Multivitamins don't provide the human body calories that contribute to an increase in fat, so they alone don't cause you to gain weight. In order to ensure proper absorption of vitamins, it's best to consume foods rich in vitamins and nutrients, according to Dana-Farber Cancer Institute.
A human body needs six nutrients in order to remain healthy: fat, carbohydrates, proteins, minerals, vitamins and water, reports FoodPyramid.com. Three of those nutrients, fat, protein and carbohydrates, provide calories. Any vitamin that a human body consumes, whether it comes from a multivitamin or directly from a food source, is noncaloric, and is extremely vital for proper growth, cell function and development.