Let's first review what carbo-loading is and why you would want to do it. Under normal circumstances, the body is fueled by free fatty acids, breakdown products of stored fat. All organs except the brain use free fatty acids. Only the brain has access to circulating glucose. The main exception to that rule occurs when insulin is present. Insulin is secreted when excess carbohydrate is available, as happens after a meal. Insulin promotes the use of carbohydrate by all active tissues and especially the conversion of carbs into fats. Working muscle can access small amounts of circulating glucose in the absence of insulin but the amount is rather insignificant. As a rule, muscle uses fats and internal glucose.
Fat is an ideal energy source, light and very efficient. Unfortunately fat cannot deliver quick bursts of energy. Fat utilization takes time. When fast delivery is required, carbohydrates are used. For very fast delivery, so-called intermediates like creatine phosphate are accessed. These are loaded by burning carbs and their supply is extremely limited.
In essence, we have fat for aerobic exercise and sugar for anaerobic bursts. Endurance racers get nearly all of their energy from fats. Training improves one's capacity to use fat. If at all possible, endurance racers should avoid anaerobic power utilization. However, even under the most aerobic conditions, a small amount of anaerobic power generation is happening. Even though it is small it is a major contributor to fatigue, especially when reserves run low.
Because muscle does not have access to blood glucose, it stores a small amount of glucose as glycogen within its fibers. To carbo-load means to push that storage to its maximum capacity. Athletes who rely on anaerobic power have well defined "big" muscle. Much of that bulk is due to stored glycogen and water in the so-called fast-twitch muscle. Despite all the hoopla about fast-twitch and slow-twitch muscle, humans do not have true fast-twitch fibers. But that is another story we will cover sometime later.
The main reason for carbo-loading is to increase endurance by making more glucose available to muscle. Muscle uses glucose for instant power delivery. As mentioned, even so-called aerobic exercise always involves a small amount of anaerobic output. We are trying to extend that capability. Muscle also uses a tiny amount of glucose to burn fat. It is unlikely that carbo-loading affects the latter mechanism.
Carbo-loading relies on a biological mechanism that kicks in when you deplete reserves. After depletion the body responds with excess storage. It is as if the body says, I won't let that happen again next time, so let's put in a little bit extra. Therefore, the first step in carbo-loading is to deplete reserves. To do so, you need to exercise hard.
The traditional method for carbo-loading works as follows. You eat no carbs (essentially an Atkins diet) for a week, while continuing to exercise hard. That is not a pleasant experience, especially towards the end of the week. Fortunately, after your last hard session you can overindulge in carbs for a day or two. That loads the muscle. Traditional carbo-loading is not something you want to do all the time. You should reserve it for special occasions.
In the modern method, which some claims is equally effective, you just exercise hard for a few days and eat lots of carbs. As long as you exercise hard and deplete reserves, the actual timing seems to matter very little and so you can eat carbs concurrently.
In either case, because of the hard exercise, it is not something you do a day before the race. You may even wonder why you should do it in the week before. Don't you need to taper? You do. But the key is to exercise hard but not long. Remember the intensity matters, not the duration. You don't have a lot of glycogen storage to begin with. Glycogen storage is ninety minutes at most, and sixty seems a lot more likely. So a few brief intense bursts is all that is needed.
Because the traditional method involves eating no carbs, and hence only protein and fats, I have seen this method advertised as fat-loading. There is no such thing as fat-loading -in the sense of loading muscle with fat to increase its performance. The only fat loading is what most people practice (i.e. get fat) and it is not desirable.
For the detail-oriented, it is true that muscle stores some internal fat, especially the so-called slow-twitch fibers. Nevertheless, fat loading is not needed. Fat can be produced internally from anything you eat (protein, carbs, fat), and the best method to accumulate fat is to eat carbohydrate as any meat producer will tell you. Remember corn-fed beef?