What nutritional deficiency causes constipation?

The #1 nutritional deficiency behind constipation: a vitamin B1 deficiency. A severe vitamin B1 deficiency is called beriberi.

Constipation is essentially a problem with peristalsis—it’s not working properly, and you’re not pumping waste through the body. The autonomic nervous system is intimately involved with the gut. When you don’t have enough vitamin B1 (thiamine), a lot of things can go wrong with the autonomic nervous system—which then affects the gut.