carbs and hydration

["Beth Glace" <lb@nismat.org>]

Hi Cheryl,
You mentioned that giving carbs seemed to delay urination and to 
perhaps raise heart rate a bit.  Physiologically both scenarios make 
sense since digestion requires a little bit more "work", the heart 
must circulate blood around the digestive track and nutrients must be 
absorbed.  Heart rates and energy expenditure are almost always 
higher after eating, and that isn't  a bad thing if the carbs can 
give you a performance boost.  

Urine production is largely dependent upon the osmotic forces in the 
blood or on the "concentration" of solutes in the blood.  For 
example, when dehydrated the blood volume goes down and this 
increases the osmolality of the blood.  The kidneys respond to this 
stimulus by cutting down on the amount of urine produced, thus 
conserving fluids and preserving blood volume. This increase in blood 
osmolality or concentration also stimulates thirst.   In a similar 
way, ingesting electrolytes or carbohydrates will up the blood's 
osmolality and may temporarily decrease urine production.  In 
essence, both electrolytes and carbs [from "supplements" or 
foodstuffs] can help keep fluids in the system.  We often see large 
increases in blood osmolality after ingestion of a carbohydrate 
beverage, and the subjects report thirst despite consumption of 
substantial volumes of fluids.  They also don't have to urinate right 
away.  If your horse is drinking normally after being dosed with 
carbs, and is temporarily urinating a little less, I would not be too 
concerned.  It would be of much greater concern if he were not 
drinking well AND not urinating.  

Regards,

Beth Glace, MS
Sports Nutritionist
Lenox Hill Hospital 
New York, New York