Nutrition for Performance (Part 1): The Case for Carbs
Reading Time: 5 minutes
In this 3-part series on nutrition for human performance, we will discuss the bioenergetics and dynamics of the 3 macronutrients (carbs, fats, and proteins) as it relates to exercise and physical performance. In this part 1, we make the case for carbohydrates…
Main Take-Aways
- Carbohydrates have gotten a bad reputation as unhealthy foods
- Carbs are a QUICK source of energy for our muscles required for exercise
- Adequate carbohydrate intake is needed for optimal performance
- Without enough of it, our muscles can be broken down and our brain function impaired
- Performance is different than health
Before we dive in, it’s important to understand the difference between HEALTH, and PERFORMANCE. “health” is defined as the state of being free from injury or illness. Performance, on the other hand, is the process of carrying out or accomplishing a task which, in our case, is optimal physical performance. Therefore, this article will focus on the roles of carbohydrates on physical and exercise performance, and not health specifically. Lets get into it…
In a world that has become so fascinated with finding the “perfect” diet and optimal nutritional intake, somewhere along the line carbohydrates have gotten a bad reputation. Often linked to obesity and unhealthy eating habits, carbs have become something that many people try to AVOID out of fear of weight gain or decreased nutritional status.
This has culminated in low-carbohydrate diets such as the popular Atkins or ketogenic diets, or many other diets in the form of carb restriction. These low-carb diets have garnered a lot of discussion in the media and amongst fitness enthusiasts as being the key to improve fat breakdown, weight loss, and lower total body fat. The mechanism, which we’ve all heard before, is that excess carbohydrate intake results in increased fat storage. While this CAN be true, it is only the case when carbs aren’t used for what they are supposed to be (performance). So we need to discuss what the main ROLES of carbohydrates are in our body…
There are 4 main roles of carbohydrates in the body which will further be discussed:
1. Provide a quick source of energy
2. Spare protein breakdown
3. Fuel for Central Nervous System
4. Metabolic primer to avoid acidosis
1. Carbs as a fuel source
Let it be clear – carbohydrates are energy dense foods that are one of the QUICKEST forms of energy for our muscles. Biologically, carbohydrates get broken down into glucose, a more readily bio-available source of energy. Once absorbed, carbs enter the blood stream and can be further broken down, used as immediate energy, or stored as glycogen in a process (for you physiology enthusiasts) called glycogenesis.
Carbs get broken down into glucose,
Which can then be stored as glycogen to fuel performance
Glycogen is stored in the muscles and in the liver and serves as the primary source of energy for moderate-high intensity (60+% maximum) exercise or prolonged exercise lasting longer than 60 minutes (McKardle, Katch, & Katch, 2010). Muscle glycogen (stored glucose in the muscles) provides the major carb-energy source while liver glycogen breakdown serves to provide extra fuel. While glycogen is of utmost importance for performance, unfortunately the body stores relatively little amounts of it, with approximately 15g/kg body weight as the upper limit for stored glucose. A 24-hour fast, or low-carb diet nearly COMPLETELY depletes glycogen stores. In contrast, a carb-rich diet for several days can almost double your glycogen stores. But how is this important for performance?
During exercise, and at increasing intensities, the breakdown of glycogen stores provides the major source of energy for the working body. Eventually, during high intensity exercise or prolonged exercise (>1hr), the liver and muscle’s stores of glycogen can no longer keep up with demands, and available glucose (potential energy) decreases. With depleted fuel sources, exercise and performance is inhibited (i.e. the energy – ATP - isn’t made available quick enough).
Just 1-hour of high intensity exercise or prolonged (>60mins)
moderate intensity exercise decreases liver glycogen by ~55%
Carbohydrates are traditionally thought of as a fuel source that is used during high intensity anaerobic activity lasting LESS than 2 minutes, and this is true. But what about during prolonged (>1hr) exercise? While glucose breakdown is the major source of fuel for anaerobic performance, it also provides the necessary breakdown for prolonged endurance exercise as well. Fats (which will be discussed in another blog) are often associated with being the fuel for endurance activities, but this only true at lower intensities (<65% max) and not during high intensity or longer duration exercise, which is why when it comes to performance – CARBS ARE KING!
A carb-deficient diet rapidly depletes muscle and liver glycogen and negatively affects performance in short term anaerobic exercise and prolonged (>1hr) intense activities
2. Carbohydrates spare protein breakdown
Your muscles are built of amino acids, which make up proteins. We don’t want to lose our muscles – right? Well in order to preserve your muscle tissue protein, an adequate carbohydrate intake is needed (McKardle, Katch, & Katch). When glycogen is depleted, such as during high intensity or prolonged exercise, the body needs to find energy to sustain movements, bodily functions, and performance. And when glycogen stores ARE depleted, it finds this fuel in the form of your own muscles!
In this case, glucose can (and IS) formed by the breakdown of protein in a process called gluconeogenesis (literally translating to creating NEW glucose from non-carbohydrate sources). This takes away from the availability to build new protein while also breaking down the protein (muscle) you already have! As stated above, it doesn’t take much to deplete your glycogen stores which makes it even more important to ensure adequate carbohydrate intake without losing muscle and the ability to perform!
3. Fuel for the Central Nervous System
Carbohydrates are the main fuel source for the central nervous system (CNS), including the brain. In fact, the CNS requires an uninterrupted stream of carbs for proper function. Under normal conditions, the brain uses the glucose from the breakdown of carbs almost exclusively over any other fuel source. Your body regulates blood glucose levels to maintain this fuel for the brain, but also because glucose is the primary source of fuel for your nerves, and red blood cells. Without adequate carbohydrate intake, your central nervous system function is inhibited.
4. Metabolic Primer to avoid Acidosis
The role of carbohydrates as a way to avoid acidosis (including ketosis) was purposely left for last on this list, because at this point the keto-zealots have probably given up reading. But if you’ve made it this far, you’re ready to hear a relatively well-known secret amongst exercise physiologists…
Sustained low-carbohydrate intake, glycogen depletion, and ketogenic diets impair both anaerobic and aerobic performance, can contribute to CNS fatigue, and trigger brain damage.
Here’s why: When there are insufficient carbs to be broken down for energy, there is less availability of glucose in the blood, uptake by the muscles, and can cause glycogen depletion. When sustained over several days (8+), a ketogenic or low-carb diet causes incomplete fat breakdown and increased ketone bodies. In excess, ketone bodies increase the acidity of your body (thereby impairing bodily functions) and produce a potentially hazardous condition. By maintaining adequate carbohydrate intake, acidosis will not occur.
But what about exercise performance? Ketone bodies CAN be used as fuel by converting to an aerobic fuel precursor (known as Acetyl-CoA). And yes, it’s true that by following a low-carbohydrate or ketogenic diet for extended (8+ days) periods of time, the body will provide enzymatic adaptations to allow for greater fat and ketone breakdown and use. However, this improved ability to use fats for fuel does NOT relate to increased performance in moderate-high intensity or prolonged exercise performance. In fact, in several studies, high carbohydrate diets have shown to increase endurance performance by up to 3 times that compared to a high-fat diet (STUDY LINK). These observations provide the basis for the importance of adequate carbohydrate intake to both high intensity anaerobic training but also long duration endurance exercise.
CONCLUSION
There’s many physiological benefits in maintaining carbohydrate intake and when it comes to exercise and physical performance (especially high intensity or prolonged endurance activities), adequate carb intake is of utmost importance for optimizing performance.
Thanks for reading,
Kalan Anglos, Msc., cscs
Founder: KFit Conditioning
Disclaimer: I am NOT a nutritionist, dietician, or medical doctor who may provide specific dietary advice and modifications for health. However, I AM a human performance expert with extensive knowledge in how nutrition can improve or impair physical and sport performance, and when it comes to performance, diet plays a BIG role. Also, these are just my thoughts (backed by science, of course), but take it for what it’s worth. As always, I’m happy to answer any questions.
