Milk and all its byproducts have always been considered to be a bodybuilder's weight-gaining staple. Many people have actually gained
weight just by drinking excess amounts of milk.. and yes, it can be fattening.
Years and years ago many of the physique men drank lots of milk to put on weight, but they only drank large quantities between bodybuilding contests. Then just weeks before a competition they would cut back on their milk-drinking and train with more intensity so that they could maintain their size and bring out a dramatic muscle definition. Now that I think back, I remember reading about Jack Delinger drinking lots of milk to put on additional size along with his muscularity.
Did it work for him? I'd say so since he was one of the few guys who ever beat the legendary bodybuilder Bill Pearl, for the Mr. Universe in London, England. The thing about Jack was that he trained hard all the time. I remember that, when he trained at the old York gym, he never rested too long between exercises or sets, but went from exercise to exercise, especially when he was training for the '49 Mr. America contest. So no matter how much milk he drank he seemed to burn off any excess fat that might have accumulated on his body.
I remember in 1967 IFBB superstar Chuck Sipes saying that he drank a few quarts the night he posed for and won the IFBB Mr. World contest Larry Scott is a big milk drinker too, and who can forget that Vince Gironda shouts loud its praises as well?
On the other hand, I have known of people who drank more milk than any of the guys I just mentioned and still were unable to gain decent weight from their efforts. I can remember one Olympic life/bodybuilder in particular who wanted to go up and lift in a heavier weight Division. He drank milk nearly nonstop all day long every day for six months but showed only a three-pound gain - though if he hadn't drunk the milk he probably wouldn't have made the three-pound gain. It's ironic that after he quit competition and wasn't interested in gaining weight, he put on over 30 pounds of extra bodyweight and then was frantically looking for a way to lose the weight!
If your body can tolerate milk and you want to gain some extra weight, then by all means use skim or low-fat milk… or by buttermilk. These varieties offer all the important ingredients of whole milk and eliminate the fat. Milk is an excellent source of the protein, calcium and minerals that most people need, especially bodybuilders who train very hard. Calcium in fact helps muscles to contract more strongly so developing muscles are aided by ample calcium. It also helps to regulate the heart beat, and it contains a number of very good vitamins. Don't gulp your milk down but sip it slowly.
Remember though, that lots of milk with less than intense and frequent training can result in a layer of adiposity (fatty tissue smoothing out your muscles… more in some, than others.