Carbohydrates (Carbs/CHO) are one of the main macronutrients that we can utilise via oxidation for fuel, and they have been found to be highly beneficial for sporting performance and a very effective ergonomic aid (Cermak & van Loon, 2013). They’re a big topic of debate in the world of fuelling, nutrition and health so I’m going to delve into them more and hopefully give you plenty of useful information on why carbs should be an essential part of your fuelling regime for sports.
The way we get energy to perform an action, such as riding a bike, is through oxidation of substrates. We get this normally through oxidation (aerobic respiration) of lipids (fat) or carbs via the tricarboxylic acid (TCA) cycle (aka Krebs or citric acid cycle), glycolysis (can occur aerobically or anaerobically) and oxidative phosphorylation. Going into detailed explanations of these processes would require a fair few essays! The main point is that we derive energy from the breakdown of these fuel sources, but there are some key differences between the two. Fats are more energy dense and can be broken down into more ATP (adenosine triphosphate – an organic compound which provides energy to cells) than carbs. ATP complete oxidation of one palmitate molecule (fatty acid containing 16 carbons) generates 129 ATP molecules, whereas for each molecule of glucose that is processed in glycolysis, a net of 36 ATP molecules can be created by aerobic respiration. The caveat here is that fat requires more oxygen to breakdown, which is fine when we have that oxygen available. But what happens when exercise intensity increases and oxygen demands increase? Well, if we’re talking primarily aerobically (since anaerobic processes still occur during low intensity the same way aerobic processes still occur during high intensity), as exercise intensity increases, our capacity to break down fats decreases, and we need to start oxidising more carbs as they require less oxygen to breakdown. However, whereas we can store 5420 - 6670MJ of potential energy in fat stores (Newsholme & Leech, 1994), we can usually only store approximately 10.4MJ potential energy from carbs, while the liver contains approximately 80 g glycogen representing 1.25MJ potential energy (Martin & Klein, 1998). This means that, for long duration high intensity exercise, we need to consume exogenous carbs to fuel our performances, be it training or racing, as our endogenous supplies of carbs are very limited whereas fat stores can last us for days of low intensity exercise.
With the focus of this piece being on carbs, let’s consider how much carbohydrate we use during different exercise intensities. This is very difficult to determine as, although there are general rules of thumb (e.g. exercise beyond 70% of Critical Power is often seen as the turn-point for carb metabolism over fats, AKA supposed lactate threshold/LT1 or Ventilatory threshold/VT), there is no ‘one size fits all’ answer. One consideration is training status, as more well-trained athletes have been found to oxidise fat at higher rates even at higher intensities than amateur athletes (Hetlelid et al., 2015), suggesting that less well-trained athletes use more carbs relatively at lower intensities. But another part of this is that the higher-trained athletes are likely performing at a higher pace relatively so are therefore using more energy. For example, Rider A (amateur) and Rider B (professional) are both riding at their Critical Power. For Rider A that is 200 watts (720 kcal/hour), for Rider B that is 400 watts (1440 kcal/hour). Let’s say that Rider A requires 90% of their energy to come from carbs and Rider B only needs 70%. That’s 648 kcal from carbs for Rider A but a massive 1008 kcal for Rider B, and as carbs are ~4 kcal/gram, that’s 162g compared to 252g, a huge amount of energy required. (This isn’t exactly how it works but it does give a good idea of energy requirements and demands). Then, finally, there’s the variability at the same power levels to consider. It was found that in 10 professional riders riding at 300 watts, carb oxidation ranged from ~2.8 - ~4.3 g/min and fat oxidation ranged from ~0.3 - ~0.8 g/min (From James Spragg of Spragg Performance), which shows us that carb usage varies considerably from person to person. We can measure the rate of carb oxidation but it requires specialist equipment. Therefore, what we do is work off ranges of carb intake that we prescribe in training (as part of our nutrition package) and then monitor various metrics and assess if the carb intake needs to be increased or decreased for different kinds of sessions.
For the sake of this piece, we’re going to assume exercise is going to last longer than 90 minutes (less than that and your body’s carb stores should be ample). So we’re talking A Sunday in Hell, Hammer Climb 1, It Seemed Like Thin Air and, worst of all, Kitchen Sink Mash-up! For these sessions, if you don’t ingest exogenous carbs, you will not be able to maintain a high intensity of exercise and will experience the dreaded ‘bonk’ where performance will drop off a cliff. But how many carbs do we need to consume? This is where things get complicated as this is variable from person to person, as well as being variable within one person themselves. The first things to consider are the duration and the intensity of the exercise that we are doing, as carbs will still be used during low intensity so, if doing long duration, you should still consume exogenous carbs. Another factor is the rate at which you are burning carbs, as a lighter rider will use fewer carbs at a given intensity (or percentage of carbs as contribution to energy production) than a larger rider if both are doing the same W/Kg. Next, we must consider the types of carbs that are consumed, as the type of carbs makes a difference to the amount that you can oxidise and the rate at which this happens. Previous thought was that the maximum oxidation rate of ingested carbs was 1g/min using glucose as the carb source. However, more recent studies have found that 1.75g/min is possible with mixed sources of carbs consumed, often at a ratio of 2:1 glucose to fructose (Jeukendrup, 2013). However other studies have found that the tolerable level of carb intake can be as much as 144g/hour with an oxidation rate of 105g/hour, again a mix of difference carb sources (Jeukendrup, 2007). But this is an extreme amount, and it has been found that not all individuals can tolerate this quantity of carbs being consumed, even with gastrointestinal (GI) training. GI training is an important part of training which is often neglected, as doing so can increase our tolerance to ingested carbohydrates significantly. However, even with GI training, we each have a maximum carb intake that is tolerable for us as individuals that is unlikely to be able to be surpassed without negative impacts on performance.
We know that we need carbs for performance, but they are also vital for health. One of the biggest risk factors in sport is when riders want to lose body fat and reduce either their total intake of food or limit their carbohydrate intake. Often, it is carb intake which is limited and fasted rides are performed regularly in order to try and improve the rate of fat oxidation and therefore, reduce body fat. However, this is not a good habit to get into as, when we overtrain our fat oxidation capacity, we lose some of our carb oxidation capacity (glycolytic enzyme reduction), and you’re left with decreased high intensity aerobic capacity which means that, race performance will likely be limited. Additionally, if calorie intake is limited, then recovery is impaired as well as cognitive function. In chronic low energy availability (LEA), which often occurs as a result of not consuming sufficient carbs to properly fuel training and recovery, there are big risks to be aware of including low bone mineral density and serious hormone imbalances (Viner, Harris, Berning, & Meyer, 2015). The impacts of these in the short term can be significantly impaired performance and, in the long term, the early onset of osteopenia or osteoporosis, as well as the possibility of becoming infertile. However, consuming enough carbs in and around training does help reduce the likelihood of these conditions occurring as a result of LEA. Additionally, strength/resistance/impact training can help with maintenance of bone mineral density.
To summarise:
- Carbs are a vital source of energy for cycling
- We all burn them at different rates due to a multitude of factors
- We all need to consume different amounts and can train the gut to tolerate more - to a limit
- Carbs are vital to help maintain adequate energy availability and maintain health
References
Cermak, N. M., & van Loon, L. J. (2013). The use of carbohydrates during exercise as an ergogenic aid. Sports Medicine , 43 (11), 1139-1155.
Martin, W. H., & Klein, S. (1998). Use of endogenous carbohydrate and fat as fuels during exercise. Proceedings of the Nutrition Society , 57 (1), 49-54.
Hetlelid, K. J., Plews, D. J., Herold, E., Laursen, P. B., & Seiler, S. (2015). Rethinking the role of fat oxidation: substrate utilisation during high-intensity interval training in well-trained and trained runners. BMJ open sport & exercise medicine , 1 (1), e000047.
Jeukendrup, A. E. (2013). Multiple transportable carbohydrates and their benefits. Sports Science Exchange , 26 (108), 1-5.
Jeukendrup, A. (2007). Carbohydrate supplementation during exercise: does it help? How much is too much. Sports Science Exchange , 20 (3), 1-6.
Viner, R. T., Harris, M., Berning, J. R., & Meyer, N. L. (2015). Energy availability and dietary patterns of adult male and female competitive cyclists with lower than expected bone mineral density. International journal of sport nutrition and exercise metabolism , 25 (6), 594-602.
