Fueling During

Energy expenditure is extremely high in endurance sports and a lot of athletes are failing to adequately fuel their bodies. Under-eating is commonly seen among endurance athletes.

Hydration During Exercise

Hydration is a complex topic and deserves a special attention. Hydration before, during and after exercise is discussed in the post that can be accessed here: Performance Oriented Hydration Guidelines for Athletes. If you haven’t read it yet, I highly suggest you do so, before we move on.

To refresh your memory, the goal of hydration is not to completely stop your body from loosing fluids, but to prevent it from loosing more than a 2% in body weight from fluid loss.

Drink to thirst! Quite simple. BUT make sure you don’t over drink. There is such a nasty thing called hyponatremia (low sodium concentration in the body fluids). Over drinking during exercise can cause dilution of sodium which eventually leads to health problems and even death. Utilizing sodium in your hydration solution helps to maintain blood plasma volume and reduce the risk of over-hydration.

For high intensity and workouts longer than 60 minutes we begin to utilize hydration solutions or sports drinks. When choosing a isotonic drink for your workout aim for the following carbohydrate & electrolyte concentration:

Carbohydrates: 4% – 8%. To calculate it, divide the grams of carbs (sugar) per serving size by the volume of the serving size (in mL), then multiply by 100 . So let’s say you want to fill your bicycle flask of 500 mL with a 6% solution we will need 30 grams of carbs (sugars).

Electrolytes. Formula: 0.5-0.7 grams of sodium per 1 liter (34 oz) of water. So in the same 500 mL cycling water bottle you will need to add 250-350 milligrams of sodium.


Fat or Carbohydrate for Fuel?

Adequate glycogen storage in your body is the key for optimal performance. Exercising at low and moderate intensities your body uses both: muscle glycogen and fat. The higher exercise intensity, the more carbohydrates are required to fuel your engine. Below is an example of how your body shifts from using fat for fuel towards running purely on carbohydrates. This is my VO2 test, conducted in professional laboratory:

Red column on a left is my heart rate (HR), measured in beasts per minute (bpm). Two columns on the right show the carbohydrate (CH) and fat (F) utilization related to heart rate. What we see here is that at lower intensities (<111 bpm) my body works primarily on fat as fuel. However the higher intensity of the exercise becomes, the faster my heart beats, more carbohydrates are being burned. Approximately at 111 bpm my body’s engine works as a pure hybrid, utilizing about the same amount of fat and carbohydrates for fuel. As the intensity increases, less and less fat is being used to fuel the exercise, until my body shifts completely to using carbs for fuel at 163 bpm.

Maintaining optimal carbohydrate stores is essential not only for good performance, but for protecting your health. Your nervous system, and brain primarily use carbohydrates for fuel. When glycogen stores get depleted and blood sugar level is low your brain will do anything to stop you from continuing exercising. You “hit the wall”. Pack your bags buddy, cause you are not going anywhere at this point. Consider yourself lucky if you just finish the race.


90 to 120 minutes are the short exercises and include sprint distance triathlons, time trials, some mountain bike races, 5-K, 10-K etc. What sets such events from longer-distance races is the high intensity.


The main focus is on hydration and the greatest nutritional need is WATER. Assuming adequate nutrition in the days and hours before the race, the athletes body is well prepared with glycogen stores. 
Taking in solid foods not necessary. 


During prolonged or high-intensity physical activities, blood flows out of the least important organs, such as gut to supply your muscles and skin for optimal performance. It impairs your gut’s ability to process and absorb the foods, therefore carbohydrates at this duration is best in a liquid form. Solid foods are better tolerated at very low intensities or exercising in cold-weather.

Don’t sacrifice your health and performance training in a low-carb states during competitive seasons or when the training load is high. Training in such way will reduce your body’s ability to utilize it’s preferred source of fuel (carbohydrates) for energy, which leads to the series of health issues: hormonal disturbances (low testosterone in men), injuries, sickness, sleep disorders, burnouts, mood swings (moodiness). Don’t work AGAINST your body, but work WITH it. Fuel appropriately and it will pay you back with those fitness gains you are chasing for.

The main focus is to maintain adequate glycogen levels in muscles and liver to delay fatigue and prevent “bonking“. Some research shows that including small amounts of protein may be beneficial in improving performance during endurance events. Protein requires additional energy to break down in your gut therefore it can be too much of a load on your digestion system, causing diarrhea and nausea. Test it on yourself!

Inadequate carbohydrate fueling during high-intensity and prolonged exercise can result in muscle wasting.

High glycemic index drinks with much greater maltodextrin or glucose than fructose are preferred. Caffeinated sports drinks or gels has been shown to enhance the utilization of the glucose in sports drinks. Be careful, too much coffee may cause you shit your pant, when intensity get’s higher. Distribute 200-300 calories per hour in equal chunks, ideally from liquid sources. As always, drink enough to satisfy thirst


Marathon, ultra-marathon, half-Ironman (70.3), Ironman (140.6), cycling races like century rides, ultra-marathon cross-country ski and rowing events fall under this category.

Participating in such events your main focus has to be on your health. Such long exercises is a true test of your nutrition and hydration strategies. Longer durations give more opportunities to fuck up, either catching hypoanatremia, or bonking on the field, running out of fuel.

Typically intensities during such events are staying low, meaning that your body is going to rely on fat for fuel. It doesn’t mean that we don’t need carbs anymore, we do, but not as much as at higher intensities. As discussed previously your body is a hybrid when it comes to fueling the muscles and the ratio of fat:carbs being utilized depends on intensity of physical activity, therefore your heart rate.

Fat burns in a carbohydrate fire”, meaning that if carbohydrate stored as muscle glycogen runs low, the body will gradually lose its capacity to produce energy from fat.

If the athlete is behind the carbohydrate intake versus expenditure curve, catching up is difficult and may be accomplished only by slowing dramatically or stopping the exercise. 
Carbohydrate must be taken in right from the beginning.

I shared my personal experience with bonking and how it impacts the performance here: IRONMAN 70.3 Marbella, Spain. February 23, 2019

The highest rate at which an athlete can expand the energy is 1000 calories/hour. It is almost impossible to replace all of it during the exercise, therefore it is really important to eat enough carbohydrates in 24-48 hours prior to the event. If you did, you’ve got about 1500-2000 calories in form of carbohydrates stored in your body, which should last you for a good part of the race, depending on intensity and time exercising.

During 4-12 hours events:

1. Consume approximately 200-400 calories / hour equally distributed every 10-20 minutes.
2. Calories should be coming from carbohydrates, ideally in liquid form.

Formula:
min 0.25 – max 0.33 grams carbs per hour x body weight (lbs)
Or
min 1 calorie/hour x body weight (lbs)

Therefore 150 lb (68 kg) athlete should aim on consuming no less than 150 calories per hour just from carbohydrates)

In addition, taking in some protein could help to the onset of nervous system fatigue. But keep in mind that protein requires extra hydration for absorption and digestion. Too much of it can cause nausea.

Use the guidelines above as the starting point and experiment. Every body is different, therefore go and test things out yourself.


If you’re up to something like this – you’re crazy and I wish you good luck! Nutrition is critical for such events Recommendations are the same as for the previous section, but in addition the solid foods now become necessity. It may include bananas, cookies, jelly sandwiches, fruit juices etc. Foods with higher glycemic index should be chosen. Don’t forget the protein, especially branched-chain amino acids.


That’s all I had in regards to fueling during the exercise. I hope you learned something new and implementation of it will improve your performance. Below I also attached a “cheat sheet” to be used as a quick reference for Pre, During and Post race fueling.

In addition, I uploaded the Race Pace Predictor, Fueling & Bonking Calculator for cycling, made by FASTFITNESSTIPS. This is a great tool to use while planning your race fueling strategy. Here are the video instructions of how to use it: Fueling Science! How to pace your cycling to avoid bonking.

If you have anything to add or correct me, please don’t hesitate to do so. I welcome any constructive comments and looking to learn.

Take care and stay strong my friends!

One thought on “Fueling During

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.