Will yeast make my mother gain weight? please answer asap!?

My mother got very sick and someone told her she should drink 1 teaspoon of yeast every morning in a glass of water. But we're afraid it makes her gain weight. I don't know anything about yeast and wikipedia doesn't really answer my question. My logic tells me, it makes bread swell, it'll do the same to her? I need to be sure that it won't make her gain weight. If anyone knows, please help.