It's often touted that body mass is all calories in and out -- that nothing else has any play besides calories. What about things like stimulants with calorie surplus? Testosterone boosters? Different foods? Exercise types? Genetics? Compounds? Many things speed up/slow metabolism so I'd figure you could get the benefits of increased nutrition and nutrient + calorie surplus while calories still exceed TDEE and not gaining body fat. I ask because some people will say it's just in and out and nothing else matters, but I don't think the body is 100% this simple -- and it will vary based on different genetics and a myriad of other factors.
Some people exponentially gain muscle on diets and never seem to get fatter -- for example, gaining LBM while keeping solid, six pack abs. That's one good sign that bodyfat isn't going up and may even go down over time too.
Others bloat up and eat slightly above maintenance and mostly get fatter. I've heard this is related to P-ratios -- how the body utlizies protein for muscle synthesis vs. just storing it as fat.
All things are never EQUAL. This is why some guys look good after cutting and can even gain muscle/lose fat on minimal splits and not make massive diet changes.
Some guys eat everything right and workout, and barely gain muscle -- others eat crappy and have more muscle despite not working out as often. Calories in vs. out cannot be the only thing at play.
So no two bodies are alike. How could calories in vs. calories out be 100% equal for all then?
Stimulants have the ability to make the metabolism faster, but do not affect nutrient absorption per se. Given such, one could temporarily eat high calorie food and still lose fat and gain muscle over time because stimulants increase things like lipolysis, heart rate, blood flow, nervous system, etc.
In short, it would seem fat loss/muscle gain are not two ends of equal coins and do not 100% respond in all individuals directly to the exact proportion of calories on a day-to-day basis. I'm not saying calories don't matter, as they're really important.
But isn't it wrong to say they're 100% of the picture when evidence disagrees based on individuality and different body responses?
What are all of the factors really involved in whether one gains, loses, or etc., and to what degrees?