The stomach is not essential for human survival. Many people have survived and adjusted to total or partial surgical removal of the stomach because of diseases like stomach cancer.
The stomach is essentially a reservoir that allows people to eat the quantity of food they want, emulsify it, and pass it gradually into the small intestine.
Without a stomach, one cannot consume the quantity of food one was able to before and usually must have more frequent and smaller meals. There is an initial uncomfortable transition period after the loss of the stomach that may last months before the person can adjust to the loss.
The operation to remove a stomach is called a gastrectomy. It can be either partial or total. Doctors often place an external feeding tube directly into the small intestine, a procedure called a jejunostomy, to provide consistent nutrition during recovery and acclimatization.
If a person without a stomach eats too much, it is very quickly apparent because of an uncomfortable feeling of being very full.
Other possible problems include diarrhea, a condition called dumping syndrome, in which nutrients are moved too rapidly into the small bowel, and upward reflux of bowel contents.