Before you can even begin to construct your diet, you need to decide what your overarching goal will be. Dr. Layne Norton often uses the analogy of bodily processes being akin to dimmer switches instead of On-Off switches, and that definitely applies here.
While you can gain muscle and lose fat at the same time, your energy intake is going to dictate which side of the switch you're going to be on.
It's exponentially easier to gain muscle in an energy surplus; it's all but a necessity to create an energy deficit in order to lose fat. This is nothing new.
But how do you decide which goal is the one you want to push your dial towards? It goes beyond the conventional wisdom that you “bulk” in the winter and try to get shredded for beach season.
The choice largely depends on your current body composition; namely your body fat percentage. Research shows that it's much easier to gain lean mass while overeating when you are already lean to begin with (study, study).
In short, males should be below ~15% body fat and females should be below ~25% before even considering trying to bulk.
A lower BF% is going to be permissive to greater gains in lean mass while in a surplus. You're going to gain muscle and fat when overeating; your body fat percentage is going to dictate the ratio in which you do so.
Why? A lower BF% is much more favorable for nutrient partitioning, as explained in the citations above. You also have a much more favorable hormonal profile & generally have less inflammation at lower levels of body fat. If you start eating a hypercaloric (more calories than you expend) diet at a higher body fat percentage, a higher proportion of those calories will end up being stored instead of utilized.
This will basically exclude a large portion of the population from trying to gain mass right away. Bummer, I know. But this knowledge will prevent you from wasting your efforts and digging yourself further into that hole.
Prioritizing fat loss (while still trying to gain some lean muscle) will make gaining muscle "easier" when that time finally comes.
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