After the previous blog where we discussed what sugars are, in this blog we will look at the role of sugar for energy provision. We will discuss the biochemistry in simple terms, as well as differences between different sugars. We will also look at the journey from the moment a carbohydrate is consumed till it is used in the muscle and exhaled as carbon dioxide.
From sugar ingestion to energy
We often say that we use carbohydrate as fuel, but it is actually mostly glucose (or sugar) we use. Put another way, we ingest a range of carbohydrates, but it is primarily glucose that is used by our cells. When we use glucose as a fuel, this happens in a number of steps. The first step is that we break down glucose in a process called glycolysis, where the end product is pyruvate. Each glucose molecule can produce 2 pyruvate. This pyruvate can be broken down even more in the tricarboxylic acid (TCA)-cycle (or Krebs cycle). Both these steps generate a lot of adenosine triphosphate (ATP), which we can use for muscle contraction or for other processes. In the end, all 6 carbon atoms from our glucose molecule end up as CO2 in the gases we expire.
During high intensity exercise, we generate a lot of pyruvate through glycolysis. The TCA cycle cannot remove pyruvate fast enough. This would mean that pyruvate would accumulate and this in turn would stop glucose breakdown… the result would be that we cannot produce ATP anymore and we fatigue. Therefore, the body has another mechanism to remove pyruvate so that we can keep producing ATP. The pyruvate is converted to lactic acid. So contrary to what many commentators on TV will tell us, lactic acid is not causing fatigue, it is actually preventing fatigue. Lactic acid dissociates into lactate and hydrogen ions. The accumulation of hydrogen ions may eventually cause a burning sensation in the muscle and may indeed contribute to fatigue. Lactate, like pyruvate are simply intermediates of carbohydrate metabolism and can also be used as fuel by the muscle.
From sugar ingestion to utilisation in the muscle
When carbohydrate is ingested, it is first digested (this means broken down to its smallest components; usually mostly glucose). This glucose will then be absorbed through the portal vein into the liver. The liver can either store the glucose or pass it on to other tissues. A lot of the glucose will be taken up by the muscle, especially during exercise. When sugars like lactose or sucrose are ingested, these are also broken down into their smallest units. The glucose will follow the fate we just described but the fructose and galactose are handled a bit differently. Fructose and galactose are converted to glucose or lactate in the liver and then released into the circulation. This is one of the reasons that these sugars are a bit slower than glucose, as there is an extra step before they can be used as a fuel.
Muscle contraction needs fuel
As we mentioned earlier, the fuel our muscles use is called adenosine triphosphate or ATP. We have different ways to generate ATP. Some ways are faster, some are slower, and some ways require more oxygen, some require less oxygen, and some don’t require oxygen at all.
The fastest way to generate ATP is through the breakdown of phosphocreatine, but this is really just a small energy source for short sprints. The next fastest way to produce ATP is through the process of glycolysis. The process is called glycolysis if glucose is the starting point or glycogenolysis if glycogen (the storage form of glucose) is the starting point. Glycolysis does not require oxygen and takes place very rapidly. During an all-out effort of 30-60 sec, the ATP for that muscle contraction will be mostly from glycolysis. A common misconception is that glycolysis only occurs when there is no oxygen in the muscle, but this is not true. The process doesn’t require oxygen but in virtually all situation, there is plenty of oxygen available in the muscle.
As mentioned above, the end product of glycolysis is pyruvate and this pyruvate can be further broken down in the TCA cycle. The TCA cycle is closely linked to another process of oxidative phosphorylation, where oxygen is used to endure a steady ATP production.
This process requires oxygen, and also delivers ATP although not quite as quickly as glycolysis. The first step (glucose to pyruvate) is called anaerobic metabolism, the second step (pyruvate to CO2) is aerobic metabolism (as shown in the infographic above).
We can also use fatty acids to generate ATP through this same TCA cycle. This process requires another pathway, called beta oxidation, and this process requires more oxygen. It is also much slower. In the infographic below you can see the maximum rate of ATP synthesis from different metabolic pathways.
Fat burns in the flame of carbohydrate
There is an old expression that has some truth to it: Fat burns in the flame of carbohydrate. This was derived from early studies that showed that fat oxidation was less efficient in the absence of carbohydrate. So essentially carbohydrates are essential for energy generation in many different ways: carbohydrate is an important substrate but it is also an important enabler of metabolism in general.
It is certainly true that glucose (or glycogen) is the most important performance fuel for most sporting events. Some very explosive short events and some extremely long events (24h or more) may be a little less dependent on carbohydrate but usually the training for those events will still require carbohydrate.
For the discussion about sugar and whether this is important or damaging for athletes, it is extremely clear that from a performance point of view sugars (in particular glucose) are extremely important. However, in order to see the effects, we can also ingest polysaccharides like maltodextrin, or some types of starch, as these carbohydrates behave similar to glucose. In the end, however, these chains of glucose will be broken down and it is a sugar (glucose) that will provide the vast majority of the energy for performance. In a separate blog we bring this all together to tackle the question: Is sugar bad for athletes?
Jeukendrup AE and Gleeson M. Sport Nutrition 2018 Human Kinetics Champaign IL