CARBOHYDRATES AS EXERCISE FUEL
To understand carbohydrates and how to use them, we need to know what they are and why they are needed in the diet. We are going to focus on what is important to know for anyone who does aspire to eat properly and explain what will happen if we fail to eat enough carbohydrates and our glycogen levels become depleted.
FUNCTIONS OF CARBOHYDRATES
Carbohydrate is arguably the most efficient source of energy for athletes. Carbohydrates are needed to fuel almost every type of activity in the body. The carbohydrates that are stored in our muscles and liver are referred to as glycogen. The amount of glycogen stored in our body has a direct effect on exercise performance. So, a high muscle glycogen concentration is going to allow us to train to our optimal intensity and achieve more from our training session. A low muscle glycogen concentration could potentially lead to early fatigue, reduced training intensity and reduce optimal performance and that is not what we want. Carbohydrate in the form of glycogen in the muscle is used during exercise and glycogen stored in the liver is released into the blood stream to maintain a normal blood glucose level and feed the brain as well as the muscles. But it is to consider that stored glycogen in the body is in limited supply. Carbohydrates provide 4kcal/g (17KJ). The table below shows the amount of stored glycogen in the body for males and females:
|Liver glycogen||90 g||70 g|
|Muscle glycogen||400 g||300 g|
Since there is a limited supply of glycogen that we could store, we need to keep these glycogen stores at their highest level at the beginning of training and aim to replenish these stores during exercise as well.
STRUCTURE OF CARBOHYDRATES
Just a quick look at the structure of carbohydrates, this will help us understand the good and the bad carbs a lot better. So carbohydrates are saccharides. They refer to a large class of natural organic compounds. They range from monosaccharides to polysaccharides. Monosaccharides are simple carbohydrates (1 sugar molecule). They are ready absorbed in the body and used as energy since they do not need to be digested. The other carbohydrates are more complicated; they are made up of 2 or more sugar molecules. They range from disaccharides (2 sugar molecules linked together), oligosaccharides (2 to 10 sugar molecules linked together) and polysaccharides (10 or more sugar molecules linked together). So, the polysaccharides are the more complex carbohydrates.
In general, the longer the chain of sugar molecules, the harder and longer it takes for the body to break down the carbohydrates. As a result there is a slow release of the end product glucose in the body. Every carbohydrate needs to be broken down to glucose in the body in order to be absorbed. Any glucose not needed right away gets stored in the muscles and the liver in the form of glycogen. Once these glycogen stores are filled up, any extra gets stored as fat. The length of time that the body takes to break carbohydrates will have different effects on our bodies. The reason we are going through this structure because we hear people talking about good carbs, bad carbs, glycemic diets, carbs affecting our blood glucose level and it all boils down to the basic structure. Is the food made of simple carbohydrate or a complex one?
Carbohydrate digestion mainly occurs in the small intestine. The glucose is transported via the blood directly to tissues for use as fuel. So if your diet consist of mostly simple sugars, they will not be difficult to break down, the glucose will be absorbed all at once and quickly by the body and that can cause a spike in the blood sugar level. If we are consuming more complex carbohydrates, these will bring greater challenge to break down into glucose before they can be absorbed into the blood to the tissues. This indicates a slower release of glucose so it is less likely to cause spikes in blood glucose levels. This is what we would be aiming for in our diet, consuming more complex carbohydrates.
GUIDELINES FOR DAILY CARBOHYDRATE INTAKE
Several factors are going to affect the amount of glycogen that your body will need during an event or training session. Glycogen is the source of energy most often used for exercise. It is needed for any short, intense bouts of exercise from sprinting to weightlifting because it is immediately accessible. Glycogen also supplies energy during the first few minutes of any sport. During long, slow duration exercise, fat can help fuel activity, but glycogen is still needed to help break down the fat into something the muscles can use. Research has consistently recommended that when performing regular exercises we do need to consume a diet that contains a relatively high amount of our energy coming from carbohydrates. So, the amount of carbohydrates you can afford to eat will be determined by:
- Levels of muscle mass
- Volume and intensity of training
- Percentage of body insulin sensitivity
- Level of exercise training
- Initial muscle glycogen levels
The table below has different intensity levels and duration and based on that what our carbohydrate target will be according to our body weight. (These values are just a close estimation, should you want to be more accurate book an appointment with us).
|3 – 5g per kg body weight|
|2.27 – 3.18g per kg body weight|
(moderate to high intensity exercise)
|3.18 – 5.45g per kg body weight|
|Very high||Extreme commitment
(moderate to high intensity exercise)
|4.54 – 5.45g per kg body weight|
The days that we are not training we can use the lower end of the target and the days we do train we can use the higher end of the target. If you are not training hard on a particular day you could use a mid-range of the target of carbohydrates. However some people might just stick to a middle range so stick to what you are most comfortable with. For people who are trying to lose weight the calories might matter more, so you can look at a lower carbohydrate target.
FACTORS AFFECTING GLYCOGEN STORES
As we spoke earlier, glycogen is stored carbohydrate in the liver and muscles. So based on that how do we improve our glycogen stores? The only way to increase muscle ability to store more glycogen is through exercise. A trained athlete has more than twice as much stored glycogen compared to a sedentary person. They have greater endurance and this explains why athletes can exercise longer and harder at the end of a sports season as compared to when they first start. For instance, the person on the couch (in the picture) he cannot just eat loads of carbs and run the marathon.
We need to train adequately to stimulate the muscles’ ability to store more glycogen. At the beginning I showed you a table of how much glycogen we can store but not everybody stores that amount of glycogen, we have to train the muscles and liver to store such optimal amounts. While every person is unique, and our carbohydrate storage capacity will vary, on average an untrained individual can store about 13g of glycogen per kg of muscle whereas a trained person on average can store about 32g of glycogen per kg of muscle. Therefore, it just goes to show that training and exercise really do increase our glycogen stores. In summary the longer and more you exercise the more glycogen you can store and the more glycogen you can store, the more you can train. A win-win situation.
When muscles are trained energy production is increased within the cell. Untrained muscles produce more waste and trained muscles utilize more oxygen leading to:
- increased energy utilization
- deceases reliance on blood glucose
- sparing of muscle glycogen
- increased endurance
- increased resistance of fatigue
INADEQUATE GLYCOGEN STORES
Inadequate glycogen stores indicate that we have do not have enough energy because we don’t have enough fuel in our muscles to keep us going. If glucose levels drop due to depleting glycogen stores or inadequate carbohydrate consumption, the person is said to ‘hit the wall’ or ‘bonking’. The rate at which the glucose levels drop depends in part on the type of carbohydrate consumed before, during and after exercise. If we do not have enough carbohydrates to fuel us, our body starts to use protein as an alternative. Protein is broken down to make glucose to maintain a constant blood glucose level. Protein may then lose their primary function as building blocks for muscle. We do not want this as we can lose muscle mass.
Inadequate glycogen stores can lead to:
- Heavy tired muscles
- Poor performance
- General fatigue
- Negative effect on training/performance
- lead to injuries
Hence, it is important to prioritize our glycogen levels and make sure that we are fully fueled going into training and exercise. Next time we will be discussing glycemic index so keep in touch.
- The Position Statement from the Dietitians of Canada, the American Dietetic Association, and the American College of Sports Medicine, Canadian Journal of Dietetic Practice and Research in the Winter of 2000, 61(4):176-192.
- Kevin JAcheson, PhD, et al. Glycogen Storage Capacity and De Novo Lipogenesis During Massive Carbohydrate Overfeeding in Man13. The American Journal of Clinical Nutrition. l988;48:240-7.
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