If you want to make money in the business, don't be the guy who makes mass monsters, but the guy who makes ripped monsters out of massive people. The Body-Recomp specialist. When we acknowledge that even top pro BBers struggle for years to master the bulk/cut into a major contest, we gotta appreciate how difficult it can be. Meaning, while there is certainly many physiological principles that hold for everybody, knowledge of such actually shows you how complex the entire picture really is, and how it still comes down to finding what specifically works for you.
But this is what is generally accepted . . .
1) Excess fat is stored as fat, though not necessarilly as bodyfat (i.e. triglycerides in muscle.)
2) Excess carbs may be stored as fat or glycogen in muscle.
3) Excess protein (amino acids that are included in protein synthesis) may be stored as fat, glycogen, or execreted. Your body must work really hard to do the former, so it's more likely that the other two options will occur.
4) Really important: your body is constantly storing and depleting throughout the day. Although surplus and deficit holds true at the end of the day, what is stored (protein, fta, carbs) and what is depleted will not be a constant regardless of calories in - calories out.
Step 4 is what makes it theoretically possible to gain muscle and lose fat over a period of a day. Basically, during the 4-hour window of your training, you approach it as a bulk. An aggressive bulk even. You take advantage of the spike in your PS levels, AND you aggressively restore your glycogen levels. Then, through the rest of the day, you approach it as a cut. The idea goes that if your glycogen stores were at a relatively moderate-to-high level, then your body will have at least two plentiful sources -- glycogen and bodyfat -- besides your protein stores to gouge on. As a result, although the caloric deficit is the same, the macro over/under favors a net positive protein synthesis while you've depleted some of your bodyfat.
However, there are inherent problems with this method.
1) It's dependent on having this window available to you for a muscle as frequently as possible. As a result, a muscle has to be worked very often -- at least 2 or 3 times a week -- in order to trigger this partitioning effect. However, any anaerobic activity will do.
2) Because there is a caloric deficit, glycogen stores inevitably go down day-by-day however you manage your carb intake. When stores are depleted, then it's very difficult (out of keto) to stave off a catabolic response. As a result, you have to carb refeed periodically. This varies person to person.
3) If you're looking to get shredded, you'll eventually have problems with your leptin-mediated set point. This is when you arrived at a certain level of leanness where it suddenly becomes really hard for you to go into your bodyfat stores. As a result, your body dips further into glycogen stores and protein catabolism for energy. Of course, this can vary greatly person to person, and this is also where nutri-sci studs like Lyle McD and the Avant Labs crew make their bling.
But, and for some reason this isn't discussed a lot in BB circles, leptin levels is largely a reflection of the muscle's mitochondrial density. In fact, what often happens with mass gainers is that, though they work at putting on muscle through bulking and progressive resistance, they don't attend to increasing their anaerobic conditioning, which increases mitochondrial proliferation relative against their myonuclei count. Sure, they do HIIT on their bikes, which accomplishes this for their legs. And those who do core training also include their abs. But they largely leave their backs, arms, and upper torso work alone. As a result (esp. with PLers) mitochondrial density vs. myonuclei # starts to go down.
This creates logistical problems for the clean bulk, because declining mitochondrial density vs. myonuclei count decreases the muscle's ability to utilize extra amino acids in the bloodstream to address the training stress, and it decreases the body's ability to dip into fat levels in order with the higher metabolism. As a result, as you get more LBM, you have to eat more and gain more fat in proportion in your current body in order to facilitate further LBM gains.
I personally don't feel the above paragraph is a signficant problem until you're well past the intermediate phase. That is, except for the guy who goes into BBing schemes without a prior exercise background. Generally speaking, the following successful bulkers don't have problems with LBM-to-fat ratios while going through the bulk . . .
1) Those who have a sports background. Because their training involves AT conditioning elements, their mitochondrial density is right in line with their strength and LBM levels. A friend of mine who's bulking like crazy spent his college years on the row team. His mitochondrial profile is so good for his size that he can do pizza runs every w/end and keep a 4-pack all year long.
2) Those who come from a high volume background. People who were already doing 30+ worksets for a muscle group per week were already accomplishing this.
3 ) Those with a high frequency background. This includes those who do cardio during their bulk, as it produces much the same effect as discussed all the way at the top of this post, in regards to nutrient partitioning. Also, higher frequency application incurs faster AT adaptations.
4) Teenagers with rabid metabolisms. Obviously, nothing in this thread applies to these cats anyway.
5) Those who periodize properly their powerbuiding schemes with either a fatigue methodology concurrent their strength phase (i.e. DFHT, DC, additional high-rep sets, etc.) or separate density training phase.
Steps 3 and 5 is what actually applies to most of us.
1) Those with chronic problems with keeping their fat levels under control would probably stand best to allot a separate cycle in order to build their anaerobic conditioning. In other words, refrain from another strength-mass cycle, and spend 2-3 months on building conditioning. And that means going on a GVT, GBC, or EDT program on all of the major muscle groups, increase their steady state cardio intake, and eat as if bulking moderately. After about 8-12 weeks training this way, you'll be shocked at how much higher your metabolism and relative conditioning has become. Also, once this cycle is over, you can ride a few mass gaining cycles without going back to this. Beginners with no exercise training prior whatsoever should start on higher volume and more workload in order to acclimate their bodies. Your body becomes a food factory.
2) Those who are pretty successful but are just looking to bulk more cleanly can add cardio on their off days, train more frequently, and add some fatigue-inducing work after their main heavy work sets. Also, they would work on their carb timing and moderate their fat intake.
3) Those who hate doing either can just add more generic steady-state cardio on their off days, leaving one or two days a week of inactivity to carb refeed. Just 25-30 minutes is needed to bump up noro-ephedrine levels for the day and improve nutrient absorption and circulation.
Again, this is my 2 cents. An alternative "training-oriented" viewpoint of this time-honoured problem.