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A lot of confusion has been created since the advent of the 'low-carb diets', and while some carbs are 'good' and others 'bad', you definitely need them if you want your body to perform at its peak

March, 2004

Shameen Adams If the right type of fuel is not added to your car at the right times, you are going to have either poor performance, or none at all. Carbohydrates form the fuel source for your body over the short term for the immediate need of movement. Being a short-term fuel source, you can easily run out of it if not taken at the right times, and in the right amounts. Just like more fuel in your tank will not make your car faster or more powerful, carbohydrates when used correctly, will make you last longer, but will not increase strength, power, or VO2 max.

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
100 200 300 400 500 600 700
200 300 400 500 600 700 800
300 400 500 600 700 800 900
400 500 600 700 800 900 1,000
500 600 700 800 900 1,000 1,100
600 700 800 900 1,000 1,100 1,200
700 800 900 1,000 1,100 1,200 1,300
800 900 1,000 1,100 1,200 1,300 1,400
120 900 1,000 1,100 1,200 1,300 1,400 1,500
Your body should still act as your guide. Should you find that you put on some weight, cut back slightly on the carbs.

Table 2: indicates which carbohydrates to pick and which to shy away from:
Eat Less
Eat More
Glucose 100 Fructose 20
Carrots 90 Soybeans 15
White Potatoes
70 Kidney beans 30
    Lentils 25

  Sweet potatoes 48
Bananas 65 Apples 36
68 Oranges 40
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
Now we get to the portion of carbs you need to take prior to exercise. Because it takes time to digest, you need to take 100 to 150g of carbs 3 to 4 hours before your session. Don't use simple carbs like sugar, take complex carbs like maltodextrin. Then when the session starts, you can immediately start taking 40g to 90g of carbs per hour of training. Research indicated that a 7% solution permitted the best supply of both water and carbs. Remember, the higher the concentration of the carbs, the slower gastric emptying occurs, hampering the absorption of the carbs. Sip continuously during the session to prevent having to drink a full glass worth every 15 minutes, which would leave you with a slosh in the stomach every time.
—Article sponsored by USN - Ultimate Sports Nutrition

USN products guide
  • Muscle Fuel - protein-carb meal supplement (individuals wanting to add additional protein, complex carbohydrates, vitamins and minerals to his diet)
  • Muscle Fuel Mass - meal replacement (Any individual wanting to add additional protein, complex carbohydrates, vitamins and minerals to his or her diet)

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