Does diet really influence 80% of your fitness objective? I don't really know of any empirical evidence. I'm the last person to admit that I am academically inclined. But it does seem true.
Take fat loss for example. It's pretty much simple mathematics. Calories in should be less than calories out. Either limit your calorie intake or maintain the calorie intake but exercise more. Either way you're bound to lose weight.
Muscle gain is similar as well. You need to take more calories in order to build more muscles. Oftentimes in the past I have gassed out because I didn't take enough food for an intense workout. And then sometimes I didn't even eat after a workout. Which is stupid. So I maintained the same size for ages and ages. The weights I carry, while it has gotten heavier, wasn't as progressive as it should be.
But since ramping out my food intake. Forcing my tiny tummy to take in more food (it's all about strategy), I've also discovered that I don't gas out as quickly, I could lift heavier weights and so far... I have not gained much fat (if any).
By the way, I just weighed myself a few hours ago at the good old clinic. I'm 48kg now. Gained 3kg of what I believe to be mostly muscle (don't have a body fat caliper, but all I can say is I don't have any jiggly bits on me) in about 1.5 months after I ramped up my calorie intake.
So maybe it's true that diet plays a huge part in your training regime:
Eat more = More energy
More energy = Train harder
Train harder = Eat more
Crude equation. I know. But it's the easiest way to explain what I think.