Hydration Statistics: Daily Needs and Performance Impact

The Adequate Intake for total water is 3.7 L a day for men and 2.7 L a day for women aged 19-30

Total water includes drinking water, all other beverages and the water in food. It is an Adequate Intake, set from typical intakes, not a minimum.

Food provides about 19% of total water intake; drinking water and other beverages supply about 81%

From US survey data behind the reference intakes. Roughly a fifth of daily water arrives without being drunk.

The ACSM advises drinking during exercise to prevent a water deficit of more than 2% of body weight

Because sweat rates and sweat electrolytes vary widely between people, the stand recommends customized plans; the change in body weight across a session, plus what you drank, estimates your sweat rate.

In studies of prolonged aerobic exercise, performance was consistently reduced once hypohydration reached 2% of body mass; muscle-function studies found decrements at 3-4%

NATA position statement summarizing the research on hydration and performance.

Adult sweat rates during exercise range from 0.5 to 4.0 L an hour

Fluid needs vary that much between people and conditions, which is why the statement recommends measuring individual sweat rates.

Sweat sodium concentration ranges from 10 to 100 mEq/L, and measured sodium losses during exercise from 0.2 to 7.3 g an hour

Variation this large calls for individual replacement strategies rather than one standard drink.

Drinking about 200 mL every 15 to 20 minutes during exercise may be ideal

Hard to manage in sports with long stretches between breaks. Exercise above 80% of VO2max may slow gastric emptying.

Replacing fluid within a short recovery window (under 4 hours) can take up to 150% of the estimated fluid deficit

The extra volume covers urine produced while rehydrating.

Hypohydration consistently lowered strength by about 2%, power by about 3% and high-intensity endurance by about 10%

Critical review; high-intensity endurance means maximal efforts lasting 30 seconds to 2 minutes. How the loss depends on the degree and rate of water loss remains unclear.

13% of 488 Boston Marathon finishers had hyponatremia (serum sodium 135 mmol/L or less), and 0.6% had critical hyponatremia (120 mmol/L or less)

Blood samples at the finish of the 2002 race. Hyponatremia had emerged as an important cause of race-related death and life-threatening illness.

Runners finishing in more than 4 hours had 7.4 times the odds of hyponatremia of those under 3:30, and weight gain during the race raised the odds 4.2-fold

Weight gain means drinking more than was lost. Drinking more than 3 litres during the race and drinking at every mile were also associated with hyponatremia.

Only large doses of caffeine (at least 250-300 mg) caused a short-term rise in urine output, and only in people who had gone without caffeine for days or weeks

Tolerance develops quickly, and standard servings of tea, coffee and soft drinks appear to have no diuretic action, so caffeinated drinks count toward fluid intake.

Drinking 500 ml of water before each main meal produced a 44% greater decline in weight over 12 weeks on a calorie-reduced diet

48 adults aged 55-75 with a BMI of 25-40; the water group lost about 2 kg more. The pre-meal water cut meal intake at the start of the study.

32.6% of US adults aged 18-64 were inadequately hydrated by urine osmolality

NHANES 2009-2012, 9,528 adults; inadequate hydration meant urine osmolality of 800 mOsm/kg or more. Those adults had a 1.32 higher BMI and 1.59 times the odds of obesity, an association rather than proof of cause.

Dehydration of 2% impairs tasks that require attention, psychomotor skills and immediate memory

Review. Long-term and working memory and executive function held up better, especially when the dehydration came from moderate exercise.

Glycerol hyperhydration retained 7.7 mL/kg more fluid than water alone and improved endurance performance by 2.62%

Meta-analysis; the performance figure rests on only four studies, so the authors say it needs further investigation.

For exercise lasting 2-3 hours, athletes are advised to take in 60 g of carbohydrate an hour; well-trained athletes racing beyond 2.5 hours can use up to 90 g an hour

The 90 g rate needs multiple transportable carbohydrates such as glucose plus fructose. Small amounts may also help shorter, intense efforts of 45-60 minutes.