Is "Carbo-Loading" as straightforward as we think?

Having recently returned from the Ironman World Championships in Kona, “Carbo Loading” in the race week was something mentioned is just about every conversation. Like it was a fact. However, carbohydrate loading prior to an endurance event is indeed ubiquitous – but why? And how should we do it? Let’s discuss.

Energy stores

This will be a refresher for regular readers of my blog, or of our monthly summary of the latest endurance science research, TSS – but understanding the whys and hows of pre-exercise carbohydrate loading requires some knowledge of the energy stores we have in the body, and how we use them during exercise.

During endurance exercise, particularly relatively even-paced events like marathons or Ironman triathlons, almost all of the energy production we need to power muscle contractions come from aerobic metabolism, or the breakdown of fuels in oxygen-dependent pathways. That’s why we have lungs, our heart, blood, and blood vessels, to deliver oxygen to our tissues for use in aerobic metabolism.

Aerobic metabolism uses energy stored in carbohydrates and fats, and, accordingly, we have carbohydrate and fat energy stores in the body. Carbohydrate energy is stored as glycogen, primarily in the muscles themselves, but also in the liver (1). These glycogen stores are finite, and we typically have less than 3000 kilocalories of carbohydrate energy stored in the body (2). That’s not enough energy to complete a marathon, assuming typical values for running economy. Accordingly, during exercise of sufficient length and intensity, glycogen stores can be depleted to very low concentrations (3–8).

In contrast, we have an enormous amount of energy stored as fat – even if you’re very lean. If you are 70 kilograms, and have only 10% body fat, you have 7 kilograms of fat energy stored, which is closer to 70,000 kilocalories of fat energy. We reckon that’s enough energy to – theoretically – complete six or seven full-distance Ironman triathlons back-to-back (9). There are probably many reasons why you might not be able to complete six or seven Ironmans back-to-back, but running out of energy stored as fat is not one of them.

During endurance exercise, we burn a blend of carbohydrate and fat to support the required rate of energy expenditure, which is determined by the work rate, power output, or running speed (10). Our highest rates of fat breakdown usually occur at moderate intensities, right around that first threshold or moderate-to-heavy intensity transition (11), although this does vary quite a bit from athlete-to-athlete. In contrast, the overall rate of glycogen breakdown increases as exercise intensity increases, and our reliance on glycogen breakdown increases at higher exercise intensities (12).

 Glycogen stores and performance

We have long known that glycogen stores have a powerful influence on exercise performance. A classic study, published by Jonas Bergström and colleagues way back in 1967, provided the first evidence for this (13). In this study, the pre-exercise muscle glycogen content of nine volunteers was manipulated by consumption of low, moderate, and high carbohydrate diets, after a glycogen-depleting bout of exercise. When the subjects returned to the laboratory, they rode to exhaustion at 75% of their V̇O₂max. In all three trials, muscle glycogen was very low at exhaustion – the participants therefore lasted longer when they had more muscle glycogen at the start of exercise. Therefore, a logical interpretation of this study is that depletion of muscle glycogen is implicated in fatigue during endurance exercise, and that having a bigger muscle glycogen store before an event is probably helpful.

Our understanding of the role of muscle glycogen in fatigue during exercise has evolved since 1967. Recently, we have come to learn that glycogen is stored in specific sites within muscles, and that glycogen in these different sites has different roles in muscle contraction (14). We won’t get bogged down in the subcellular details here, but a series of recent studies suggest that depletion of specific muscle glycogen stores impair the mechanisms of muscle contraction, which gives us a mechanistic link between muscle glycogen depletion and fatigue (15). Studies published over the last year or so suggest individual muscle fibres become inexcitable – unusable – when glycogen-depleted beyond a certain level, which has implications in the latter stages of long events, as the most-aerobic, type I fibres are used and therefore depleted first (16, 17).

It's not all about muscle glycogen, though. As I have mentioned before, liver glycogen stores are also burned during endurance exercise (6–8). Liver glycogen has an important role in the maintenance of blood glucose concentrations, as liver glycogen is broken down to glucose and released into the blood. This is important, as we don’t like our blood glucose getting too low – that’s the light-headed feeling you can get when you’re running low on fuel. During prolonged endurance exercise, your blood glucose can start to drop (18, 21, 23, 25, 26) – Bergström and colleagues saw this in 1967, too (13) – and this hypoglycaemia may also be part (if not the most important part) of the fatigue equation (19).

Carbo-loading

Therefore, given our carbohydrate energy stores are finite and can be depleted during endurance exercise and given that this depletion is implicated in fatigue during endurance exercise, it makes sense that we might want to start our race with as much glycogen in our muscles and liver and possible. That’s essentially the rationale behind pre-event carbohydrate loading. Bigger tanks take longer to empty right?

There is merit to this. As discussed, Bergström et al. found that time-to-exhaustion at 75% of V̇O₂max was longer when glycogen stores were highest (~189 min) rather than moderate (~126 min) or low (~59 min).

Therefore, sports nutrition guidelines typically recommend high rates of carbohydrate ingestion in the days immediately prior to an event, to ensure that muscle glycogen stores are topped-up, or rather ‘supercompensated’ to higher-than-normal levels, and that liver glycogen is high and therefore adequate to maintain blood glucose concentrations throughout the event. For example, the 2016 guidelines in Medicine and Science in Sports and Exercise call for a whopping 10-12 grams of carbohydrate, per kilogram of body mass, per day, in the 36-48 hours in advance of a competition lasting longer than 90 minutes (27). That’s a lot!

However, the equation is a little more complicated than it first appears. When you have more glycogen in your muscles, you also tend to burn it more quickly, which means that starting exercise with 20% more glycogen doesn’t mean you’ll still have 20% more after an hour – the gap will narrow as exercise progresses (20, 22, 28). We see this ‘auto-regulation’ in glycogen metabolism during long bouts of exercise, as the rate of glycogen breakdown slows as exercise progresses, as our glycogen stores are reduced, even if the power output or running speed is held constant (29).For Ironman this is an important fact, as we are talking about very long events.

Accordingly, not all studies have found that pre-exercise carbohydrate loading has a massive impact on actual performance. A study from 1981 had runners consume different diets for six days prior to a 20.9-km – almost half-marathon – run test (24). In a repeated-measures design, the runners consumed diets containing: (i) low carbohydrate for three days, then high carbohydrate for three days, (ii) moderate carbohydrate for three days, then high carbohydrate for three days, and (iii) moderate carbohydrate for six days. Pre-run muscle glycogen was higher after the first two diets than the purely moderate carbohydrate diet, by ~25-30%, but performance in the run test was not different between the trials.

How I do it

If you read my blog, you’ll know that I’m not always on the side of super high carbohydrate intakes – in the regular diet or during exercise. I try to take a more nuanced view, guided by the evidence. The same goes for pre-event carbohydrate loading.

I agree that it is important to start your event with topped-up muscle and liver glycogen stores. When you wake up, having fasted overnight, your liver glycogen stores are typically reduced by ~25% compared to when you went to bed (30). You can rescue these stores, to help you to regulate your blood glucose concentration during the race, with your pre-event meal. So, I do certainly recommend having some carbohydrates for breakfast before your race. I typically have oats soaked overnight in water with a scoop of protein powder. This is typically around 40-50 g of carbohydrates and 20 g or protein.

I don’t, however, see the need to go into endurance events with the incredibly-high rates of 36-48-hour carbohydrate ingestion recommended in the guidelines to maximise muscle glycogen stores (27). I do see value in ensuring these stores are topped-up, and I typically recommend doing this through a combination of reduced training load during the taper and slightly-higher-than-normal carbohydrate intake for a couple of days. As I said, stretching muscle glycogen to their absolute maximum will accelerate the rate at which they are used during the race, so differences in muscle glycogen content after an hour-or-two are not as pronounced as you might think.

For example, a classic study from 1985 compared muscle glycogen content before and after 105 minutes of cycling at 70% of V̇O₂max – likely close to Ironman power – on two occasions, once after a 16-hour overnight fast, and once four hours after consuming a meal containing two grams per kilogram of body mass of carbohydrates, along with some protein. The pre-exercise meal successfully increased pre-exercise muscle glycogen content – the cyclists had ~42% higher muscle glycogen immediately prior to their ride in the fed trial. However, they also used it ~50% faster in the fed-state trial, such that muscle glycogen content in the two trials was not statistically different after 105 minutes in the saddle. Not even the halfway point of an Ironman distance bike ride. I appreciate this isn’t strictly a study of carbohydrate loading, but you get my point – having more muscle glycogen before exercise doesn’t mean you’ll have more left in the muscle after a couple of hours or so.

My approach to competition and muscle glycogen is focused instead on slowing the rate at which it is used, by training the body to improve the rate at which those limitless fat energy stores are used at competitive intensities. If we are able to use fat at higher rates during exercise, this reduces the burden placed on those precious muscle glycogen stores, and delays fatigue. You can do that by limiting carbohydrate intake to what is necessary during training, or what I call the “right fuel, right time” approach – this switches on genes involved in fat metabolism (31), and helps the body to develop the adaptations required to burn fat at high rates during exercise. When I was racing at my best, I aimed to consume around 100–150 g of carbohydrates per day during regular training. In the 3 to 4 days leading up to a race, I increase this to approximately 200–220 g per day. While my approach may not be typical, it demonstrates a modest increase in line with a significantly reduced training load (50% of normal load). This method has always worked well for me.

It is also worth noting that better trained athletes also seem to sit at higher resting muscle glycogen concentrations (32), so getting properly fit is another means of boosting your pre-exercise muscle glycogen contents.

Summary

In summary, pre-exercise carbohydrate loading – or carbo-loading – is synonymous with preparation for an endurance sport event. Athletes do this because our carbohydrate energy stores – glycogen stored in the muscle and liver – are finite, becoming depleted during prolonged exercise. As this glycogen depletion is thought to be related to the development of fatigue, it makes sense that we should want to stand on the start line with as much glycogen in our muscles and liver as possible.

In my view, we do certainly want to ensure these stores are topped-up, but I don’t think that crazy-high rates of carbohydrate intake in the day or two prior to an event are necessary. There’s a number of reasons for that, not least because glycogen is used at rates in proportion with how much glycogen we have stored. I prefer to focus instead on stimulating adaptations that develop our capacity for fat oxidation during exercise in training, and the boosting of resting muscle glycogen content through years of training.

In the few days in advance of your race, you can ensure your glycogen stores are topped-up by reducing your training load – you’ll do this anyway as you taper – and eating slightly more carbohydrate than normal.

EndureOn!
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