The best explanation for what’s really happening is that you alternate between periods of caloric surplus (anabolism) and caloric deficit (catabolism) and the net result is a gain in muscle and a loss in body fat.
You see, if you stay in a calorie surplus, it’s the body’s natural tendency for body fat and lean body mass to go up together. And if you stay in a calorie deficit, it’s your body’s natural tendency for body fat and lean body mass to go down together.
There may be exceptions, but the general rule is that it is usually very difficult to gain muscle and lose fat at the same time – the mechanisms are mostly antagonistic to one another. When big increases in muscle and big decreases in fat are seen at the same time, it’s almost always the result of “unusual conditions” – I call them X factors.
There are 4 "X factors" as Tom calls them:
1) Training age - it is always easier for a beginner to make gains compared to someone who's been training for many years. The further away you are from your genetic potential, the easier it is for your to make gains.
2) Muscle memory - it's always easier to gain back muscle you've lost than it is to build new muscles.
3) Genetics and body type - your genetics and your body type ultimately determines how easily it is for your to build muscle and lose fat.
4) Performance enhancing drugs.
Build Muscle and Lose Fat At the Same Time
Whether you're able to build muscle and burn fat at the same time will depend on several key factors such as your training, and your diet and nutrition. Traditionally, calorie balance is looked upon on a 24 hour basis, and calorie surplus and deficit calculations were based on this. However, if we break the day into shorter periods, it is possible that you go through with-in day calorie surpluses and deficits. As Tom says:
It’s entirely possible that you might pass through periods of “within-day” surplus where you were in a highly anabolic state (for example, you eat the biggest, highest carb meal of the day after your workout), and you were in a deficit the rest of the day.
Exercises, specifically high intensity weight training sessions dramatically changes this dynamic.
Certain types of training will alter your hormones and physiological response to how you handle calories and nutrients. If you did intense weight training, and you timed your nutrient intake just right, isn’t it possible that you could gain a small amount of muscle during those anabolic hours, while losing fat the rest of the day? Granted it might only be grams or ounces – but what if you kept that up for a week? A month? Three months?
Ah-ha! This is a very revealing explanation, and certainly makes a lot of sense. Immediately following intense workouts, you have a very anabolic window of opportunity to feed your muscles, and this must be taken advantage of. Throughout the week, there are days where you have a caloric surplus, and days where you have a deficit for the entire day. Tom calls these within week and within day phases microcycles and mesocycles. This is known as nutritional periodization, or cyclical dieting. The key to this is the manipulation of your calorie intake by controlling your carbohydrate intake, also known as carb cycling and carb targeting - leading to periods of calorie surplus when you need it most to build muscle, and alternating with periods of calorie deficit to create an environment conducive to fat loss.
While these ideas relating to nutritional periodization are important, even more important factors are hormones and nutrient partitioning. As Tom said in his Holy Grail Body Transformation program:
"The key is to partition as much of that surplus [calories] into lean tissue and have as much of the energy deficit withdrawn from fat tissue as humanly possible. Anabolic and catabolic hormones are primary players in the partitioning of energy and nutrients."
So, if you were able to control your within-day energy balance, use nutritional periodization, and control your hormones with food and lifestyle strategies, then muscle gain and fat loss is possible. While this is a difficult goal to achieve, it is not impossible, and Tom has proven it with real people in his total body transformation contests, where it's not uncommon to see contestants lose fat and gain muscle at the same time. Tom has put all of this into his book titled "Holy Grail Body Transformation". In his program, he shows you all the details about nutritional periodization, cyclical dieting, hormonal manipulation, within day energy balance, nutrient partitioning, and all the keys to building muscle and losing fat at the same time.