Re: gain muscle, keep fat off
calories in vs calories out
you can't make significant muscle gains without putting on some fat because you need to have a surplus calorie intake to increase muscle mass, unfortunately. unless you do it very gradually. if you're looking to bulk up and put on 3-4 kilos in muscle in a few months then you're going to have to put a little bit of fat on. doesn't mean you can't still be fit, if you keep your running and boxing training up.
to avoid putting on extra fat when "bulking" you should try to eat as healthily as possible (lots of lean meat, eggs, vegetables, fruit, good carbohydrates and water)