Carbohydrates and training
Updated: Feb 19
Why do you need carbs?
This article deals specifically with the importance of carbohydrates (a.k.a. “carbs”) to your exercise/workout regimen. It does not address weight loss goals or other potential health concerns, but rather workout performance, including endurance.
There are 4 sources of energy in our diet, broken down by our body as follows:
· Carbohydrates – broken down into sugars
· fats – broken down into fatty acids
· protein – broken down into amino acids
· alcohol – mostly absorbed directly into the bloodstream
Once broken down into sugars, carbs are stored as glycogen in our liver (up to approx. 100g) and muscles (up to approx. 400g), along with roughly 3 times that weight in water. (Weight loss on low carb diets are primarily glycogen and water in the early days of the diet.) Glycogen stores must be restocked daily, since these amounts equate to roughly 1,600 – 2,000 calories, or one day’s worth. The purpose of liver glycogen is to maintain steady blood sugar levels. Muscle glycogen is used to fuel physical activity. Therefore, increased muscle mass results in an increased capacity for glycogen storage.
Though you may be able to increase your glycogen storage capacity to a point (known as carbohydrate loading, a topic beyond the scope of this article), your glycogen stores are not endless. By maximizing your glycogen stores and reducing the rate at which you use up muscle glycogen (by increasing your aerobic fitness level), you can delay fatigue during your workout.
Alcohol cannot be used for energy by the muscles, and it is broken down by the liver at a fixed speed, whether exercising or not. Protein is not a substantial source of energy for exercise, unless carbohydrates are in short supply. Carbohydrates are the main source of fuel for exercise, along with fats.
The use of carbs relative to fat during exercise varies based on a number of factors.
1. Intensity – high intensity (sprints, heavy weight training) uses mostly muscle glycogen; intermittent bursts use more glycogen than fat; low intensity uses more fat than glycogen.
2. Duration – because of the limited storage capacity, glycogen cannot supply energy indefinitely unless replenished. On average, glycogen can last for 90 – 180 minutes of endurance training, 45 – 90 minutes of interval training, or 30 – 45 minutes of high intensity training. The amount of fat used will increase over the course of the workout, but the body needs the presence of carbs to burn fat. Once glycogen stores are depleted, protein will be used for energy.
3. Fitness level – over time, aerobic training increases your body’s ability to burn fat by increasing the capillaries supplying blood to the muscles, transporting fatty acids to the muscles thereby sparing glycogen, allowing you to exercise longer.
4. Pre-exercise diet – low glycogen stores (resulting from a low carb diet) reduces your ability to sustain exercise for more than an hour at moderate intensity and affects your ability to perform at maximal power output (high intensity bursts).
In summary, for most activities, performance is limited by low pre-exercise muscle glycogen stores, leading to early fatigue, lowered exercise intensity and lower training gains. All this to say that glycogen (and therefore, carbohydrates) is the most important and valuable fuel for any type of exercise.
How much do you need?
Your requirement for carbohydrate intake will depend on activity level, and is best calculated using body weight rather than as a percentage of calories per day. In general terms, the recommended daily carb intake per kg of body weight for various intensity levels are as follows:
· Very light training – 3 to 5g/kg
· Moderate intensity (60 minutes) – 5 to 7g/kg
· Moderate to high (1 -3 hours) – 7 to 12g/kg
· Very high intensity (>4 hours) – 10 to 12g/kg
You may have heard of the “train low, compete high” theory of training the muscles to adapt to using fat stores instead of carbs, but there is not enough research evidence to support this theory.
Does it matter when you get them?
Numerous studies have concluded that carbohydrate intake before exercise leads to improved performance compared to exercising on an empty stomach. They have also shown that a low GI (see next section, “Are all carbs created equal?”) carbohydrate food/meal is most effective at helping endurance and delaying fatigue.
Some experts recommend between 1 and 4g/kg of body weight, within 1 to 4 hours before your workout, while others recommend roughly 2.5g/kg of body weight approximately 3 hours in advance. The right amount for you will depend on your fitness level, and the type, duration and intensity of your training. You don’t want to feel full, but you also don’t want to feel hungry. If you wait too long after eating before starting your workout, you risk low blood glucose, which can make you fatigue sooner, and if you get light-headed as a result, you also risk injury.
For workouts lasting less than 45 minutes, there is no proven performance advantage to carbohydrate intake during exercise. For intense exercise lasting from 45 to 75 minutes, there may be some advantage in getting very small amounts of carbs during the workout. However, for training sessions longer than 60 minutes, research shows carb intake of 30g – 60g in total (depending on intensity and duration, but not body weight) helps maintain blood glucose, can spare muscle glycogen stores, delay fatigue and increase endurance.
The amount of time it takes to refuel your glycogen stores depends on: 1. How depleted your glycogen stores are, 2. Extent of muscle damage, 3. Your training and fitness level, and 4. Amount and timing of carbohydrate intake. Let’s discuss this last point further.
Glycogen storage is faster in the 2 hour “window” after exercise than at any other time of day – 150% faster than normal. The more carbs you eat within this window, the faster you can refuel, which is particularly important if you train daily. Research suggests 1g/kg of body weight of moderately high GI carbohydrates within the first 2 hours after your workout is most effective.
Research also shows that combining high quality protein, such as whey, with carbohydrates in exercise recovery enhances muscle protein synthesis following resistance training compared to carbohydrates alone.
Are all carbs created equal?
A very basic categorization of carbohydrates is: simple and complex. Simple carbs are very small molecules consisting of 1 or 2 sugar units – glucose/dextrose (metabolized form of sugar in the body), fructose (fruit sugar), galactose (from metabolized lactose), sucrose (white sugar) and lactose (milk sugar). Complex carbs are much larger molecules, with anywhere from 10 to several thousand sugar units – starches, amylose, amylopectin, and fibre such as cellulose, pectin and hemicellulose. In reality, many foods contain a mixture of both simple and complex carbohydrates.
It is misleading to conclude that simple carbs give fast-released energy and complex carbs give slow-released energy. For example, apples produce a small and prolonged rise in blood sugar, despite being high in simple carbs, whereas potatoes are digested very quickly and result in a rapid rise in blood sugar, despite being very starchy. For exercise, what is more important is how rapidly the food is absorbed from the intestine to the bloodstream. The faster this happens, the more rapidly the carb can be taken up by cells, including muscle cells, and have an effect on training and recovery.
Enter the glycemic index, or “GI”. GI ranks food from 0 to 100 based on its immediate effect on blood sugar levels. It is a measure of the speed at which food is digested and converted into glucose. The faster this occurs, the higher the GI. This link takes you to a list of the GI of various foods that can help you make the proper choices for your needs.
Glycemic load takes into account the GI as well as the amount of carbohydrate consumed and thus provides a measure of total glycemic response to a food or meal.
Are you getting enough?
If you are feeling fatigued during your workout, lacking the strength and/or endurance required (barring illness or injury), you are most likely not getting enough carbohydrates for your training intensity and duration.
So what do I recommend?
The question of adequate carbohydrate intake required for your specific training program is fairly complex and highly personal. Are you training for a marathon, a weight-lifting competition, a body-building competition, your first 10K or just to stay in shape? Are you trying to lose or gain weight? Are you otherwise active during your day or do you have a desk job? This article is really only hitting the generalities in terms of the importance of carbohydrates, and the amount you as an individual require to get the results you want. Customized advice is the best advice, and can take into account your individual goals. Contact us at M'agine Nutrition or M'agine Fitness for a consultation and we would be more than happy to help you!