Osmoregulation Question?

Question

some diets call for protein-rich foods and the complete elimination of carbs. people on these diets often complain of frequent urination and increased thirst. what is the likely source of these symptoms?

Answer 1

They are absorbing too many proteins into the blood. The proteins are solutes, and the blood plasma is the solvent. Water migrates from the inside of blood cells (erythrocytes, leukocytes, thrombocytes) to the outside of the cells into the plasma, causing the cells to dry up. This is because water travels from areas of low solute concentration to areas of high solute concentration. This is dehydration, which causes increased thirst. The frequent urination results from the body trying to get rid of these excess solutes (proteins) in the blood plasma in the form of urea.

Answer 2

A key part of this question is that part of the break down product of proteins is nitrogen, unlike carbs, which break doen into carbon, oxygen and hydrogen. Nitrogen mixed in water can form ammonia (NH4), which is toxic, so humans use energy to convert it to urea. Thus, people on a high protein diet have more nitrogen, so more urea, so more urination. It requires water to eliminate the urea, so too much water is lost, so the blood osmolarity becomes higher, and the result is thirst.