When your body is deformed, it is going to affect the function of the organs inside of the structure. One of the system of organs that gets affected by poor posture is your digestive system.
Often people will try to fix digestive problems through diet alone, and a lot of digestive problems are caused by the food you eat, but not all of them. This is unfortunately a cause for poor digestion that many people are unaware of.
And often this is going to lead to a frustrating process of attempting to fix your digestion problem with a certain diet and getting modest or disappointing results.
If your lower back is overarched, that will put a large amount of abnormal pressure on your digestive system. It is effectively going to press your entire digestive system into your abdominal wall.
Imagine eating a meal after which you lay on your back and someone's puts a brick on your abdomen. How well do you think that you are going to digest and absorb that food?
You might even be able to see how your belly is portruding, if you are lean enough you can be sure that is not fat, it's your digestive system being squished into your abdominal wall.
Other than the direct pressure on your digestive system, the nerves that supply your digestive system can become impinged.
There are a lot of muscles that are part of your digestive system, like the ones that control various openings, empty your stomach into your intestine, or push the digested food through your intestine.
If the electrical supply through the nerves that supply these muscles is not adequate, this muscles can atrophy, become weak, or even paralyze.
This is why sometimes old people lose control over their bowls and blader function, and have to wear diapers.
Also the blood vessels that supply, your digestive system can get kinked by the mechanical deformation of your body. Which will starve those organs of important nutrients. Causing them to malfunction, and large amounts of dead tissue which could lead to infection.