Kwashiorkor: why protein deficiency causes oedema
Kwashiorkor is a disease where a low protein diet with near normal energy intake lowers blood albumin, which mainly explains pitting oedema (swelling in which pressing leaves a dent).
Remember
A diet short of protein, with energy intake near normal, lowers albumin in the blood, which weakens the pull that keeps water in the vessels, and this is mainly why fluid leaks into the tissues as pitting oedema. The swelling can hide loss of muscle.
Kwashiorkor can develop in a child whose diet is low in protein while energy intake is near normal. Severe swelling is often seen around the feet, ankles and abdomen.
Albumin is a major protein in blood plasma. Without enough protein in the diet, the liver makes too little albumin, so its level in the blood falls.
Plasma proteins, especially albumin, give the plasma its oncotic pressure, the pull that holds water inside the blood vessels. When albumin falls, that pull weakens and fluid leaks out into the tissues. The result is pitting oedema: swelling in which pressing leaves a dent.
The fluid can cover up a loss of muscle, so a child can look puffy rather than thin even though muscle has been lost. The liver is fatty and enlarged too, since lipid transport molecules are not made in adequate amounts.
Marasmus is a deficiency of all energy and nutrients, not just protein, and the child is severely wasted and thin without oedema. Remember: kwashiorkor is wet and swollen, marasmus is dry and wasted.
Low albumin is not the whole story. Research suggests that sodium retention, leaky capillaries and damaged connective tissue also play a part, but low plasma protein and the low oncotic pressure that goes with it remain the central explanation.
Key facts
- Cause
- a diet low in protein, with energy intake close to normal, can lead to it.
- Mechanism
- low plasma albumin lowers oncotic pressure, and more fluid then stays in the tissues.
- Oedema
- severe and pitting, often seen around the feet, ankles and abdomen.
- Hidden muscle loss
- the fluid can mask the loss of muscle, so a child can look puffy rather than thin.
- Liver
- a fatty liver, because lipid transport molecules are not made in adequate amounts.
- Marasmus
- a deficiency of all energy and nutrients, with a severely wasted, thin child and no oedema.
Exam traps
- A general shortage of calories describes marasmus, where the child is severely wasted and thin without oedema, so a pure calorie deficit does not explain pitting swelling.
- Heart failure raises the pressure in the capillaries and veins, so fluid is pushed out of the vessels. That makes it a cardiac cause of oedema, not the nutritional cause seen in kwashiorkor.
- Lymphoedema is swelling from blocked or damaged lymphatic vessels, the vessels that drain excess fluid from the tissues. It tends to be firm and is often non-pitting, though it can pit in its early stages, and it is not linked to low blood protein, so it does not explain the generalised swelling of kwashiorkor.
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