It's mostly about net calories, for managing weight loss or gain. That's what most people are interested in. A forty day fast is gonna deplete fat reserves, simple as that. When I hop on an exercise bike and burn through a few hundred calories, that's great. Then if I turn around and go eat a 900 calorie (I'll be talking in terms of the SI "kilo-calorie" unit, like on food labels) meal, what good was it, if I could have eaten a 600 calorie meal that was just as filling? I wasted my time doing cardio. OK, not exactly, your cardiovascular system would be the more healthier for it, but your weight wouldn't change much. To put things into perspective, a pound of fat is around 3500 calories. Burning 10 calories per minute on the exercise bike is a brutal pace, and that's 350 minutes. So you've got six hours or more per week at the gym vs. altering your diet to eat 500 calories below your BMR each day. One seems markedly easier to me than the other. The trick is getting all your micronutrients in on a restricted diet. To finally answer the question; 70% is a reductionist estimation that merely serves to emphasize how altering your diet will probably be the easiest way to see results. Even in the case of trying to gain muscle (or any mass), I would again try to relate the importance of diet.
Thank you! Could you expound on your point of eating a less caloric meal that's just as filling? I think I know that one's feeling of fullness isn't 100% correlated with caloric density, but what meal is two-thirds the calories but just as filling?
Like veen said, eating at a slower pace can help. Our processed food, fast food being the prime example, is so dense in calories that it's easy to consume more than your body required before the signal of "fullness" arrives to the brain from the stomach. And the signal delay makes sense, evolutionarily. A delayed response will give a person the opportunity to overeat, when the food was plentiful enough, and store the excess within their fat reserves for when times get tough. Unfortunately, it's 2015 AD, not BC. Try to eat things that occupy large amounts of volume, even though they're not dense in calories. Vegetables are great. You can make a huge salad, with chicken and a tasty vinaigrette, and you'll only tally up a few hundred calories. Personally, I cheat, and drink a fair amount of muscle milk dissolved in regular skim milk, and I'll substitute that for a meal pretty often, at least lately, during my "cutting" phase.