was followed for a year using tap water, it would cost a total of $1.10 (based on my personal water costs). That’s over a one thousand fold difference (try spelling Evian backwards if you haven’t already). There are various myths surrounding bottled water that I won’t go into, but pause to consider the bottle and the environmental impact of its manufacture, distribution and disposal.

Then there’s the myth that by the time you are thirsty you are already dehydrated. Again, this has been disproven. The body is pretty serious about maintaining its water content, and the thirst mechanism has evolved over millions of years to be finely tuned to our needs. Thirst is a warning sign that is delivered early enough to prevent dehydration.

A related myth is that performance declines if we are even slightly dehydrated — that’s wrong too. It has even been shown to be wrong in professional cyclists racing in heat (33C) and humidity (40%). Modern hunter-gatherers run for hours in heat without taking food or water with them. The first person (Abebe Bikila) to win consecutive gold medals in the Olympic marathon (1960/64) drank nothing.

He also ran both races barefoot (but thats another story).

Another myth is that coffee and tea should not be counted. The diuretic characteristics of these beverages has been over-promoted. In regular casual drinkers, the body adapts and the beverage is not significantly diuretic. Likewise, moderate alcohol consumption (beer and wine) can be included in any estimate of water consumption. Finally, food is high in water — not just food that seems watery. Carrots have the same or higher water content than full-cream milk. Zucchini are higher in water than watermelon (95% vs. 92%). Even bread, baked in the oven, is around 25% water (even when it is stale). As far as the body is concerned, water is water.

No one needs to worry about any of this. Just drink when thirsty.

Further reading:

Heinz Valtin: Is there scientific evidence for “8×8”?
Margaret McCartney: “Waterlogged?
Tim Noakes: “Is drinking to thirst optimum?”, and “Waterlogged - the serious problem of overhydration in endurance sports”
Spero Tsindos “What drove us to drink 2 litres of water a day?



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