I was cycling with a group, it wasn't a particularlly hard ride and it wasn't terribly hot outside. During a break one of the riders didn't look good at all and commented that he didn't feel well. We offered him some fluids and some food, which he declined.

Now there could have been an infinite number of things wrong with this guy but it raised a question in my head...

What are the symptoms of dehydration and can you visually tell if someone is dehydrated?

  • If they're declining water then that's usually the best sign they're not dehydrated. Possibly just unfit and a bit embarrassed about it
    – Chris S
    Mar 9, 2011 at 17:48
  • 1
    @ChrisS The body actually tends to reject water when you are very dehydrated. Source: Been there, done that, got the IV scar.
    – mootinator
    Mar 9, 2011 at 18:15
  • Could someone add a [cycling] tag? Mar 9, 2011 at 19:02
  • 1
    @neilfein - I don't think a cycling tag would be appropriate for this question, it could have been any exercise and isn't important to the question.
    – Walter
    Mar 9, 2011 at 19:03
  • This site will undoubtedly get a lot of people from the Bicycles site, and a [cycling] tag would help us find questions that relate to the activity we love. Mar 9, 2011 at 19:05

4 Answers 4


This is by no means definitive, but often a skin turgor test is performed to check for dehydration.

To determine skin turgor, the health care provider grasps the skin on the back of the hand, lower arm, or abdomen between two fingers so that it is tented up. The skin is held for a few seconds then released.

Skin with normal turgor snaps rapidly back to its normal position. Skin with decreased turgor remains elevated and returns slowly to its normal position.

Source: http://www.nlm.nih.gov/medlineplus/ency/article/003281.htm

  • 1
    I agree with Benjamin, this is a quick and effective way of visually telling if someone is dehydrated.
    – 에이바
    Mar 9, 2011 at 19:11

Cramps would be a dead giveaway especially while riding a bike. Some other signs to look for are confusion, dizziness, tiredness, irritability, and a swollen tongue. None of these are necessary or sufficient to diagnose someone as dehydrated however. You'll need to play it by ear.

A more serious condition that you should certainly be on the look out is heat stroke. This will develop quite quickly in individuals that are dehydrated, out in the head, and exercising. Symptoms of heat stroke are the same as I listed above, but most easily identified by a lack of sweat. Once somebody has developed heat stroke they are no longer able to cool their body and require immediate attention (cold water, shade, fan, etc).


The signs and symptoms of dehydration and the signs and symptoms of hyponatremia are very similar. However, they can both be fatal if not treated and the treatment is very different for both. In fact, the treatment for one can cause death if the problem is the other.

This whole topic is how to keep water and salt balanced in the body. Putting one or both out of balance causes various problems.

  • Dehydration is lower than normal amounts of water in the body.
  • Hyponatremia is lower than normal amounts of sodium in the body (and is normally associated with hypervolemia (too much water in the body) because the athlete has been over-drinking).

Ultra-runners know of these dangers and carefully monitor the fluid they take and supplement with salt tablets. For example, @Sparafusile said, "Cramps would be a dead giveway". I would disagree. If I see cramps then I would be wondering if the salt balance was high enough (because the body uses sodium and potassium as part of the signalling pathway for muscle contractions - along with calcium and magnesium in lower concentrations).

The best way to learn how to manage these issues is the nine-way hydration/salt-balance table from the scientist Karl King. This table shows the interaction between hydration being low, ok or high and electrolytes being low, ok or high (hence nine-way).

In determining dehydration vs hyponatremia I would be looking at weight changes and puffiness in fingers. If in doubt, I would transport the athlete to hospital and make sure the serum sodium levels were taken before the medical staff assume dehydration (often they treat for dehydration instead of hyponatremia and the patient dies).

The scientist Lulu Weschler posts on the Ultra mailing list quite a lot and was a co-author on the 2008 consensus statement on Exercise Associated Hyponatremia. I will list some of her papers here from her email to that list on Dec 14, 2008. That email is available in the password-protected but free Ultra mailing list archive. In that email she gives a nice summary but I won't breach her copyright by repeating it here without her permission.

  • Hew-Butler T, Jordaan E, Stuempfle KJ, Speedy DB, Siegel AJ, Noakes TD, Soldin SJ, and Verbalis JG. Osmotic and Non-Osmotic Regulation of Arginine Vasopressin during Prolonged Endurance Exercise. J Clin Endocrinol Metab 2008.
  • Hew-Butler T, Noakes TD, and Siegel AJ. Practical management of exercise-associated hyponatremic encephalopathy: the sodium paradox of non-osmotic vasopressin secretion. Clin J Sport Med 18: 350-354, 2008.
  • Rae DE, Knobel GJ, Mann T, Swart J, Tucker R, and Noakes TD. Heatstroke during Endurance Exercise: Is There Evidence for Excessive Endothermy? Med Sci Sports Exerc 40: 1193-1204, 2008.
  • Hew-Butler T, Ayus JC, Kipps C, Maughan RJ, Mettler S, Meeuwisse WH, Page AJ, Reid SA, Rehrer NJ, Roberts WO, Rogers IR, Rosner MH, Siegel AJ, Speedy DB, Stuempfle KJ, Verbalis JG, Weschler LB, and Wharam P. Statement of the Second International Exercise-Associated Hyponatremia Consensus Development Conference, New Zealand, 2007. Clin J Sport Med 18: 111-121, 2008.
  • Weschler LB. Sweat electrolyte concentrations obtained from within occlusive coverings are falsely high because sweat itself leaches skin electrolytes. J Appl Physiol 105: 1376-1377, 2008.
  • Hew-Butler T, Noakes TD, Soldin SJ, and Verbalis JG. Acute changes in endocrine and fluid balance markers during high-intensity, steady-state, and prolonged endurance running: unexpected increases in oxytocin and brain natriuretic peptide during exercise. European journal of endocrinology / European Federation of Endocrine Societies 159: 729-737, 2008.

Mayo Clinic offers a great list of symptoms associated with dehydration: http://www.mayoclinic.com/health/dehydration/DS00561/DSECTION=symptoms.

Can you visually tell if someone is dehydrated? That depends. If they show visual symptoms, such as lack of sweat, yes. However, from experience, the answer is not always yes. Occasionally I get dehydrated and my symptoms are not visual. It usually includes headaches and full body aches. A bystander would not be able to assume I was dehydrated just by looking at me.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.