Some sports drinks, and even vegetables claim to be 'more hydrating than water'. What does this even mean? How is 'hydration' even measured? If I ate enough cucumbers could I survive without ever drinking anything at all?
They seem to measure hydration by seeing how much of the fluid stays in the body instead of being secreted.
For milk there a study with describes the process:
Urine samples were collected before and for 5 h after exercise to assess fluid balance. Urine excretion over the recovery period did not change during the milk trials whereas there was a marked increase in output between 1 and 2 h after drinking water and the sports drink. Cumulative urine output was less after the milk drinks were consumed (611 (sd 207) and 550 (sd 141) ml for milk and milk with added sodium, respectively, compared to 1184 (sd 321) and 1205 (sd 142) ml for the water and sports drink; P < 0·001). Subjects remained in net positive fluid balance or euhydrated throughout the recovery period after drinking the milk drinks but returned to net negative fluid balance 1 h after drinking the other drinks.
Question: Can anything be more hydrating than plain water?
- Yes, fluids that contain certain amount of sugars and sodium can be more hydrating than plain water, which can be beneficial in prevention or treatment of dehydration during prolonged endurance exercise (Nutrition & Metabolism, 2009).
- In everyday life, when you are not dehydrated and when you are not very physically active, there is probably no practical benefit from regular drinking "more hydrating" beverages, which may be even harmful, because of potentially excessive sugar and sodium intake.
Question: How can be a hydration potential of a beverage measured?
- By measuring the speed of absorption of water from a certain beverage. For example, water from sports drinks with 4-8% of sugar can be absorbed faster than water from plain water, so they are intended for hydration during exercise (Journal of Athletic Training, 2000).
- By measuring fluid retention, that is the time during which the water from a beverage stays in your body before being excreted (IJSNEM, 2017). Sports drinks with sufficient amount of sodium and oral rehydration solution (ORS) can improve water retention.
1) What is good (appropriate) hydration of the body and how it can be evaluated?
"At home" you can say that you are well hydrated when you don't have symptoms of dehydration, which include dry mouth, thirst, strong yellow urine, prolonged skin turgor and sudden loss of body weight (1 L water = 1 kg weight) within hours/few days (MedlinePlus).
A doctor can further evaluate your hydration status by a combination of tests, such as the levels of sodium in your blood and urine (but this is beyond the scope of this answer).
2) Can anything be more hydrating than plain water and how can you measure that?
Beverages that contain sugars and sodium can be more hydrating than plain water. Sugars increase the rate of water absorption (but probably not more than ~10%: Fig 1: blue (G0) = plain water, yellow (G3) = a solution with 3% glucose) and sodium prolong the time by which the water stays in your body before being excreted. Both effects have been measured in studies:
The absorption of glucose in the jejunum will lead to increased fluid absorption. Sodium is also important for rehydration after a period of dehydration as sodium helps with fluid retention. (Nutrition and Metabolism, 2009)
The purpose of this study was to examine the efficacy of water and a 50 mmol/L NaCl solution on postexercise rehydration when a standard meal was consumed during rehydration. Eight healthy participants took part in two experimental trials during which they lost 1.5 ± 0.4% of initial body mass via intermittent exercise in the heat. Participants then rehydrated over a 60-min period with water or a 50 mmol/L NaCl solution in a volume equivalent to 150% of their body mass loss during exercise. In addition, a standard meal was ingested during this time which was equivalent to 30% of participants predicted daily energy expenditure. Urine samples were collected before and after exercise and for 3 hr after rehydration. Cumulative urine volume (981 ± 458 ml and 577 ± 345 mL; p = .035) was greater, while percentage fluid retained (50 and 70%) was lower during the water compared with the NaCl trial respectively. (International Journal of Sport Nutrition and Exercise Metabolism, 2017)
3) What exactly can be more hydrating than plain water?
a) Sports drinks
If CHOs are included in the fluid, the concentration should be 4% to 8%. Concentrations higher than 8% slow the rate of fluid absorption.
Small quantities of sodium may enhance palatability and retention...Sodium concentration should be approximately 0.3 to 0.7 g/L.
CHOs = carbohydrates = sugar; 0.3 to 0.7 g sodium/L = 13.2-30 mmol sodium/L; (National Athletic Trainers’ Association Position Statement: Fluid Replacement for Athletes, Journal of Athletic Training, 2000)
b) Oral rehydration solution (ORS), released by World Health Organization is similar to sports drinks, but contains more sodium (75 mmol/L) than most sports drinks. ORS is mainly intended to treat dehydration in children "at home," for example, due to diarrhea. It's main purpose is to prevent hyponatremia, which could develop after drinking high amounts of plain water, which contains almost no sodium.
4. How hydrating can be vegetables and fruits?
Vegetables and fruits, which can contain more than 90% of water, are hydrating, but because they are solid, they pass through the stomach into the small intestine slower than liquids (Colostate.edu), so they are absorbed slower and do not hydrate you as quickly as liquids. But yes, you can get enough water by eating only cucumbers (probably from at least 1.5-2 kg of them per day...).
5. How can sodium be beneficial for hydration if it is known to cause body swelling (edema) and dehydration?
Sodium in beverages, where it is present in relatively low amounts (<1 g/liter) can be beneficial during a prolonged exercise, like a marathon, because:
- it replaces the sodium lost by sweating and prolongs the time during which water stays in your body and thus decreases the frequency of drinking and urination.
- prevents dilutional hyponatremia after drinking large amount of fluid.
Sodium, acutely or regularly consumed in large amounts (>5 g/day) is usually excessive sodium, which can be harmful, because: