Building muscles involves extensive calorie burning compared to burning fat. Use should use foods which are rich in vitamins, by increasing the intake of super food to enjoy healthy weight gain and youth for long.
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You need a weight gainer like nitrotech or whey powder. Basically, after you workout you need to down some serious protein to help your muscles regenerate the tissue you've torn so they build up thicker and heavier. My brother had great success with nitrotech from GNC but it's pretty pricey... Good Luck