Carbohydrates In Sports

Fatigue in sports can occur from a range of factors: lack of sleep, lack of energy from poor nutrition, fitness, physiological capacity and stress. With so much negative media coverage surrounding carbohydrates and fad diets demonising this food group, a lot of athletes have put themselves at a disadvantage by taking up these new crash diets.
In my private practice in Sydney as a a sports dietitian, I see athletes who are just trying to maintain weight and eat “healthier” for sport, with an intake of carbohydrates that is insufficient for their performance goals.
When we talk about fuelling for optimal nutrition and sports performance, we need to understand what energy systems are being used at what 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.
Adenosine Triphosphate (ATP) is the “fuel” currency used by muscles. It is the energy 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 now explore the different energy systems:

Phosphocreatine (PCr) system is used for very short bursts of intense activity. It operates optimally as a fuel source for up to 5-6 seconds of maximum effort.  
A weights workout initially relies heavily on this system. It is short duration, high intensity exercise. Phosphocreatine stores deplete entirely after 20-30 seconds of intermittent lifts. This also covers many sprint sports such as; 100 & 200m track races, 50m swims, as well as the high intensity components of many intermittent sports such as karate, football & cross fit. Stores of phosphocreatine can also be restored fairly quickly in between sprint or lifting efforts within 30 seconds.
The biggest fuel supply we have is glycogen, the carbohydrate that is stored within muscles and liver. Glycogen stores are also greatest in muscles that are used during training, which is a training adaptation. Carbohydrate can be used to produce ATP both aerobically (with oxygen) & anaerobically (without oxygen). Carbohydrate is an important fuel in exercise, but glycogen stores are fairly limited.
Glucose in the blood is a form of carbohydrate. This is whats commonly referred to as blood sugar. All carbohydrate in food (whether starch or sugar) is digested into glucose, fructose or galactose & is absorbed from the gut into the blood. The fructose & galactose are then converted to glucose in the liver. Blood glucose is absorbed from the blood into muscles as well as other organs. If you 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 mainly to prevent low blood sugars during exercise.

When exercising there are several factors influencing fuel selection:

Intensity – The higher the intensity, the greater the contribution of carbohydrate. At very high intensity (ie. sprint efforts, short duration high intensity) you use only muscle glycogen, not blood glucose.
Duration – as fuel stores in the muscle are depleted, you rely more on fuels provided through the bloodstream. If carbohydrate stores are running low, you rely more on fat & protein. This is also 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 & altitude all tend to increase the reliance on carbohydrate as a fuel source.
In endurance based sports, carbohydrate is a limiting factor in sports performance. Hence, if used correctly, can be an ergogenic aid. It is known that at a fixed pace, the higher the intensity, the more carbohydrate (grams/hr) you need to use to maintain that pace.
Also, over a fixed distance or time, the more carbohydrate (grams/hr) available to the body from glycogen & food, the higher the intensity you can sustain over that distance. Limited carbohydrate availability to the muscle 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 supplying ATP is too slow to fuel high intensity exercise.
Due to the limitations in oxygen delivery, fat can’t produce energy fast enough to be the main fuel source for high intensity activity. So, when carbohydrate is limited, the body relies more on fat as an energy source, which requires a reduction in intensity.
If glycogen stores run out in one muscle group, you can’t use stores from another muscle group e.g. in runners, if leg muscle glycogen runs low, can’t transfer glycogen from arm muscles – can only rely on delivery from blood glucose.
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. This is when you have “hit the wall”.

What does hitting the wall mean?

Hitting the wall is generally an experience of blood glucose becoming too low. This happens because muscle glycogen stores become used up & blood glucose is drawn into the muscle. When enough glucose is transported from the blood into the muscle, the blood glucose level falls.  You can still exercise once you’ve hit the wall but you generally have to reduce the speed / intensity so as to utilise more fat.
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 according to time for optimal performance
CHO required
Recommended intake
Carb type
Glu + Fru
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
Absorption of carbohydrates is a limiting factor in exercise lasting <2hrs. Glucose transporter SGLT1 becomes saturated. We can overcome this by using multiple transportable carbohydrates (glucose and fructose).

Multiple transportable carbohydrate can increase gastric emptying, carbohydrate oxidation into ATP, fluid delivery, reduce GI distress, reduce fatigue and improve exercise performance.
For endurance or longer duration events or exercise, the goal is to top up carbohydrate stores. If you’ve carbohydrate loaded in the days before, this shouldn’t require much, as you don’t use much carbohydrate overnight when you’re sleeping. It’s also important to still feel comfortable – not too hungry or too full – and note food can be an important part of psychological readiness and preparation.
If you’re a recreational athlete, it may be time to rethink your low carb diet plan. You may be putting yourself at a performance disadvantage, when you should be taking the stage!

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