Six Hydration Myths: What Studies Actually Say

Myth 1: “You must drink 8 glasses of water a day.”

There’s no scientific basis for the “8×8 rule.” The origin is a 1945 U.S. Food and Nutrition Board recommendation that was misinterpreted — the original text even stated that most of this water comes from food.

Modern research shows hydration needs vary massively based on:

• climate

• sweat rate

• diet

• body size

• activity level

The National Academies now emphasise that fluid needs are individual and should be guided by thirst, activity, and environment. For active people, hydration needs can be far above 2 litres per day. use our hydration calculator to see what you need.

References:

1. Institute of Medicine — Dietary Reference Intakes (water).
   https://www.ncbi.nlm.nih.gov/books/NBK56076/

2. European Food Safety Authority — Water Intake Opinion (2010).
   https://efsa.onlinelibrary.wiley.com/doi/10.2903/j.efsa.2010.1459

Myth 2: “Caffeine dehydrates you.”

This myth is persistent — and wrong for most people.

While caffeine has a mild diuretic effect, multiple studies show that regular caffeine consumers are no more dehydrated than non-consumers. Coffee, tea, and caffeinated beverages do count toward daily hydration.

One study even found that moderate coffee intake hydrates just as effectively as water.

References:

1. Killer et al., 2014 – No Evidence of Dehydration With Moderate Coffee Consumption
   https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0084154

2. Armstrong LE et al., J Appl Physiol (2005). Caffeine and fluid-electrolyte balance.
   https://pubmed.ncbi.nlm.nih.gov/16131696/

Myth 3: “If you’re thirsty, you’re already dehydrated.”

Thirst typically begins when you’re around ~1–2% down in body mass (mild dehydration). That level is common and considered mild; it usually has small effects on performance and little health risk. Thirst is an effective physiological cue for most day-to-day situations. (Athletic contexts sometimes require planned drinking strategies, but even there thirst is a useful guide.)
Thirst is not failure — it’s your body’s normal reminder to drink. For most people and activities, drinking to thirst is safe and effective.

References:

1. Maughan RJ & Shirreffs SM (2010). Dehydration and rehydration in competitive sport. Scand J Med Sci Sports.
   https://pubmed.ncbi.nlm.nih.gov/21029189/

2. Kenefick RW (GSSI Sports Science Exchange). Planned drinking vs drinking to thirst.
   https://pubmed.ncbi.nlm.nih.gov/29368181/

Myth 4: “More water = better.”

Overhydration is real — and dangerous.

Drinking excessive water without electrolytes can dilute blood sodium levels, leading to exercise-associated hyponatremia (EAH). Symptoms include nausea, confusion, headaches, and in severe cases, seizures.

Large endurance events (London Marathon, Ironman triathlons) have documented EAH cases even in recreational runners following “stay ahead of thirst” advice. The safest approach?
Drink to thirst and replace electrolytes when sweating heavily. Our hydration calculator works out your electrolyte need too, and you can read more about why our formulation is optimal here.

References:

1. NEJM – Hyponatremia in Marathon Runners
   https://www.nejm.org/doi/full/10.1056/nejmoa043901

2. Hew-Butler et al., 2015 – Consensus on Overhydration Risk
   https://bjsm.bmj.com/content/49/22/1432

Myth 5: “Salt is bad for you.”

For sedentary populations, excess sodium can contribute to hypertension — but for athletes and heavy sweaters, the story is completely different.

Sweat contains between 400–1,200 mg of sodium per litre depending on the individual. Endurance athletes losing large volumes of sweat often require additional sodium, not less.

A 2023 review found that sodium replacement during exercise improves fluid retention, protects performance, and reduces risk of hyponatremia.
For active people, the problem is usually under-consuming sodium, not over-consuming it. use our ‘hydration calculator’ to make sure you’re getting the correct amount of salt.

References:

1. Institute of Medicine. Dietary Reference Intakes — Sodium.
   https://www.ncbi.nlm.nih.gov/books/NBK538102/

2. Noakes & Speedy – Sodium Loss and Endurance Performance
   https://pubmed.ncbi.nlm.nih.gov/15233599/

Myth 6: “Clear urine means hydrated.”

Urine colour is useful — but not perfect.

• Completely clear urine can actually indicate overhydration.

• A pale straw colour is generally ideal.

• Darker urine may reflect dehydration, supplements, or certain foods.

The American College of Sports Medicine recommends combining urine colour + thirst + body weight changes for a more accurate hydration picture.

References:

1. Armstrong LE (1994). Urinary indices of hydration status. Int J Sport Nutr.
   https://pubmed.ncbi.nlm.nih.gov/7987361/

2. American College of Sports Medicine position stand (2007). Exercise and fluid replacement.
   https://pubmed.ncbi.nlm.nih.gov/17277604/

3. Belasco R et al. (2020). The effect of hydration on urine color objectively assessed. Nutrients (review).
   https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7649145/

Bottom line — how to think about hydration (practically)

• For most everyday situations, drink to thirst, eat normally (food contributes water), and use urine colour/weight checks for extra signals.

• For longer duration, hot, or high-sweat activity, plan to replace both fluid and electrolytes — sodium is especially important.

• Don’t fear moderate caffeine: if you normally drink coffee or tea, it still contributes to hydration.

• Finally: context wins. The advice that’s right for a sedentary office worker isn’t the same as for a marathoner — tune your approach to your activity and environment.