Glycogen: what it is, how it works and why it’s important for endurance athletes

Glycogen is stored energy. Just like a car needs petrol the human body needs a source of fuel to provide it with the energy required to perform its many functions. Food contains energy in the form of calories, which are gleaned from carbohydrates, fats and proteins, otherwise referred to as macronutrients, and when it comes to sport performance ‘carbs are king’.

What is glycogen?

Carbohydrates are broken down into glucose and what the body doesn’t use is converted to glycogen and stored in the muscles and liver, while the remainder gets stored as fat.

When you exercise at a high intensity, glycogen particles are converted to glucose which is then oxidised by muscles cells to produce adenosine triphosphate (ATP) used to fuel muscle contraction.

How is glycogen used during endurance exercise?

The most dominant energy system used during endurance sport is the aerobic system which utilises both carbohydrates and fats in the presence of oxygen. However, when the intensity of exercise increases, oxygen becomes less available, which is when the anaerobic system kicks in to play. This system utilises your stores of glycogen to make ATP more quickly, but by-products of this process include lactate and hydrogen ions. If the rate of lactate production outweighs its removal from the body, then muscle fatigue can occur, which jeopardises performance.

The point at which this occurs is called the lactate threshold, which defines the upper limits of sustainable effort during training and competition.

How can endurance athletes maximise their glycogen stores?

There are a number of ways in which endurance athletes can manipulate their training and diet to create adaptations in the body that help to maximise the storage of glycogen in the muscles.

Thirty-six-48 hours before racing an event lasting more than 90 minutes, athletes should try to increase their carbohydrate intake to the tune of 10-12g/kg body weight per 24hrs.

‘Train low, compete high’ is a method that involves training after an overnight fast or withholding carbohydrate intake during, and for 2 hours after, a hard training session to promote adaptations that lead to glycogen ‘super compensation’.

‘Train low, sleep low’ involves training with high carbohydrate availability in the evening but no carbohydrate refuelling before bed, then training with low carbohydrate availability in the morning to promote glycogen super compensation. The usefulness of these strategies needs to be weighed up against the ability of the athlete to perform at their best, both physically and mentally, in the absence of carbohydrates.

Certain training techniques can help athletes increase their lactate threshold to improve performance while endurance training increases the capacity for muscle to store glycogen.

How can you replenish your glycogen stores during an event?

During events lasting between 1 and 2.5 hours, it’s recommended that you consume 30-60g of carbohydrate per hour and you should start refuelling 20mins into your event (so, pretty much after the swim in the triathlon). The best way to consume your carbohydrate during a triathlon is a sports drink on the bike (6g carb per 100ml) and gels if you need them during the run

Ultra-endurance events lasting more than 2.5-3 hours may require a greater intake of up to 90g of carbohydrate per hour. As well as sports drinks and gels, athletes will likely need to explore the use of energy bars, chews and other products high in carbohydrates.

Any food or supplement must be tested before an event to ensure gastrointestinal (GI) compliance, palatability and ease of use. Drinks and gels should offer a combination of 2:1 glucose to fructose to maximise carbohydrate oxidation (glycogen replenishment) and reduce the risk of gut discomfort.

While increasing carbohydrate intake may help to improve performance there’s a practical consideration of how much an athlete can take on board without experiencing GI distress. Any increase in carbohydrate should be done slowly and testing for carbohydrate tolerance may be useful.

How can you replenish your glycogen stores after an event?

Immediately after an event, muscle cells which have sustained a significant depletion in glycogen become metabolically prepped for rapid replenishment, as the glycogen used during exercise switches on its synthesis during recovery.

Consuming carbohydrates shortly after exercise triggers an increase in insulin sensitivity and glucose uptake in muscle cells, which is a response that can remain elevated for up to 48 hours. It’s recommended that athletes repeat an intake of 1-1.2g of carbohydrate per kg of body weight per hour for the first 4 hours post-event to stimulate high rates of glycogen synthesis, and then resume to normal carbohydrate intake to meet their needs for that day.

So to summarise, carbohydrates are essential for performance in endurance events. Maintaining adequate carbohydrate in the diet before, during and after training and competing will help to equip the body with the glycogen required to fuel the body’s energy systems for a successful performance.

Rob Hobson is the head of nutrition at Healthspan Elite