I haven't followed Mentzer's routine, but I do agree with this 100%....as soon as I backed off significantly I realized I can both retain and even build with less. I've also been the strongest when I back off. Also I liked that Mentzer said that the outrageous amounts of food bodybuilders consume is unecessary. You don't need 6000-8000 calories per day, just need slightly increased protein intake and that's sufficient for growth....I also find this to be true.
Mentzer's advice has sent more people to the hospital than it has to the posing dais. Besides, HE (and his brother) may not have needed higher calories. But other people do. I would have never even broken 200 lbs, had I not pushed my calories beyond the 5000 calories mark in my younger years.
Some people can grow on lower calories; other need higher amounts. Plus, once you get to a certain size (and a certain age), you don't need as much. I certain don't need as much as I did 10-15 years ago (thank goodness!!).
If you're skinny, steak/ground beef, whole eggs and milk should be on one side of your diet and weight gainer shakes should be on the other. Throw in the veggies to get the fiber, vitamins, and minerals; and business will pick up.
It's about food, FOOD, and more FOOD. When you're not eating it, you should be drinking it. That's the basics for size.
You'd be amazed how many natural guys turn out not to be such "hard gainers" after all, when they take such an approach.