The first source of energy to be used for any movement, like when you get up and walk to the door, is called ATP (adenosine triphosphate). Less time and oxygen is needed to break down muscle glycogen into ATP, than would be the case with fat, or protein.
During anaerobic activity, carbohydrates can be converted into ATP up to five times quicker than fat can. This explains why you need to do low intensity exercise for an extended period if you want to burn fat. It is also true that a fitter person can more readily use fat as an energy source than an unfit one. The mechanism for the utilisation of fat for fuel in an unfit person is simply underdeveloped or non-existent. Like anything, if you do not use it, you lose it, but the more cardiovascular training anyone does, the more this mechanism develops. That is why you really need to get fit before you can start burning fat through a cardiovascular session.
Studies have also indicated that an endurance session combined with "sprints" in between, will increase the metabolism for a longer time following the workout, not only during the workout. For athletes aimed at a medal rather than fat-burning, forcing your body to run out of glycogen to ensure fat burning is not advisable, since performance suffers when fat is the major source of fuel. If you need high quality training sessions, you still need proper carb intake. This applies to endurance as well as strength activities, since they all need high quality training sessions.
Now the question is: When should I take carbs; before, during, or after my training session? The answer is all three. But certainly muscle glycogen is more readily available to be converted to ATP than blood glucose. And also, the enzyme called hexokinase, which is responsible for the conversion of blood glucose to glycogen, is the limiting factor. So the most important portion of carbohydrates would then be the muscle glycogen. These are the carbohydrates that had been replenished following the end of the previous workout, right up to the start of the next one.
In order to start a training session with the maximum amount of muscle glycogen, you should start to consume carbohydrates immediately after you finish a session. Muscle glycogen synthesis occurs in two phases after exercise, the most rapid phase for the first 2 hours, and rather rapid for 4 to 6 hours following the workout. Then it slows down drastically for the next 24 hours. Glycogen synthase, the enzyme that controls glycogen storage, is very active straight after exercise, which is the time when you really need to hit the carbs. Feed your body about 225g of liquid glucose polymers (complex carbs) during the first four hours, which should replenish your stores. It is also advisable to add a bit of glucose and/or fructose right at the end of a session, which at this time will also be absorbed by the muscle and converted into glycogen, without even spiking insulin.
To maintain your glycogen state, you can consume about 650g of carbs per day, but if you are a serious endurance athlete, this figure can exceed 1000g. The timing of how to consume the rest of the carbs is also important. To maintain full glycogen stores, you have to maintain glycogen synthesis. And to do that, you need a constant flow of carbohydrates across the intestinal wall. If that flow is disrupted during the 24 hours following the workout, glycogen storage is reduced, and carbs may be stored as fat. This happens because the enzyme glycogen synthase is dependent on the constant flow of insulin. So the best way to achieve complete replenishment is to constantly eat carbohydrates in small meals throughout the day. Remember, you can only absorb 50g to 100g per hour, and every gram of carbohydrate needs 2.7g of water to store it.
The type of carb eaten should be those with low glycemic indexes, which will not spike and then drop your insulin and thus prevent further glycogen synthesis. To give a more complete indication of the amount of carbs needed.
Table 1: Correlates body weight and training hours per day to determine your carbohydrate need.
| Body Weight | Daily Training Hours | ||||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | |
| 40 | 100 | 200 | 300 | 400 | 500 | 600 | 700 |
| 50 | 200 | 300 | 400 | 500 | 600 | 700 | 800 |
| 60 | 300 | 400 | 500 | 600 | 700 | 800 | 900 |
| 70 | 400 | 500 | 600 | 700 | 800 | 900 | 1,000 |
| 80 | 500 | 600 | 700 | 800 | 900 | 1,000 | 1,100 |
| 90 | 600 | 700 | 800 | 900 | 1,000 | 1,100 | 1,200 |
| 100 | 700 | 800 | 900 | 1,000 | 1,100 | 1,200 | 1,300 |
| 110 | 800 | 900 | 1,000 | 1,1000 | 1,200 | 1,300 | 1,400 |
| 120 | 900 | 1,000 | 1,100 | 1,200 | 1,300 | 1,400 | 1,500 |
Table 2: indicates which carbohydrates to pick and which to shy away from:
| Eat Less | GI | Eat More | GI |
| Food | Food | ||
| Sugars | |||
| Glucose | 100 | Fructose | 20 |
| Honey | 87 | ||
| Vegetables | |||
| Carrots | 90 | Soybeans | 15 |
| White Potatoes | 70 | Kidney beans | 30 |
| Lentils | 25 | ||
| Sweet potatoes | 48 | ||
| Fruit | |||
| Bananas | 65 | Apples | 36 |
| Raisins | 68 | Oranges | 40 |
| Grains | |||
| White flour spaghetti | 56 | Whole wheat spaghetti | 40 |
| Cornflakes | 85 | Oats | 48 |
| White rice | 70 | Brown rice | 60 |
| White flour pancakes | 66 | Buckwheat pancakes | 45 |
| White Bread | 76 | Whole wheat bread | 64 |