multivitamins are micronutrients and are not intended to be used for providing energy. They are used to correct certain nutritional deficiencies like sore angle of mouth, red tongue dry and scaly skin and certain bone disorders like rickets. In normal health person taking balanced diet which includes carbohydrates fats and proteins and mineral and vitamin rich fruits and vegetables there is no need to take multivitamins. In order to increase your muscle mass and weight you have to take protein rich foods like potatoes nuts meat eggs dhal varieties etc and muscle toning weight bearing and stretching exercises. consult your nutritionist for more details.