Naturally getting fat by eating daily food? You'll most likely die by clogged arteries if you keep that up. Why one earth would you want to load yourself with fat? Fat cannot transmute into muscle.

First, you want to gain healthy weight, not fat. And healthy weight is muscle. For muscle you'll need protein. You can get them from supplements, or from chickens, beef, eggs and milk. And protein cannot magically turn into muscle either. When you workout, you purposely tear down and damage your muscle. Therefore when your body goes into repair mode, they will use the protein to repair those muscle by making them bigger and stronger so that you cannot damage them as easily. That is when you switch to heavier weight, so your muscle can be damaged and your body can repair it bigger and stronger each time.

supplements isn't a requirement to build muscle mass, but it sure does help you if you know what you are doing. You just need to workout smart and hard, consume proper food and nutrition and have enough rest. That's the 3 basic principles of bodybuilding.