The study conducted by Canadian researchers, reported in the journal Psychology and Health, claims that any person who taxes his or her brain on word games uses up energy needed to exercise.
To reach the conclusion, two groups of volunteers were sent for evening gym exercises for eight weeks but one were set a series of mental tests needing concentration by day, reports The Daily Express.
They did not exercise as hard, said Professor Kathleen Martin Ginis.
She added: "They were more likely to skip their exercise sessions. You only have so much willpower."
The professor said: "Willpower is like a muscle. It needs to be challenged to build itself."