Hebrew’s expression for “on a full stomach” is a direct translation from the English – על בטן מלאה

.

For example: 

עשיתם כושר פעם על בטן מלאה? זה כואב.

Ever exercised on a full stomach? It hurts.

Notice that בטן

– stomach – is grammatically feminine, as you can tell by the word that describes it – מלאה

, not מלא

.