What is Tea and Toast Hyponatremia?

KidneyAide

 The so called tea and toast Hyponatremia has always fascinated us.  The kidney can not expel water without solute (stuff like Urea, Salt, and Potassium).  The classic presentation of this is an elder female who is eating very little but continues to drink plenty of liquids. As we age our ability to dilute the urine becomes impaired1.  If, for example, one can only dilute the urine to 200 mosm/L and they are drinking 2.5 liters/day of liquids, with only 300 mosm of solute/day, this will start to dilute the serum sodium level.  With the example above the first liter of liquid will go out with the first 200 mosm of solute.  Now we have 1.5 liters still to get rid of and only 100 mosm to do it. Another 500 cc out with the last 100 mosm.  Now what?  We are left with 1 liter of liquid.  Insensible losses will help, that is sweat, feces and respirations.  In fact in this scenario those losses may equal 1 liter and we could break even for the day. If this same person, however, drank another liter the dilution effect would begin to take shape.  Recall Serum Sodium=Sodium +Potassium (exchangeable)/Total Body Water. 

If this 50 Kg elder F is half body water, 25 Liters, we can calculate her sodium after that excess liter.  

Na=138 meq/L (L is L of TBW) 

138 x25=3450 meq Na 

Now take 3450 meq Na and divide not by 25 L but by 26 (the extra liter) 

3450 meq Na/26L=132 meq/L 

There you have it Tea and Toast diet leads to Hyponatremia 


References 

1.  

Aging and physiological changes of the kidneys including changes in glomerular filtration rate

Carlos G Musso, Dimitrios G Oreopoulos
Nephron Physiology 119 (Suppl. 1), p1-p5, 2011

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