Here we’re getting into some good crunchy sciencey stuff. Given that human energy metabolism is fiendishly complex, I certainly won’t pretend to know everything about what I’m writing about, but this understanding has been good enough for me so far. I got most of it from Lyle McDonald, Kiefer, Silverhydra, Prof. Dr. Andro, and Martin Berkhan. They (and their sources) don’t always agree, and odds are excellent that I’ve made mistakes in trying to understand their writing, too. Take this with a pillar of salt.
I repeat: I am going to simplify outrageously and not cite my sources (except as I’ve done above).
Let’s start here. In metabolically healthy people (fixing diabetes is not part of the game plan), insulin is a storage hormone. It encourages cells to take up nutrients from the bloodstream and build stuff with ’em. Insulin levels are elevated by three things: Carbohydrate consumption; protein consumption; and high blood sugar. Eating protein will cause a short, sharp jump in insulin levels; eating carbs will cause a similar jump, and then a longer second jump as the sugar or turns into blood glucose and gets shuttled away to wherever it’s going.
For the purposes of losing fat specifically, which is what we’re after, insulin is not our friend. It inhibits lipolysis (the breakdown and release of fatty acids from fat tissue) and encourages fat storage. We want to keep insulin levels chronically low (again, please don’t do this if you’re diabetic) in order to support as much lipolysis as we can possibly manage. We do that by cutting out carbs and going ketogenic.
Tissue burns glucose preferentially. Deprive the body of carbs, and the liver will release glycogen (“animal starch”), which gets broken down into glucose. Muscle cells have their own stores of glycogen. In general it takes the liver about a day, maybe less, to burn through its glycogen supply if you’re starting from a normal carby diet. Once it does, provided that blood triglyceride levels are sufficient, it’ll start producing ketones from those trigs, which can be burned in place of glucose by most tissues in the body. (It’ll also start producing small amounts of glucose from fat, which is good because parts of your brain really need it.)
Ketone uptake and metabolism is sufficiently different from glucose uptake and metabolism that it takes a few days for your body (in particular, your brain) to adapt to burning ketones instead of glucose, so you feel like shit for a little while. This is the “low-carb flu” you might’ve heard of, although I’ve never found it to be nearly as bad as actually having the flu. In any case the solution is to embrace the suck. After a few days your cells will have adapted to uptake and process ketones, and you’ll feel fine again.
Fat metabolism, both directly and through ketone production, is significantly less efficient than glucose metabolism. Since we’re trying to burn as many kcal worth of fat as possible, that’s a good thing. But we don’t want to stay in ketosis absolutely forever: Liver glycogen levels act as a metabolic regulator, so if the liver’s out of glycogen (which it has to be in order to produce ketones) your metabolism will slowly drop, which is thoroughly counterproductive. If you try to ramp up your workouts to compensate, you’ll find that without muscle glycogen you can’t lift for shit. Also, when you’re in ketosis your pee will probably smell funny; maybe stay away from asparagus.
The solution is to carefully and selectively reintroduce carbs. We can be clever about this by timing our carb-ups right. General insulin sensitivity is highest in the morning, when the government and the food lobbies want you to be eating Cap’n Crunch and drinking orange juice, and lowest in the evening. But muscle is most sensitive to carbs right after a good hard workout, particularly if it’s empty of glycogen. So by lifting in the afternoon and slamming a bunch of carbs and protein immediately thereafter, we can ensure that the majority of those carbs go right into muscle glycogen, which is right where we want ’em in order to promote muscle protein synthesis. The remainder will get turned into glycogen by the liver, giving metabolism, thyroid function, and &c. a bit of a kick and probably getting burned up overnight, so we’ll wake up the next morning in ketosis again. This is the general approach taken, to various extremes, by Lyle McD’s targeted ketogenic diet, SilverHydra’s Cheat Mode, and Kiefer’s Carb Backloading: A one- to four-hour (…ish) carb load after each workout.
Another option is to drive the body into deep glycogen depletion over the course of several workouts and low-calorie low-carb days, say over the course of a workweek, then after one final workout to raise muscle insulin sensitivity carb-load like a motherfucker for 30-48 hours. The idea is that any glucose you consume won’t get stored as fat until your liver and muscles are packed full of glycogen, and any small amount of fat you put on from fructose and dietary fats will get dealt with during the next week’s low-cal glycogen depletion hell. While you’re carb-loading you hit the weights as hard as you goddamn can. This, in vastly insufficient detail, is the method behind Lyle McD’s Ultimate Diet 2.0.
About the only downside to carb loading is that you probably shouldn’t drink during the carb-up. The idea is that, if your liver’s processing alcohol, it isn’t doing anything with carbs — it’s not clear to me whether alcohol uptake and conversion to acetaldehyde and acetic acid in the liver actually inhibits glucose uptake; if it doesn’t, it’s plausible that you’ll be burning alcohol (really acetic acid) for energy while you’re storing even more carbs (like the maltose and maltodextrin in your favourite beer) as glycogen. Alcohol does accelerate ketosis, so separating a few glasses of whiskey from your carb up by an hour or two might be a good plan. Kiefer has some interesting things to say on the topic; he’s more sanguine than I am. Clearly this calls for some n=1 experimentation. Anecdotally, when I was running UD2.0, I timed the end of my carb-load for Friday night, wherein I ate tons of chips and popcorn and drank beer and whiskey, limited more by having to squat the next day than by dietary concerns, and I still managed to lose a good pound or more of fat per week.