OK, now for an enterology lesson! In a healthy gut (actually the small intestine − see first figure below) there are tiny, finger-like projections called ‘villi’ that allow the body to absorb nutrients from the food we eat into the blood − the average healthy villus is around 1.0 mm long and around 0.5 mm in diameter.
This figure shows the villi in a healthy gut:
In a child with tropical enteropathy the villi become inflamed and flattened − this is termed ‘villous atrophy’ (see next figure below). With the villi damaged in this way, the body can’t properly absorb all the nutrients from food − a process called ‘malabsorption’.
This figure shows a photomicrograph of healthy villi on the left and one of atrophied villi on the left:
[First three figures from here; last from Google Images here.]
And this is a slightly more detailed explanation (from here):
Good health requires proper digestion and absorption. Digestion is the mechanical and chemical breakdown of the food we eat. As food is digested it needs to be absorbed. Absorption is the process of bringing the nutrients from our gastrointestinal tract into the rest of our body’s tissue. Digestion is initiated when we chew food and begin to break it down with digestive enzymes. Food then enters the stomach where further breakdown occurs due to the presence of hydrochloric acid and pepsin, which together begin the breakdown of proteins. From the stomach the products of digestion enter the small intestine.
The small intestine is called “small” because it is smaller in diameter than the large intestine. However, it is in fact longer and in many ways more crucial to our health than the large intestine. The lining of the small intestine consists of villi − finger-like projections that stick out from the wall of the intestine into the lumen. These villi are between ½ and 1½ mm long, just barely visible to the human eye. On the ends of the villi are microvilli. These two adaptations, villi and microvilli, increase the surface absorption area of the small intestine up to 1,000 fold. It’s estimated that the entire absorptive area of the small intestine is roughly the size of a basketball court [i.e., ~435 sq. m − amazing!].
This total area for absorption can be compromised by any condition that irritates the lining of the small intestine. This leads to poor digestive function and affects many vital structures on the intestinal wall. Inadequate absorption of nutrients is referred to as malabsorption − the inability to get the vital nutrients your body needs delivered to your cells.
So, if a child has malabsorption then most of the nutrients in the food (s)he eats just passes through and out. Thus malabsorption → malnutrition → low weight-for-age and low-height-for-age → impaired cognition and then reduced productivity in adult life. Exactly what’s not needed for socio-economic development in developing countries.