I recently made a post on Instagram that got a lot of attention and questions, and it was one that compared the percentage efficiency of carbohydrate (CHO) ingestion when was taking 120 g of CHO per hour. There were a lot of questions, and of course it’s hard to explain in enough detail via an Instagram story (see below).
So, as such, in this blog, I am going to have a look at the emerging literature suggesting it may be possible to ingest carbohydrate at rates as high as 120 grams per hour during exercise (that’s ~5-6 gels!). Carbohydrate ingestion during training and racing has a long history in exercise physiology, sports nutrition, and endurance sport. Despite decades of research investigating the optimum dose, type, and form of carbohydrate to support performance, research is still emerging that challenges our practices. So, let’s get into it.
Why carbohydrate during exercise?
We have often discussed how whilst the body’s fat energy stores are vast and effectively limitless during exercise, our carbohydrate energy stores – muscle and liver glycogen – can be depleted to low concentrations during prolonged, vigorous exercise. Therefore, a vast literature has emerged over the last ~100 years on strategies to preserve our carbohydrate stores during exercise (our endogenous stores), such as through consumption of exogenous carbohydrate in sports drinks and gels (8, 13, 15). As we will discuss, consuming exogenous carbohydrate during exercise provides an alternative fuel source and accordingly reduces the rate at which our own endogenous carbohydrate stores are depleted (14). Ingesting carbohydrates during exercise has consistently been shown to improve performance (13), and I certainly recommend that carbohydrates are part of your race-day nutrition strategy in Ironman and Half-Ironman events.
For completeness, we should acknowledge that carbohydrate ingestion during exercise has the potential to improve endurance performance beyond simple effects on carbohydrate availability. In fact, the simple presence of carbohydrates in the mouth stimulates receptors that seem to have some effect on the brain that improves performance – as evidenced by mouth rinsing studies, in which carbohydrate-containing solutions are swilled around the mouth and spat out. These trials have shown that even this can improve performance (13).
So, we have plenty of evidence to support carbohydrate ingestion during competition. An obvious question is, how much?
A question of dose – rate-limits and multiple-transportable carbohydrates
Research on the optimum dose of carbohydrate – that is, the rate of intake in grams per hour – has generally sought to determine the rate of carbohydrate ingestion that maximises exogenous carbohydrate oxidation rates. By that, I mean the rate at which carbohydrates in the sports drinks are metabolised; there seems little point in ingesting carbohydrates in sports drinks and gels that aren’t even metabolised – this just increases the risk of gastrointestinal issues. More isn’t always better.
The early research found that when we ingest our carbohydrates as glucose, we maximise our exogenous carbohydrate ingestion rates at ~60 grams per hour (6). However, when we ingest a mixture of glucose and fructose – two slightly different simple sugars – we can increase exogenous carbohydrate oxidation rates up to ~90 grams per hour (3, 4, 14). This increased exogenous carbohydrate oxidation with combined glucose-fructose ingestion likely occurs as glucose and fructose are trafficked across the gut and into the blood using different transport proteins, and the rate-limit for glucose alone is ~60 grams per hour. More glucose would back up in the gut. Fructose gets across using a different transporter.
The new data
To some extent, we thought the research in this space was relatively settled. Ingesting more than 90 grams per hour seemed unlikely to result in greater exogenous carbohydrate oxidation or further improvements in performance, and, if anything, may make gastrointestinal stress more likely.
The new study showed that ingestion rates of 120 grams per hour during three hours of moderate-intensity cycling (95% of the lactate threshold) was tolerated well by a group of male endurance athletes (2). If that sounds like a lot of carbohydrate, it is. Carbohydrates were ingested as combined glucose/maltodextrin (effectively the same thing – maltodextrin is just glucose, but less sweet) and fructose in different forms – drink, gel, or as a semi-solid chew, these comparisons were the main purpose of the study. These results therefore suggest that multiple transportable carbohydrate approaches may allow even greater rates of carbohydrate ingestion to power performance than previously thought.
However, as is the case in most studies of exogenous carbohydrate oxidation rates during exercise, oxidation efficiency – the percentage of ingested carbohydrate that was used for oxidation – was only ~75%. We can’t be sure where the remaining 25%, which is substantial at ~30 grams per hour, went; it may have been slowly building up in the gut, which has the potential to cause problems later, or it might be that the method used to estimate exogenous carbohydrate oxidation rates – which involves labelling ingested carbohydrates with high molecular weight carbon that is tracked on the breath - underestimates the absolute rate of exogenous carbohydrate oxidation. My best guess is that it is a combination of the two. This aligns also with other work showing worse oxidation efficiency at 120 vs. 90 grams per hour (~76 vs. ~86%), and therefore evidence that the extra 30 grams per hour might not be benefitting you from a metabolic or performance perspective (12).
We should also consider some of the simple details of the study. The study was conducted in cycling rather than running, and we know that gut tolerance is much better during cycling. We can’t say what would have happened to gut comfort had participants needed to jump off their bikes and run a marathon, either. Importantly, the study was conducted in male athletes; we would need to see these data replicated in females before recommendations could be made for female athletes. It is quite possible, but by no means certain, that males may be able to tolerate carbohydrate ingestion at higher rates during exercise due to having (on average!) larger stomachs, intestines, and livers (carbohydrates are first taken to the liver after entering the bloodstream). Also, whether the carbohydrates would have been tolerated so well at competitive intensities above VT1, we can’t say – my hunch is probably not.
Another study published last year, this time led by leading sports nutritionist Tim Podlogar, adds to the story (12). Tim and colleagues measured exogenous carbohydrate oxidation rates during three hours of cycling at 95% of VT1, so not dissimilar from Ironman power. They had participants ingest glucose-fructose mixtures at 90 and 120 grams per hour, and compared exogenous and endogenous carbohydrate oxidation rates.
The higher dose did increase exogenous carbohydrate oxidation rates; however, it didn’t decrease endogenous carbohydrate oxidation rates – it just increased total carbohydrate oxidation rates overall. This means that the extra carbohydrate ingestion wasn’t sparing any muscle or liver glycogen, so it wasn’t really worth the bother. Therefore, the extra exogenous CHO seems likely to be blunting fat oxidation, rather than sparing any glycogen.
To my mind, these two new studies show that, whilst it may be possible to ingest carbohydrate at the mighty ingestion rate of 120 grams per hour – at least in some people – it’s probably not worth the bother if you’re trying to boost performance, as it doesn’t seem to help us preserve our finite endogenous carbohydrate stores. It might be useful for multi-day road cycling events, where you might need to use the hours on the bike to help get in the mega-calories required to maintain energy balance and body mass.
Adding to that, in triathlon, 120 grams per hour is a logistically challenging regimen. Where do we store all that extra carbohydrate? Sure, it seems quite feasible professional cycling, where domestiques can collect sandwiches, drinks, and gels from the team car. Anyone who has done a triathlon competitively knows we don’t have that luxury.
There is also some interesting research that suggests we should be cautious to avoid ‘overdosing’ on carbohydrate, rather than just ingesting as much carbohydrate as we can tolerate. In 2018, Andy King and colleagues reported that ingesting 112.5 grams per hour glucose and fructose in a 2:1 ratio during two hours of cycling at 77% of V̇O2max resulted in GREATER (yes greater) muscle glycogen use, compared to 90 grams per hour; this higher ingestion rate may also have had negative effects on performance in a subsequent 30 min time-trial (10). These results were mirrored in a follow-up study in which glucose and fructose was ingested at 80, 90, and 100 grams per hour during three hours of cycling at 60% of V̇O2max; muscle glycogen oxidation was lower and subsequent time-trial performance better with 90 compare to 100 grams per hour (9). Once again, more isn’t always better. In fact, in these two studies, it appears less is more.
Gut training: Can we unlock higher exogenous oxidation rates?
When we perform repeated exercise training, we challenge our muscles and cardiovascular systems such that they adapt and improve their capacity to respond to the exercise stimulus next time. What about our guts? If we repeatedly challenge our gut to absorb exogenous carbohydrate at the highest rate possible, will it adapt and allow greater rates of exogenous carbohydrate absorption and oxidation in the future? This is the principle behind ‘training the gut’ (1, 7). Can gut training ‘unlock’ the benefits of ingesting carbohydrate at a rate of 120 grams per hour?
The research on training the gut suggests…probably not. A very recent systematic review found that gut training may improve gut discomfort during exercise with high rates of carbohydrate ingestion, and possibly explaining that, reduced carbohydrate malabsorption (undigested carbohydrate making its way into the colon, which causes issues) (11). The two studies that assessed gastric emptying (the rate at which carbohydrate passes through the stomach) have not seen benefits, although we should note that it’s probably what’s going on in the intestines that is important in this context.
But what about other high-profile long-distance triathletes known to have great success with 120 grams per hour carbohydrate ingestion rates? Well, I have a couple of thoughts here. First, there is a large variation in exogenous carbohydrate oxidation rates. This variation exists both within individuals and between individuals. It means that certain environments and situations can change maximal exogenous oxidation rates (within-individuals) – e.g. heat stress reduces exogenous carbohydrate oxidation rates (5), and some athletes can oxidise exogenous carbohydrate at higher rates than others (between-individuals) (1). Figure 1 from the Hearris et al. (1) shows the large variability in oxidation rates between subjects, with one individual reaching close to 1.9 grams per minute, while most are around 1.6 grams per minute, and the lowest at approximately 1.3 grams per minute. To be the best in the world at a sport like triathlon, it undoubtedly requires some "freakish" genetics, so it might be that higher oxidation rates are occurring here. However, we can be sure that it's most likely genetics that are driving these higher oxidation rates, and not any "gut-training." What's important here, from my perspective, is that these individuals are not the norm. Therefore, it seems nonsensical for other athletes to blindly copy the same strategy. Second, we must consider that it is possible that these athletes are winning despite this nutrition strategy, not because of it. So, at present, we don’t yet have evidence that ingesting carbohydrates at rates above 90 grams per hour spare our endogenous glycogen stores, or improve performance, and we don’t yet have evidence that training the gut will help with this. I remain open-minded if new evidence comes to light, but, for now, I think the extra carbohydrates simply aren’t worth the risk. Particularly for Age-group triathletes who likely have lower habitual CHO oxidation rates, and lower overall energy outputs generally. And moreover, “gut-training” is an expensive habit. As I have said is previous blogs, we do have to consider both sides of the coin. Attempt to train your gut or improve your fat metabolism.
Figure 1. Taken from Hearris et al. (2), peak oxidation rates of all 9 subjects from the study for: drink (circle), gel (square), chew (inverted triangle) and mix (triangle).
That was a lot. Here are a few key takeaways:
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