You've no doubt heard this phrase in the fitness community:
"Diet is 70% of your results!"
Nobody will argue the importance of diet when it comes to achieving your physique goals. A nutrition plan that's not in line with your goal will make it much harder to reach it.
If your objective is to get shredded and you're consuming too much food or the wrong kind of foods it'll make the process harder. If you're mostly after size and strength, then not consuming enough protein and overall food will slow the process down.
So the overarching message of this quote is good – if you want to progress optimally, pay attention to how you eat. Problem is, this old saying is not very accurate.
Not So Fast
"Diet is 70%" means that training is only responsible for 30% of your gains. Heck, it's actually less than that because we must also factor in sleep and supplementation when it comes to getting results.
I prefer what Dorian Yates said: "Nutrition is 100%. Training is 100%. Recovery is 100%." Basically if you want maximum results, everything counts.
The one you're NOT paying attention to will be responsible for your lack of progress.
The "70%" quote can also lead to diet extremism. People who get too obsessive about eating perfectly believe they'll lose their gains if they stray every once in a while. While you should definitely shoot for eating as much healthy food as possible, you don't have to be that OCD about it.
Studies have shown that as long as protein intake is high and caloric intake falls within a reasonable range, your nutrition plan will work. I always joke that I could get someone in bodybuilding contest shape by eating only fast food and having protein shakes. For health and longevity reasons, this isn't a great idea of course.
Nutrition Is Not More Important Than Training
Training triggers muscle growth; food alone can't do it. Training makes you strong; food doesn't, unless you're deadlifting crates of frozen chicken breasts.
And while your diet is crucial for fat loss, it's the training that helps you retain muscle. Muscle is the main contributor to the power of your metabolism, and you can't "eat on" muscle without training.
A perfect diet without hard training will never yield optimal results. And hard training with an improper diet also won't lead to the best progress. So don't make up percentages and attribute them to either element.