Demystifying Hydration—and How Much Water We Really Need
The dangers of severe dehydration are well-documented—professional athletes and military personnel largely have avoiding it down to a science. For the rest of us, common sense indicates that staying hydrated is essential, with 64-ounces a day being the standard…though activity levels, diet, gender, age, et al seem to suggest that maybe it’s a bit more nuanced than that. And then there’s the fact that most of us are consistently slightly dehydrated—often without realizing it—and the effects of this mild dehydration are less talked about. Below, hydration expert, Lawrence E. Armstrong, Ph.D., professor, and director of UCONN’s Human Performance Lab, shares some research that surprised us. Beyond the oft-repeated, overly simplified eight-glasses-a-day recommendation, he explains how to be sure you’re actually hydrated, and what the real deal is with water and weight loss.
A Q&A with Lawrence E. Armstrong, Ph.D.
Q
Is the eight-glasses-a-day recommendation valid? I.e. how much water do we really need to be drinking, and can eating fruit, drinking liquids other than water, etc. make a real difference?
A
Every person’s water requirement is different because water needs vary on the basis of body size, activity (exercise or labor), amount and type of foods consumed, and air temperature.
Three international organizations have systematically determined the daily water requirement of humans: the Institute of Medicine, National Academies of Science, USA; the European Food Safety Authority (28 member countries); and the National Health & Medical Research Council of Australia. These organizations published lengthy reports, considered numerous factors, and provided recommendations for the daily water requirements of children, adults, senior citizens, and pregnant/breastfeeding women. The daily adequate intakes (ml/day of all fluids) for these organizations appear in the table below.
Eight 8-ounce glasses of water are equivalent to 1.9 liters (1900 ml) of water. Thus, for adults, the 8 glasses of water recommendation is below the adequate intakes of these three health organizations.
| Group | Age | European Food Safety Authority(ml/day) | Australian National Health & Medical Research Council(ml/day) | U.S. National Academy of Sciences, Institute of Medicine(ml/day) |
|---|---|---|---|---|
| INFANTS | 0-6 months | 680 ml (23 oz) via milk | 700 ml (23.7 oz) from breast milk or formula | 700 ml (23.7 oz) |
| 7-12 months | 800-1000 ml (27.1-33.8 oz) | 800 ml (27.1 oz) from all sources; 600 ml (20.3 oz) as plain water | 800 ml (27.1 oz) | |
| CHILDREN | 1-2 years | 1100-1200 ml (37.2-40.6 oz) | 1000 ml (33.8 oz) | 1300 ml (44 oz) |
| 2-3 years | 1300 ml (44 oz) | 1000-1400 ml (33.8-47.3 oz) | 1300 ml (44 oz) | |
| 4-8 years | 1600 ml (54.1 oz) | 1400-1600 ml (47.3-54.1 oz) | 1700 ml (57.5 oz) | |
| 9-13 years Boys | 2100 ml (71 oz) | 1900 ml (64.2 oz) | 2100 ml (71 oz) | |
| 9-13 years Girls | 1900 ml (64.2 oz) | 1900 ml (64.2 oz) | 2100 ml (71 oz) | |
| 14-18 years Boys | Same as adults | 2700 ml (91.3 oz) | 3300 ml (111.6 oz) | |
| 14-18 years Girls | Same as adults | 2200 ml (74.4 oz) | 2300 ml (77.8 oz) | |
| ADULTS | Men | 2500 ml (84.5 oz) | 3400 ml (115 oz) | 3700 ml (125.1 oz) |
| Women | 2000 ml (67.6 oz) | 2800 ml (94.7 oz) | 2700 ml (91.3 oz) | |
| PREGNANT WOMEN | 2300 ml (77.8 oz) | 2400-3100 ml (81.2-104.8 oz) | 3000 ml (101.4 oz) | |
| LACTATING WOMEN | 2600-2700 ml (87.9-91.3 oz) | 2900-3500 ml (98.1-118.3 oz) | 3800 (128.5 oz) | |
| ELDERLY | Same as adults | Same as adults | Same as adults |
The amounts in this table represent all daily water intake, including beverages (juice, tea, coffee) and water in solid foods (watermelon, soup). Approximately 20-30% of your total water intake is consumed as solid foods. Therefore, to determine the daily adequate intake you should drink in the form of water or beverages, multiply the numbers in the table by 0.8.
Q
How can you tell if you’re adequately hydrated?
A
Hydration changes from day to day, and within a day. Many hydration indices exist. The best method should be simple, inexpensive, and scientifically valid. For the average person, three checks are recommended: thirst, urine color, and body weight change.
Thirst: When you’re thirsty, you’re already 1-2% dehydrated.
Body weight change: For seven consecutive days, weigh yourself when you wake up in the morning. Rather than taking an average weight, look for three similar body weights—this is your body weight baseline. Going forward, check your body weight to see if it changes. A pint is a pound the world around—so if your body weight is one pound less than your baseline weight, you are one pint dehydrated.
Urine color: You can use a chart or estimate the color of your urine. If your urine is pale yellow or the color of straw, that means your body is releasing water and your urine is dilute; you are adequately hydrated. If the urine is dark, your body is retaining water and that means you need to drink more. If your urine is clear every time you measure it or several times a week, you’re probably drinking more than you actually need.
Another way to think of these three methods is as a Venn diagram. Thirst is one of the circles, body weight is a second, and urine is the third circle. If all three are indicating the same thing—that you’re dehydrated—it’s very likely that you are indeed dehydrated.
Q
Is it possible to over-hydrate?
A
Yes. If you consume a large volume of water or a dilute beverage such as a sports drink, it is possible to dilute the body fluids to the point that you become ill. The brain maintains the concentration of blood and intracellular fluids within a narrow range. If the concentration diverges from this preferred range, a variety of functions may be negatively affected (i.e., movement of fuels and waste products into/out of cells, muscle contraction, nerve conduction).
Known as “water intoxication” or “hyponatremia,” this condition is diagnosed by measuring blood sodium levels, and verifying that serum sodium is low (diluted). The signs and symptoms of hyponatremia include lightheadedness and nausea, but this illness usually involves a headache, vomiting, fluid in the lungs, and/or altered mental status (e.g., confusion, seizure) resulting from brain swelling. Death results in severe cases.
The hydration indices (body weight, urine color, thirst) listed above can be employed to determine if you are over-hydrated. Urine that is constantly the color of straw or pale yellow (two to four times per day or more) suggests that you are habitually consuming too much water. It is normal and reasonable to have urine that looks like water a few times each week but don’t overdo it. Similarly, if you never experience thirst during one week, you may be drinking too much water each day.
Q
How do women’s hydration needs change when pregnant or breastfeeding?
A
Due to the baby’s watery environment, it is essential that a pregnant mother consumes adequate water. As shown in the table above, pregnancy increases a woman’s water requirement by 300 ml per day.
Breast milk contains 87% water, and the average milk production is 780 ml/day (700 ml water) during the first 6 months of breastfeeding. As shown in the table above, a breastfeeding woman’s daily water requirement is 600-700 ml/day greater than a woman’s who is not breastfeeding.
The hydration markers described above are valid during pregnancy and breastfeeding.
Q
Can you talk to us about mild dehydration—how it’s defined, how common it is, what the implications are for our systems, both physical and mental?
A
The term “mild dehydration” refers to a body weight loss (i.e., due to water) of less than 2%, and the term “moderate-to-severe” dehydration refers to a body weight loss of more than 2% (as water).
The average adult experiences mild dehydration several times each week, as signaled by thirst. Recent research indicates that men and women experience negative effects on cognitive performance during mild dehydration, in areas like problem-solving, vigilance, mood, headache, and increased task difficulty. Exercise performance is reduced, beginning at approximately 1.5% dehydration, and degrades with increasing dehydration up to a 4% loss of body weight and beyond. This includes muscular endurance, strength, and power.
Q
Are there long-term health issues associated with being consistently dehydrated?
A
Recent epidemiological studies conducted in Scandinavia report that chronic low water consumption is related to an increased risk of developing diabetes, metabolic syndrome, and cardiovascular disease. Other published studies have demonstrated that low daily water consumption is a risk factor for urinary tract infection, kidney stones, and chronic kidney disease.
Q
What’s the antidote to chronic dehydration? Particularly for people who don’t find themselves typically thirsty, is there a way to stoke thirst?
A
If normal individuals are rarely or never thirsty, they likely are drinking an adequate amount of water each day. I’ve never met a person who hasn’t experienced thirst—I think that’s quite rare. But thirst is blunted in elderly individuals. In cases of reduced or abnormal thirst sensations (i.e., elderly men and women), individuals can use daily body weight measurements and urine color to check their hydration status, as described above. On days where they sense thirst, rehydration should be a priority.
To be sure you’re hydrated, use the chart above to set personal hydration goals that you can measure and meet.
Q
What are the best vehicles for hydration besides water? Are electrolyte-y drinks really all that? Is coconut water more hydrating than plain old tap?
A
A recent investigation in the United Kingdom evaluated the effectiveness of various fluids to maintain hydration status, 1-4 hours after intake. Compared to plain water (i.e., the reference standard fluid), the following beverages were superior rehydration agents: whole milk, skim milk, and orange juice. In other words, drinks containing the highest macronutrient and electrolyte contents were the most effective at rehydrating the body. Interestingly, although some fluids often are faulted as “dehydrating the body” or as “diuretics,” the following beverages rehydrated similarly to water and sports drinks: lager (beer), coffee, hot tea, cold tea, cola, and diet cola.
These research findings support a study published by our research team at the University of Connecticut in 2005. We observed that caffeine did not dehydrate young men across 11 days of observations when consumed in amounts up to 452 mg per day (i.e., approximately four 8-oz cups of coffee).
Q
Does water intake have an impact on weight loss?
A
A complex and dynamic network of behavioral, metabolic, and physiological factors influence food intake and weight gain across days, weeks, and months.
Two studies have reported that consuming water with one meal resulted in decreased hunger and increased satiety. Normal weight, overweight, and obese (older) adults consumed fewer calories (one meal, ad libitum food choice), when given a 500-ml water preload, 30 minutes before the meal. Young adults did not consume fewer calories after a water preload. This suggests that age-related differences may exist.
Some studies show greater weight loss after consuming 500 ml water before all meals, whereas other studies show no effect of consuming water before meals. The evidence does not provide a clear answer.
Increasing total daily water intake by 1.0 liter (1.06 qt) throughout a day (added to habitual daily intake) resulted in an increased weight loss of 2.0 kg (4.4 lb), during a 12-month feeding study.
In summary, caloric intake each day is influenced by a complex interaction of the time of water intake, volume of fluid consumed, method of fluid presentation, chronological age, and personal food/fluid preferences. Regarding weight loss and weight management, future research is required. Science cannot adequately explain the ways that water consumption, before or during meals, may help overweight individuals manage their body weight.
