Carbs & performance

Carbs & performance

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Carbs and Performance

By Gabrielle Maston


You’ve got a big race ahead of you but your performance has been less than ideal in the last few training sessions; you feel exhausted but just can’t put your finger on why!

Fatigue in sports can occur from a range of factors: lack of sleep, lack of energy from poor nutrition, fitness, physiological capacity and stress. One mechanism an athlete can have control over is nutrition quality to optimise training and decrease fatigue. With so many mixed messages surrounding carbohydrate intake and fad diets demonising this food group, a lot of athletes end up with poor nutrition due to an insufficient carbohydrate intake. This in turn places an athlete at a significant performance disadvantage.

When we talk about fueling for optimal nutrition and sports performance, we need to understand what energy systems are using what and at which times. After we understand the physiological mechanisms then we can start backing up our activities with the right fuel to feed these systems.

There are several fuel sources used during exercise. These fuel sources can come from within the muscle (stored) or from the blood. ATP is the ‘fuel’ currency used by the muscle. It is what muscles use to contract.  There is a very small store of free ATP in the muscle at rest – only a few seconds of energy, in fact. During exercise, ATP is generated from other fuel sources (fat, carbohydrate, protein or lactate).

Let’s explore these different energy systems:

The phosphocreatine (PCr) system is used for very short bursts of intense activity; PCr operates optimally as a fuel source for up to 5-6 seconds of maximum effort, so for short, sharp efforts.  

A weights workout initially relies heavily on this system due to the effort often being of a short but high-intensity nature. PCr stores deplete entirely after 20-30 seconds of intermittent lifts. Stores of PCr can be restored relatively fast, though – usually within about 30 seconds between sprint or lifting efforts. This also covers many sprint sports, including the 100-200m track events, 50m swimming events, as well as the high-intensity components of many intermittent sports such as combat sports and team sports.

A bigger carbohydrate fuel supply we have is glycogen – the carb that is stored within our muscles and liver. Glycogen stores are also greatest in muscles that are used most during training, which is part of training adaptation. Carbohydrates can be used to produce ATP both aerobically (with oxygen) and anaerobically (without oxygen). Carbs are an important fuel in exercise; however, glycogen stores are fairly limited.

Glucose in the blood is another form of carbohydrate. This is what’s commonly referred to as ‘blood sugar’. All carbohydrates in food (whether it’s starch or sugar) is converted into glucose, fructose or galactose and is absorbed from the gut into the blood. The fructose and galactose are then converted into glucose in the liver. Blood glucose is absorbed from the blood into muscles as well as other organs. When we are not exercising, the glucose is stored as glycogen when it enters the muscle; however, during exercise it can be used as a fuel source straight away. The liver also stores a small amount of glycogen which can be released into the blood – this is mainly to prevent low blood sugar levels during exercise. If our glycogen stores are full and there is excess glucose in our blood stream, it can be stored as fat.

When exercising there are several factors influencing fuel selection:


Intensity – The higher the intensity, the greater the contribution carbohydrates make (compared to fat and protein) to fuel exercise. At a very high intensity (ie. sprint efforts or short-duration high-intensity exercise) you use only muscle glycogen, not blood glucose.

Duration – As fuel stores in the muscle become depleted, you rely more on fuels provided through the bloodstream. If carbohydrate stores in the muscles and blood are running low, you rely more on fat and protein. This can be the case if your carbohydrate stores are not replenished sufficiently prior to exercise.

Training status – As people become more trained, they will utilise more fat as a source of fuel at the same absolute exercise intensity because they can get more oxygen into the muscles. However, as people get fitter they will generally be able to exercise harder for longer – at least they should be if they’re following a progressive training program.

Environment – Heat, cold and altitude all tend to increase the reliance on carbohydrate as a fuel source.

In endurance-based sports, carbohydrates are a limiting factor in sports performance; however, if used correctly they can be an ergogenic aid. It is known that at a fixed pace, the higher the intensity, the more carbohydrates (grams/hr) you need to use to maintain that pace.

Also, over a fixed distance or time, the more carbohydrates (grams/hr) available to the body from glycogen and food, the higher the intensity you can sustain over that distance. Limited carbohydrate availability to the muscles will result in either not sustaining a high intensity for as long or not being able to complete a set distance or time at the same intensity. This is because the fuel source to working muscles converting and suppling ATP is too slow to fuel the high-intensity efforts.

Because of the limitations in oxygen delivery, fat can’t produce energy fast enough to be the main fuel source for high-intensity activity. So when carbohydrates are limited, the body relies more on fat as an energy source, which requires a reduction in intensity.

Normally, if stores of glycogen are reasonably high, these stores will last 1-2 hours of moderate- to high-intensity endurance exercise if you exercise at a fixed pace without slowing down.

To get optimal sports performance out of carbohydrates you need to consume enough and time them right. This table is an idea of how many carbohydrates an athlete may need in relation to exercise time for optimal performance.



CHO required

Recommended intake

Carb type






Very small amounts

Small sips

Any form


Small amounts

Up to 30g/hr

Any form


Moderate amounts

Up to 60g/hr

High-GI only


Large amounts

Up to 90g/hr

High-GI only


Beware that absorption of carbohydrates is a limiting factor in exercise lasting less than two hours. Glucose transporter SGLT1 becomes saturated but you can overcome this by using multiple transportable carbohydrates (glucose and fructose). Sports drinks and gels are designed to use both glucose and fructose transporters for optimal absorption.

Using multiple transportable carbohydrates can increase gastric emptying, carbohydrate oxidation into ATP and fluid delivery, reduce GI-distress and fatigue, and improve exercise performance.

For endurance events and training, the goal is to top up carbohydrate stores regularly and start off each training session with full glycogen stores. If you’re an athlete on a low-carb diet, it may be time to rethink your nutrition plan. You may be putting yourself at a performance disadvantage when you could be taking the stage!



1.   Romijn JA. et al. (1993). ‘Regulation of endogenous fat & carbohydrate metabolism in relation to exercise intensity & duration’. Am J Physiol. 265(3 Pt 1):E380-91

2.   Burke & Read 1987, Sherman et al. 1981


Gabrielle Maston BSc Hons
Exercise physiologist, clinical and sports dietician at Changing Shape diet and exercise specialists in Sydney, health and wellness writer, consultant and lecturer at FIA fitness college. 

E-mail Gabrielle at  or visit for more information.

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