Fueling Workouts

Are you taking in carbohydrates during your training sessions? To start, let’s cover what a carbohydrate actually is. A carbohydrate is a macronutrient that consists of carbon, hydrogen, and water (CHO). CHO’s are found in all living cells and serve as the primary fuel source for human cells to make energy (ATP). Since we are talking about training here, I am mainly going to cover how CHOs pertain to the exercising body.

The human body has a limited CHO store in two locations. 1.) The liver (~100g) and 2.) The muscles (~500g). This means that at a maximum, the body stores roughly 600g of CHO which equates to ~2,400 calories of CHO energy. Our fat stores contain a much larger energy reserve and even lean individuals store 100,000+ calories from fat. After we eat, our digestive system breaks down the CHOs in food, which are then transported via the bloodstream to be stored in the liver and muscles. Once CHO is stored in the liver and muscles, it is there for later use. We also store a small amount of CHO in the blood stream (~4g) which is used to fuel immediate energy demands of human cells. The role of CHO stored in the liver is to maintain a stable blood glucose concentration which is regulated by the nervous system’s feedback loops and the use of the hormones insulin (store CHO) and glucagon (release CHO). The nervous system relies exclusively on blood CHO for energy, and when it runs low we experience “the bonk” or “hypoglycemia” in the form of dizziness, nausea, weakness, and hunger. CHO stored in the muscle is locked into the muscle and can only be used to make energy once the muscle begins working.

Now that we have covered what a CHO is, where it is stored in the body, and in what amount, let’s cover why it is important to take in CHO during your training sessions.

Working muscles (when you are training) prefer to use CHO as a fuel source to make energy (ATP). This energy (ATP) is what allows muscles to continue contracting. During exercise, our muscles are using up this finite CHO fuel source at a rate that is coincides with the intensity in which we are working, and when we run out, we bonk (or can’t continue at the same work rate). When we run out of CHO in the body, the reason we bonk is primarily because there is not enough CHO to fuel the brain and nervous system. The goal of taking in carbohydrates during exercise is to maintain a stable blood glucose so that the brain and nervous system continue to have energy. When the brain and nervous system register that energy is running low (low blood CHO), they (brain and nervous system) have the power to shut you down. The way we reach a state of low blood sugar during exercise is when we use up all of our stored muscle and liver glucose. The muscles can do work without lowering blood glucose too much if they have sufficient CHO stored, and the stored liver CHO serves the function of regulating blood glucose while we exercise. The muscles and brain are both drawing energy from blood glucose, but when the muscles run out of stored CHO, they begin to draw from blood glucose at a higher rate.

The increased utilization of blood CHO by the working muscles can quickly drop blood CHO levels. If the stored liver and muscle CHO are both used up, muscles will use the blood CHO quickly, leading to low blood CHO. Once the brain realizes that CHO is running low in the blood, it knows that the working muscles and liver have run out, and if exercise continues, then it will run out too. For survival reasons, the brain down-regulates the working muscles and exercise capacity to preserve what little fuel is left for itself. Afterall, if we continued to exercise our way through our entire blood CHO store, it would result in death (but gladly bonking prevents this long before it could ever happen!).

When you are taking in CHO during training, you are providing your body an alternative fuel source for the muscles and nervous system. Taking in CHO in the form of sports drink, gels, bars, or chews almost serve as a “second liver”, which steadily drips CHO into the system to maintain blood glucose levels and providing an additional fuel source for the working muscles. This exogenous (from the outside) CHO intake prolongs the duration it takes for liver and muscle glycogen to run out, so your brain and nervous system allow you to continue exercising harder for longer. Yes, this means that taking in CHO during exercise extends how long you can train at a given work rate.

The reason that it is best to consume simple sugars during exercise instead of more complex CHO comes down to digestion and absorption. When we are exercising, blood is diverted from the digestive system to the working muscles. This work is part of the autonomic nervous system, in that when we are exercising (sympathetic fight or flight mode), the right organs have sufficient blood, oxygen, and nutrients to carry out their function. When we are not exercising (parasympathetic rest and digest mode), we then have the blood, oxygen, and nutrients for different organs to carry out resting functions. Since our gut during exercise has limited capacity, we need to take in CHO in a simple form, meaning without protein, fat, fiber, or other chemicals. The goal is to have your CHO contain as simple and minimal ingredients as possible. Common simple CHOs found in sports nutrition products are glucose, fructose, and maltodextrin. These CHO sources in the form of sports drink, gels, bars and chews are absorbed quickly in the gut and are immediately absorbed into the bloodstream to be transported to the working muscles.

The rate in which you ingest CHO during training depends on the intensity and duration in which you are working. Lower intensity, short duration training will require less CHO per hour, and higher intensity, long duration training will require more CHO per hour. The general recommendation is to fuel workouts of greater than one hour at a rate of 30-60g per hour. Sessions lasting more than two to two and a half hours can benefit from ingesting 60-90g+ per hour. It is important to know that if you are ingesting CHO at a rate greater than 60g per hour, you will need to use a nutrition source that contains multiple sources of CHO such as glucose and fructose. The reason for this is that glucose transporters in the gut max out at an absorption rate of 1g per minute (60g per hour). If you consume 90g per hour, ideally it would come from 60g of glucose, and 30g of fructose. Many sports nutrition products such as those from Precision Fuel and Hydration come in this 2:1 ratio of glucose:fructose.

Consistently fueling workouts at the correct rate for your given intensity and duration has other benefits as well. It has been proven that fueling workouts with CHO leads to a lower cortisol response, which means we experience less stress when we fuel our training which may lead us to be less susceptible to overtraining. Fueling during workouts also leads to more stable hunger across the day, in that your workout will create less of an energy deficit leading to a more stable appetite for the rest of the day after your workout. If you find yourself excessively hungry after rides, this is largely due to under-fueling. Don’t fear taking in sugar during exercise though…even exercising at a low to moderate intensity at an ingestion rate of 60g per hour is only replacing half of the calories you are burning. Secondly, when we think of the damaging effects of sugar, it largely has to do with high glucose and insulin levels. This damage occurs when too many simple sugars are consumed at rest. During exercise, our muscles burn the glucose we ingest before levels get too high, and the exercising state of the body shuts off the insulin production so no CHO will be stored in muscles, liver, or fat cells.

There has been a lot of debate of low carb training for fat adaptation and favorable endurance adaptations. While low carb training can train your metabolism to burn fats more efficiently, this has never been proven to be advantageous for performance for high intensity racing. Unless you are an ultramarathoner, low carb training is not for you. The adaptation of increasing fat oxidation is triggered by a state of low glycogen, not low CHO intake. Even if you fuel a workout with CHO beforehand and during, by the end of your long ride (3h+), your glycogen levels are low and this still leads to enhanced fat metabolism. So don’t waste your time going crazy with low carb rides.

Personally, I prefer to fuel training with strictly carbohydrate sources only, meaning products that do not contain fat or protein. Typically energy bars contain moderate amounts of both, but the body does not digest these macronutrients as well in an exercising state. In terms of managing body weight, nutrition products that contain fat and protein are contributing to your overall energy intake each day, and you only need the carbohydrates during exercise to support your energy needs during training. It also may help to oxidize the bodies stored fatty acids vs the fatty acids ingested from energy bars that end up in the blood stream. Overall, when you fuel with strictly carbohydrates, your gut runs smoother, empties quicker, and your energy intake for the day will be overall lower which may assist with weight loss goals.

So there you have it, these are some of the main how’s and why’s of fueling your training with carbohydrates (CHOs). I highly recommend working with a professional sports dietician and fueling your rides, runs, and gym workouts accordingly.

Example guide of CHO fueling (low intensity use the lower end, higher intensity use the higher end):

0-0.5h: 0g - mouth rinse

1.0-1.5h: 30-60g per hour

1.5-2.0h: 30-90g per hour

2.0h-3.0h: 60-90g per our

3.0h+: 90-120g per hour

Disclaimer* I am only offering general nutrition advice and I highly recommend working with a registered dietician for more professional nutrition guidance. This is not intended to be medical advice to diagnose or treat diseases.