Why does a stomach become distended due to malnutrition?

Answer 1

deficiency of protein due to malnutrition causes the osmotic pressure of plasma to drop (osmotic pressure prohibits water from diffusion through capillary walls, and it is caused by presence of plasma proteins such as albumin), this increases the influence of hydrostatic pressure ( which opposes osmotic i.e. forces the water out to the interstitial space) that's why in the case of malnutrition excessive fluid accumulates in the abdomen causing distension. this is called Kwashirkor, and it means Outcast in some african language.

Answer 2

It is not the stomach itself, but the abdominal cavity that distends. It is due to ascites, the accumulation of fluid in the abdomen. With severe malnutrition, your serum protein becomes very low. The resultant low oncotic pressure allows fluid to extravasate (move out) from the blood vessels into the tissues and abdominal cavity.

Decades ago I practiced for a short time in Haiti, and we treated many children with a condition known as Kwashiordor, where their caloric intake consisted of sugar cane, giving them enough calories to sustain life, but almost no protein. They looked like living skeletons from the chest up, but had swollen bellies and lower extremeties due to this fluid accumulation.