1 D-mannose is the active ingredient in cranberry juice, but the amount of D-mannose in cranberry juice is significantly less than its supplement form, making cranberry juice much less effective for UTIs.n a study of more than 300 women with a history of recurrent UTIs, researchers treated the women with either two grams of D-mannose, 50 milligrams of an antibiotic, or no treatment daily for six months. D-Mannose is a naturally occurring sugar that's closely related to glucose.