How much water do you actually need?

Where did eight glasses a day come from?

The eight-glasses idea most likely comes from a 1945 recommendation that also counted the water in food. Looking for the origin in The BMJ (2007), Vreeman and Carroll found a suggested allowance of about 2.5 liters of water a day for adults in most instances. The same passage goes on, “Most of this quantity is contained in prepared foods.” Without that last sentence, the recommendation reads as an instruction to drink eight glasses a day.

Heinz Valtin looked for evidence rather than an origin. He reported in the American Journal of Physiology: Regulatory, Integrative and Comparative Physiology (2002) that “no scientific studies were found in support of 8 x 8.” Valtin limited that conclusion to healthy adults in a temperate climate leading a largely sedentary life. He added that some diseases call for larger intakes, as do vigorous work and exercise, especially in hot climates.

What do the official water figures count?

The official water figures count total water: drinking water, other beverages and the water in food. The Institute of Medicine’s 2005 report on water and electrolytes set an Adequate Intake for total water of 3.7 liters a day for men and 2.7 liters for women aged 19 to 30. Both figures are median intakes in US survey data (Dietary Reference Intakes, summary). The chapter on water states that total water intake “includes drinking water, water in beverages, and water that is part of food” (Dietary Reference Intakes, chapter 4).

An Adequate Intake is not a requirement. The report says no single level of water intake would ensure adequate hydration and optimal health for half of all apparently healthy people in all conditions. The report adds that the Adequate Intake “should not be interpreted as a specific requirement.” Announcing it, the National Academies put the practical point plainly: “The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide.”

The National Library of Medicine says the same in Water in diet: there is no specific recommendation for how much water you should drink. Drinking fluids when you are thirsty and having beverages with meals should keep you hydrated.

What changes how much water you need?

Activity, heat, altitude, pregnancy, breastfeeding, illness and some medications all change how much water a person needs.

• Activity and heatThe report states that higher intakes of total water are required for people who are physically active or exposed to hot environments. At altitudes above about 4,300 meters, respiratory losses rise by roughly 200 milliliters a day.
• Pregnancy and breastfeedingThe Adequate Intake for total water is 3.0 liters a day in pregnancy and 3.8 liters during lactation.
• IllnessMedlinePlus lists diarrhea, vomiting, heavy sweating, urinating a lot (which some medicines and illnesses cause) and fever as causes of losing more fluid than usual. It also notes that some people lose their sense of thirst as they age.
• Medications and medical conditionsSome change fluid needs in either direction, and some come with fluid limits. That answer belongs with your doctor or pharmacist.
• Work in the heatThirst is not always a reliable prompt here. OSHA advises that workers in the heat be encouraged to drink water frequently, “not just if they are thirsty.”

Can you drink too little, or too much?

Both are possible. Pooling 33 studies in Medicine & Science in Sports & Exercise (2018), Wittbrodt and Millard-Stafford found that dehydration impaired cognitive performance when water deficits exceeded 2% of body mass. Attention, executive function and motor coordination were affected most. The American College of Sports Medicine sets the same line for exercise, aiming to prevent losses beyond 2% of body weight. Because sweat rates vary widely between people, it recommends customized fluid plans.

Drinking too much is less common and still real. The 2005 report notes that healthy kidneys can excrete excess water at up to about 0.7 liters an hour. StatPearls defines exercise-associated hyponatremia as a blood sodium level below 135 mmol/L, during or up to 24 hours after physical activity. Exertion itself can trigger antidiuretic hormone release, which makes the body hold on to water.

Do coffee and tea count toward your water intake?

Coffee and tea count. In a crossover trial reported in PLOS ONE (2014), 50 men who habitually drank coffee had four cups a day or the same volume of water for three days each. Total body water, 24-hour urine volume and the other hydration markers showed no differences between the two. The participants were men used to caffeine, which limits how far the result reaches.

Does water before meals help with weight loss?

Water before meals showed a modest effect in one trial of older adults who were already dieting. In Obesity (2010), 48 adults aged 55 to 75 on a reduced-calorie diet drank 500 ml of water before each main meal for 12 weeks. They lost about 2 kg more than those on the diet alone. The drop in how much they ate at a test meal after drinking water was significant at the start but not at week 12. The trial does not show that drinking water causes weight loss on its own.

What hydration research supports