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We all know that there are important benefits to good hydration. No question there.

I recently heard about a study (unfortunately I didn't catch the author's name) concluding that water is not close to an optimal hydration solution. The report was riddled with pseudo-sciency sounding stuff ("Plain water doesn't hydrate you, it goes right through you into the urine.") and I thought it was one of the worst reports I'd heard in a while.

While trying to find this study online today, I realized that while there are many ways to evaluate "hydration", from simple (thirst, urinary output, urine specific gravity) to complex (serum osmolality, isotope dilution, neutron activation analysis (?), bioelectrical impedance, and others), I am only familiar with a few of these.

Assuming an average healthy person, especially with regard to kidney function, is there any consensus on what the optimal way to measure "hydration" is?

Edited to add: For simplicity's sake, consider age to be irrelevant to this question, as with assuming normal kidney function.

Edited to clarify my intention with this question: I'm not asking about people in obvious need of fluids and how to measure success of rehydration. I'm asking more about average healthy people who know the benefits of hydration and therefore try to stay hydrated. Is there a best way to measure that they are, indeed, optimally hydrated? Sorry about the confusion.

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    This is a very good q, as in general we tend to take multiple factors into consideration to judge level of hydration, but I have not looked at the research for what factor is evidenced as the strongest indicator. – DoctorWhom Aug 27 '19 at 6:56
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The claim that "water is not close to an optimal hydration solution because it goes right through you into the urine" is grossly exaggerated. Water is just good for hydration; even if somewhat more effective beverages are available, they are not really necessary for everyday needs.

Sugars speed up water absorption.

Fructose stimulated 66-100 per cent as much net sodium and water absorption as glucose. (The Journal of Clinical Investigation, 1975)

In conclusion, solutions with multiple substrates [glucose, fructose] stimulate several different solute absorption mechanisms yielding greater water absorption than solutions with only one substrate. (Medicine and Science in Sport and Exercise, 1995)

Sodium promotes water retention.

Sodium does not stimulate water absorption but makes the water to stay longer in your body:

...increasing the amount of sodium (0–60 mmol/L) in a 6% glucose beverage did not lead to increases in fluid delivery...Sodium is also important for rehydration after a period of dehydration as sodium helps with fluid retention. (Nutrition & Metabolism, 2009)

In indoor environments, performing routine activities and even without excessive sweating, isotonic beverages [containing sodium, chloride, potassium and sugars] may be more effective at retaining fluids and maintaining hydration status by up to 10% compared to distilled water (Nutrients, 2017).

Drinking sports drinks with sugars and sodium can make sense in certain sports when quick and sustained hydration is needed. Oral rehydration solution, which contain sugar and sodium are better than plain water for treatment of severe dehydration: to prevent water intoxication (dilutional hyponatremia), especially in infants.

Plain water does not just go through you but stays in your body for a shorter time than water with added sugars and sodium. Still, for everyday use, plain water is good enough, because you can get sugar and sodium from food. Saying that, a little bit of sodium in some mineral waters can greatly improve their taste.

How can you estimate hydration status on your own?

The signs of good hydration include normal moist in mouth, no thirst, colorless or straw yellow urine, no drop of body weight within few hours or days and normal skin turgor, which means that the skin at the back of your hand flattens immediately after being pinched and released (Encyclopedia.com).

Interestingly, thirst can be an unreliable sign of dehydration - some people can be severely dehydrated without being thirsty (Mayo Clinic). The most measurable sign of dehydration is a sudden weight loss (more than 1 kg within few hours to few days).

How can a doctor evaluate dehydration?

A doctor needs to evaluate the severity and type of dehydration. Severity can be quickly estimated by weight loss: 1-3% loss = mild, 4-6% = moderate, 7% or more = severe (Emedicine).

The severity and type of dehydration can be further estimated by a combination of blood and urine tests; results can be completely different in isotonic, hypertonic and hypotonic dehydration.

Isotonic dehydration: (Lecturio)

  • Blood osmolality: 285-295 mOsm/kg (normal range)
  • Blood sodium: 135-145 mmol/liter
  • 24-hour urine volume: <800 mL/24 h (ADAM)
  • Urine specific gravity: increased

Possible causes: excessive sweating, repeated vomiting, diarrhea, severe bleeding, burns (ResearchGate)

Hypertonic dehydration: (Lecturio)

  • Blood osmolality: >300 mOsm/kg
  • Blood sodium >150 mmol/liter
  • 24-hour urine volume: decreased (in water deprivation) or increased (in diabetes mellitus or insipidus)
  • Urine specific gravity: increased (in water deprivation, diabetes mellitus) or decreased (in diabetes insipidus) (Journal of General Internal Medicine)

Possible causes: water deprivation, excessive sweating, drinking sea water, hyperventilation, diabetes mellitus or insipidus, ketoacidosis, end-stage renal failure, certain diuretics (Lecturio, ResearchGate)

Hypotonic dehydration: (Lecturio)

  • Blood osmolality: <275 mOsm/kg
  • Blood sodium <135 mmol/L
  • 24-hour urine volume: increased or decreased
  • Urine specific gravity: decreased or increased

Possible causes: treating dehydration in small children or marathon runners with fluids that contain little or no sodium, gastrointestinal obstruction or fistula, pancreatitis, Addison’s disease, chronic malnutrition, cystic fibrosis with excessive salt loss in sweat, salt-wasting nephropathy, prolonged treatment of high blood pressure with low-sodium diet and thiazide diuretics ( hydrochlorothiazide), furosemide, osmotic diuretics (mannitol) (Lecturio, ResearchGate)

In most cases of dehydration, the blood urea nitrogen (BUN)/creatinine ratio will be increased to >20:1 (ScienceDirect, Rochester.edu).

Total body water

There are various methods to measure total body water, such as flowing afterglow–mass spectrometry (AJCN, 2002), but they are not used to evaluate hydration status, because this can not be determined just by knowing the absolute or relative amount of water in the body, which vary greatly with age, body weight and the percent of fat and muscle tissue.

Accurate measurements of total body water (TBW) are of value in many physiologic and pathophysiologic circumstances. A fundamental aspect of body composition, TBW is influenced by nutritional status (1) and water homeostasis (2). In renal medicine, where both these aspects of body composition can be adversely affected (3), TBW is of particular importance because it also determines the volume of distribution of water-soluble uremic toxins (4).

In conclusion:

  • Plain water can be just fine for optimal hydration. Adding sugars and glucose into it can promote water absorption and retention (longer stay of water in your body), but this is not relevant for an otherweise healthy person in the everyday life.
  • When the hydration status is tested, the results from a combination of blood and urine tests need to be considered.
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  • Comments are not for extended discussion; this conversation has been moved to chat. – Carey Gregory Aug 27 '19 at 22:36
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    Unclear why my comment challenging your claim ("thirst may be an unreliable sign of dehydration") was removed. I stated thirst in normal healthy people not in the extremes of age is a reliable indicator of hydration status.Your Mayo article states thirst may be unreliable in the extremes of age and some other (unspecified) individuals and go on to discuss diarrhea, etc. There is no reason to suspect they are not referring to people with certain medical conditions. Cherry picking to support iffy claims is not the safest course of action medically. – anongoodnurse 42 mins ago – anongoodnurse Aug 28 '19 at 23:46
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    I even sourced it: Are we being drowned in hydration advice? Thirsty for more?: "Key points: Ad libitum drinking seems appropriate in most exercise and environmental settings, but in special circumstances of obligatory hypohydration, anticipatory drinking is warranted." – anongoodnurse Aug 29 '19 at 0:13
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    @anongoodnurse I have no explanation. I didn't delete them and they don't appear among deleted comments. – Carey Gregory Aug 29 '19 at 13:58
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    @BryanKrause - Ah, thank you! Thank you too, Carey Gregory! – anongoodnurse Aug 29 '19 at 20:32

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