Is it true that exercice can help me gain weight?

Weight gain: Weight gain occurs when your caloric intake exceeds the rate that you burn up calories. What i think you are talking about is that you are increasing muscle mass. Muscle weighs more then fat so if you exercise and build muscle you will probably weigh more. If you are replacing an area of your body that was fat with muscle your weight will increase while you appear slimmer.

Yes: Eat healthy foods that are high in calories. Eat six meals per day. Work on strength training to build your large muscle groups. Work out with heavier weights with fewer repetitions. Sleep at least 8 to 9 hours per night. Hydrate well with water.