Why should I replenish carbohydrates when I train?

You have probably heard some seasoned endurance athletes say things like "no energy is needed for 4 hours," "real athletes only run on water, carbs are not needed," "you must train the body to use fat stores"

The mantra "it works for me" combined with experience in the sport can weigh heavily when receiving tips and advice. But what does research actually say?

Let us start by answering the question: Why should I replenish carbohydrates when I train?

The Basics

Regardless of the sport, the body needs "fuel" to drive us forward during training and competition. The body uses both fat and carbohydrates for this process. In the muscle cells, fats and carbohydrates are converted into energy (ATP). The consumption of fats and carbohydrates varies depending on the level of exertion, but there is always an interplay. The higher the intensity, the more carbohydrates the body burns. This is because glycogen (stored carbohydrates) requires 7% less oxygen to extract energy (ATP) compared to fatty acids, and thus the fatty acids cannot be used as fuel when oxygen is not plentiful.

While we have nearly unlimited fat stores, we can only store carbohydrates in muscles and the liver for 1-2 hours of high-intensity activity, about 500g. Overall, glycogen constitutes only 4% of the body's energy reserves, yet it remains our dominant fuel during the majority of physical activities, which makes it interesting, especially if you want to achieve more than just to walk, casually roll, or glide during your runs.

An athlete starting with well-filled glycogen reserves will, after 2 hours of tough training, be down to levels where muscle efficiency drops drastically due to inhibited calcium handling in the muscle cells. The wall is now near! When glycogen stores run low and the intensity of the training decreases, we increasingly rely on fatty acids as fuel. 

Okay, carbohydrates are good, but shouldn't you also work on your fat burning?

Having a body that is good at using fat as fuel is certainly a good idea. But as mentioned above, the body almost always works with both fats and carbohydrates during activity. Part of the training adaptation from your long runs is that you become better at using fat as auxiliary fuel while also increasing your capacity to store glycogen when you provide energy during the session. Thus, you help the body from two fronts. More energy-efficient glycogen right from the start, and lower consumption of it when fatty acids can be used to a greater extent. A well-trained athlete who often trains with energy, however, still has high fat burning and is better at working with low glycogen reserves.

Several studies in recent years have shown that the body becomes better at using fat as fuel when training without energy or following a low-carbohydrate diet. However, most of these studies also show that this does not lead to any performance improvement. The body is smart and adapts to what we expose it to. If we provide more fat, it will work better with that. If we replenish with more carbohydrates, it will optimize the uptake and use of them. Recommendations state that during competition or truly high-intensity sessions, one should aim to consume around 90g of carbohydrates per hour. However, if you are doing a gentle long-distance session for 3-4 hours, you can aim for 30-60g/h and still improve your fat burning despite carbohydrate intake.

One of the largest and most cited studies on this subject came out last year by Louise Burke.


Photo: Jimmy Hansson

Summary

If your goal is just to have fun and training for health and performance improvement is not your priority, then you certainly "don't need" to consume carbohydrates or any other energy during any of your training sessions. You will become more worn out, hit the wall a bit more often, and not maximize your potential, but it's not dangerous to get tired. However, if your goal is to improve your performance, carbohydrate intake is of utmost importance for maximizing your training, recovering faster, and achieving higher continuity over time.

Even if you don't feel you need any energy during, for example, a 3-hour endurance session, you will be able to push harder during the session and recover better with energy intake. The interval session the next day won't suffer from empty glycogen stores. This becomes more important the higher volume of training you plan to undertake. However, it doesn't always have to be sports nutrition that you use. During a slower, longer endurance session, a banana, cinnamon bun, or candy can also serve as carbohydrate replenishment, of course. If the result is that you can maintain a slightly higher pace, your body will reward you immediately for this.


Why should I replenish carbohydrates when I train?

To answer the question: Carbohydrates are needed for efficiency reasons. They are more energy-efficient, cost less oxygen to use, and can be supplied during activity to always have "race fuel in the tank" and maximize your training.