Performance nutrition: Part 5 of 6.
Understanding why you should consider maintaining high carbohydrate availability during a race… and whether doing so will improve performance.
Thomas Solomon, PhD.
13th June 2021.
You know how much fuel is stored in your body (if you don’t, read Part 1). You know how your muscles burn fuel during exercise (if you don’t, read Part 2). You know how long your stored fuel allows you to go (if you don’t, read Part 3). And, you know you can create a high carbohydrate availability before your race (if you don’t, read Part 4). But, can you maintain high carbohydrate availability during your race? And… Will doing so improve your race day performance? Stay with me today and I will take you on a deep-dive history lesson, starting with the 1924 Boston marathon, to find out how we know what we now know...
Reading time ~20-mins (4000-words)
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So far you have learned that a high carbohydrate availability can be established before your race with a high-carb diet on the days before race day and a carb-full breakfast on the morning of your race. The obvious missing component of your race day performance nutrition equation is...
Can you maintain high carbohydrate availability during your race?
Unlike our canine companions, who can rapidly replenish muscle glycogen levels during prolonged exercise even when carbohydrate intake is low (see here and here), we are not blessed with glycogen replenishing powers when on the go — when we move for prolonged periods, we deplete glycogen. Although some evidence in endurance-trained athletes shows that, during prolonged (~3-hours) low-intensity cycling, glycogen resynthesis can occur in inactive type 2 (aka “fast-twitch”) muscle fibres when carbohydrate is consumed, the amount is small and resynthesis does not occur in the type 1 (aka “slow-twitch”) fibres being used. Consequently, if you want to increase carbohydrate availability during a race, you need to add some carbohydrate into the mix.
In 1924, yep 100 years ago, Sam Levine and colleagues at Harvard Medical School found that runners completing the Boston marathon had lower blood glucose levels than when they started and noted that the “condition” of the athletes at the end — symptoms of physical weakness, pallor, and collapse — was associated with their blood glucose levels. They described it as “a picture of shock not unlike that produced by an overdose of insulin”. Anyone who has hung out at the finish line of a marathon will relate to that description.
These observations imply that a drop in blood glucose levels — hypoglycaemia — is a cause of fatigue but it took the Earth a few “runs” around the sun for anyone to delve deeper… In their 1967 paper showing that exercise depletes muscle glycogen and that the liver attempts to keep supplying glucose to the muscle by releasing more glucose into the blood, Jonas Bergström and Eric Hultman also found that intravenous glucose infusion helped reduce muscle glycogen use during exercise but that muscle glycogen was still responsible for the greater part of the energy production during exercise even when the blood sugar level is high. Subsequent work in endurance-trained athletes throughout the 80s, 90s, and 2000s showed that the decline in blood glucose contributes to fatigue during prolonged exercise, that glucose infusion during exercise can restore healthy blood glucose levels after hypoglycemia has developed, and that glucose infusion prolongs exercise time-to-exhaustion after muscle glycogen has been depleted during long-duration (2-3-hours) low-to-moderate intensity exercise (see here and here). But, the same studies showed that infusion doesn’t prevent muscle glycogen depletion during exercise (see here and here) and doesn’t improve high-intensity (75-80% VO2max, ~1-hour) time trial performance.
To put all that in plain English, we’ve known since the 60s that muscles keep munching through their glycogen during exercise even if glucose is abundantly available in the blood and that infusing glucose during exercise prevents hypoglycemia and delays fatigue.
This sounds wonderful but intravenous glucose infusion is neither allowed (it violates WADAs ethical code) nor is it practical during a race. So, the alternative is to eat carbohydrates like a normal human, just like Levine and colleagues examined in 1924.
In 1982, John Wahren’s lab found that when untrained regular folks drank “sugar water” (either 10 or 20 grams of glucose every 15-mins) during a ride-to-exhaustion at 60-65% VO2max following an overnight fast, hypoglycemia was prevented but time-to-exhaustion was unaffected when compared to water. But, as you are probably aware, studying untrained folks is not so useful for informing athletes’ knowledge. Fear not; many studies have examined endurance-trained athletes…
Ingesting carbohydrate during exercise does not prevent muscle glycogen breakdown — muscle glycogen is always “burned” during exercise — instead, carbohydrate feeding simply maintains blood glucose levels, which reduces the need for liver glucose output, sparing liver glycogen for longer, all the while providing a continually high carbohydrate availability to the muscles.
With that knowledge in mind, since we know that faster runners burn glucose at higher rates (also see here, here, and here), faster runners will need to be more aware of their need for high carbohydrate availability during their race. Therefore, ingesting carbohydrate during a race allows glucose, the economical and rapid fuel, to be “burned” at the high rate required to provide energy to the working muscles.
For a visual representation of how all this fits together, check out my animation below:
So, yes, high carbohydrate availability can be maintained during a race. But, to make an informed decision as to whether this is a good idea, there is a very important question to consider...
Does carbohydrate intake during a race improve performance?
The answer to this question is best answered by first considering the duration of your race.
Note: The figure below will help you understand this point. To read about this in detail, read Part 3 of this series.
To examine this question in detail, let's jump back into John DeLorean’s time-travelling car and head back to Boston…
As I mentioned earlier, in 1924, while Europe was rationing sugar in the aftermath of WW1, Sam Levine and colleagues at Harvard Medical School were dishing it out to runners competing in the Boston marathon. This was to help prevent the signs of hypoglycemia they had observed in athletes at the end of the race the previous year. As you know already, they asked 17 athletes (including those who raced the previous year) to eat a high-carbohydrate diet for 24-hours before the race. But, they also asked them to start eating candy about 24 kms (15 miles) into the race. The result: athletes had a better “condition” at the finish line, higher post-race blood glucose levels than pre-race, and completed the marathon quicker than the previous year. As a fun side note, the chap who won the race in 1924 and came second in 1925 did not eat a high carb diet nor did he eat candy during the race — a fat-burning machine — but he also only ran 2:30 and 2:34 in each year, which means he would have been about half an hour slower than present-day carbohydrate-gobbling machines (even before the advent of supershoes).
Those investigators at Harvard hypothesised that by preventing hypoglycaemia during a prolonged, high-intensity race, they could prevent fatigue and improve performance. Of course, we cannot know that their intervention was the direct cause of better performance because they did not conduct a randomised controlled trial (RCT) and a lot can change in a year — better training, better taper, cooler weather…
These analyses are convincing but these systematic reviews did not use very stringent inclusion criteria and also included studies in which subjects were fasted. Because, as you already know, it is not particularly wise for an endurance athlete to line up with low liver glycogen levels on race day, it is more appropriate to examine studies in which subjects have eaten breakfast before testing.
Useful info? Yes. But, another pitfall to these meta-analyses is that many of the included trials had a long-duration (1 to 2-hours) moderate-intensity exercise bout (presumably to deplete glycogen) immediately before a performance test. I’ve never understood what scientists are trying to model with this design because it doesn’t reflect many race scenarios (except maybe an often futile, solo breakaway effort in a cycling road race but even then, said cyclist, let’s call him “Jens Voigt” ,would have been well-fed up to the point he decided to give it large). So, what about “real life” performance studies?
Also convincing, but one more nuance remains... Many studies examine “performance” with a time-to-exhaustion ride or run at a low to moderate intensity (60 to 70% VO2max) — what does that really tell us about performance? A time trial is more informative.
In 2013, Colombani and colleagues found only 17 RCTs (1 running, 1 soccer, 15 cycling) that mimicked “real life” — examining the effect of carbohydrate intake before or during a time trial in endurance-trained athletes who had eaten breakfast. But they found an unlikely effect with time trials up to ~70-minutes and a less-than-compelling ergogenic effect with time trials longer than ~70-mins. They used appropriately stringent inclusion criteria, excluding hundreds of papers, noting the general low-quality of studies... commenting that “The absence of clear evidence is, nevertheless, not clear evidence of an absent effect”. In 2016, when more RCTs were published, Pöchmüller and colleagues completed a similar meta-analysis of time trials in athletes who had eaten a pre-trial meal (breakfast). They found that ingesting carbs in a concentration range of 6–8% (6-8 g per 100 mL) before and/or while exercising longer than 90-minutes improves performance. But, due to the lack of sufficient RCTs, Pöchmüller et al. commented that findings cannot yet be confidently extrapolated to elite athletes or female athletes, and more work is needed on bouts lasting less than 90-minutes and in sports besides cycling.
This all sounds ace but if, like me, you have followed this body of evidence for nearly 20-years, it will be clear that, in general, we know a lot but there is a lack of randomised controlled trials in elite athletes, in female athletes, and in real race-like settings. This will keep scientists from gazing into blank spaces for a few more moons to come. Furthermore, most studies have examined cyclists. So, I was excited in 2016 to see Patrick Wilson publish a critical review of all known studies and meta-analyses to answer the question, “Does Carbohydrate Intake During Endurance Running Improve Performance?”. He concluded that running performance is most likely improved during events longer than 2-hours when 100–200 ml of a carbohydrate drink (5–8 grams per 100 mL; i.e. 5 to 16 grams of carbs) is drunk every 15–20 minutes, although several studies show benefits for tasks lasting 90 to 120-minutes. I was less excited to see how limited the scientific evidence base is in running — notably, there is a lack of studies in runners, including a lack of studies examining gels and foods in runners — therefore, in running, we largely apply cycling-based scientific evidence to generate running-based empirical evidence through observation in the field. And, in doing so… does carbohydrate intake during a (long) race improve performance?.
It did in the 1920s in Boston and it does now. Given what Levine and his colleagues learned at the Boston marathon 100-years ago, little has really changed — a high carbohydrate availability is key for successful race day performance.
So far in this series, I have made a deep dive into the individual scientific studies and the systematic reviews and meta-analyses that have brought us to where our knowledge is today. But, to relay the synthesis of evidence to the target audience — to coaches and athletes — governing bodies and professional societies publish guidelines. So...
What do sports nutrition guidelines say?
Older guidelines from the 1990s recommended that endurance athletes should consume a total of about 600 g every day during heavy training and to consume carbohydrates during exercise, generally in the form of solutions containing glucose, sucrose, or maltodextrins, at a rate of 30 to 60 grams per hour. The old (and now "retired") American College of Sports Medicine (ACSM) position stand from 2009 gave similar advice: daily intake of 6 to 10 grams of carbs per kg body weight (~400-650 grams per day) and 30-60 grams of carbs per hour during exercise. With their ears-to-the-ground of the emerging evidence from science and practice, in 2011, Louise Burke, John Hawley, and Asker Jeukendrup proposed an update to the recommendations, stating that “carbohydrate intake during exercise should be scaled according to the characteristics of the event”. They suggested that athletes could fluctuate daily carbohydrate intake relative to their needs — the emergence of “carbohydrate periodisation” — and that, during exercise, smaller amounts of carbs (30 grams/h) could be ingested during shorter events with higher rates of intake (60...90...120 grams/h) during longer events.
To summarise what these guidelines say about carbohydrate intake...
For general endurance athletes:
The ACSM (2016) recommends basing daily carb intake on training load:
… 3 to 5 grams of carbs per kg body weight for light training;
… 5 to 7 g/kg for a moderate training load (1 h/day);
… 6 to 10 g/kg for a high load (1-3 h/day);
… 8 to 12 g/kg for a very high load (more than 4-5 h/day).
Pre-race carb-loading with a diet containing 10 to 12 g/kg/day of carbs for 36–48 hours, for events longer than 90-mins.
Pre-race feeding 1 to 4-hours before the race with 1 to 4 g/kg of carbs.
During race carb intake based on duration:
... up to 45 mins → no carbs needed;
... 45 to 75 mins → a mouth rinse or small carbohydrate amount;
… 1 to 2½ h → 30–60 g/h of carbs;
... longer than 2½ h → up to 90 g/h of carbs.
The IAAF (2019) recommends pre-race carb-loading with 10 to 12 g/kg/day of carbs for 36 to 48-hours before the race,
a pre-race meal 1 to 4-hours before the race containing 1 to 4 g/kg of carbs,
during-race nutrition depending on race duration:
... 45 to 75 min → a mouth rinse or small carbohydrate amount;
… 1 to 2½ h → 30–60 g/h of carbs;
... longer than 2½ h → up to 90 g/h of carbs.
And the ISSN (2018) recommends consuming 30 to 60 g/hour of carbohydrate in a 6–8% carbohydrate-electrolyte solution every 10 to 15 min throughout high-intensity exercise longer than 90-minutes.
And specifically for ultra-distance runners:
The ISSN (2019) recommends a general moderate-to-high carbohydrate diet of 5 to 8 g/kg/day during training and during a race, to prevent caloric deficits, to aim to consume 150–400 kcals/hour to include 30–50 grams/hour of carbs and 5–10 grams/hour of protein from a variety of calorie-dense foods while considering food palatability, tolerance, and savoury vs. sweet preference in longer races.
While the IAAF (2019) recommends consuming ~0.8 to 1 g/kg/hour carbohydrate during exercise in a 6–10% weight to volume solution.
As you can see, there’s a little bit of variation between the various position statements, which is no surprise when different groups of experts evaluate evidence in its entirety, but, promisingly, the general sentiments are very similar and are “running down the same trails”.
What can you put in your performance nutrition toolbox?
Since the evidence shows that carbohydrate ingestion allows exercise to continue even if glycogen levels are depleted, then theoretically, you could start the race with low liver and muscle glycogen if enough carbohydrate can be ingested during a race to meet the metabolic demand of your muscles. But, this is a bloody big risk. The evidence also shows that low muscle glycogen reduces power output possibly due to a direct blunting of muscle contractility. Furthermore, if you cannot ingest enough carbohydrate during the race (due to poor logistics, aversion, or sickness etc) then you will very quickly encounter Darth Fader, the Sith Lord of fatigue. Don’t “run” that risk, especially when considering how oh-so-simple it is to start a race with high liver and muscle glycogen levels...
On your “A” race day, you will need to line up with a high carbohydrate availability and, after the B of the bang, if your race is long, you should be planning to maintain a high carbohydrate availability throughout the race. Doing so will help delay fatigue for as long as possible by sparing liver glycogen, keeping blood glucose within the normal range, and supplying glucose to the hungry muscles. As Renato Canova’s Kenyan athletes say, “when I finish the fuel, I stop” — the best distance runners on Earth understand their metabolic limitations very well.
Establishing and maintaining high carbohydrate availability during your race can be achieved with three simple steps:
Muscle glycogen “supercompensation” (aka carbo-loading) is probably best achieved by eating a high-carbohydrate diet for the 24 to 48-hours before race day while tapering your training or resting altogether. “Carbo-loading” in this way is especially important if you typically eat a low-carb diet.
Your liver glycogen store, which is depleted after a night of sleep, will be maximised if you eat a high-carbohydrate breakfast on race day, 2- to 4-hours before gun time.
Consuming carbohydrates at regular intervals from the start of a race lasting around 60-mins or longer will help delay glycogen depletion and maintain blood glucose levels for as long as possible.
So, if you haven’t already guessed it, what Sam Levine and colleagues discovered in 1924 at the Boston marathon — to eat lots of carbs for the 24-hours before a race, on the morning of the race, and during the race — is about as simple and accurate a message as anyone can put in a bottle. Some folks are just ahead of their time.
Sports nutrition guidelines for establishing and maintaining high carb availability on race day are pretty clear and very useful. They also confirm the notion that high carbohydrate availability is a key facet of your endurance performance. The various guidelines and position statements are also built on scientific evidence, which is good, but scientific evidence must always be balanced with empirical evidence from coaches and feedback from athletes. Why? Well, because there is no “one size fits all” approach. In their utility, the specifics of sports nutrition guidelines — the grams per day and the grams per hour — are rather generalised and should not be used in a cookie-cutter type way. Consequently, one key question remains... “How can you maintain high carbohydrate availability on race day?”. Stay tuned and find out in the final part of this series...
Until that time, keep training smart...
Disclaimer: I occasionally mention brands and products but it is important to know that I am not sponsored by or receiving advertisement royalties from anyone. I have conducted biomedical research for which I have received research money from publicly-funded national research councils and medical charities, and also from private companies, including Novo Nordisk Foundation, AstraZeneca, Amylin, the A.P. Møller Foundation, and the Augustinus Foundation. I have also consulted for Boost Treadmills and Gu Energy on their research and innovation grant applications. These companies had no control over the research design, data analysis, or publication outcomes of my work. Any recommendations I make are, and always will be, based on my own views and opinions shaped by the evidence available. The information I provide is not medical advice. Before making any changes to your habits of daily living based on any information I provide, always ensure it is safe for you to do so and consult your doctor if you are unsure.
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About the author:
I am Thomas Solomon and I'm passionate about relaying scientific information to the masses and helping folks meet their fitness and performance goals. I hold a BSc in Biochemistry and a PhD in Exercise Science and am an ACSM-certified Exercise Physiologist and Personal Trainer, a VDOT-certified Distance running coach, and a Registered Nutritionist. Since 2002, I have conducted biomedical research in exercise and nutrition and have taught and led university courses in biochemistry, molecular medicine, and exercise physiology. My work is published in over 80 peer-reviewed medical journal publications and I have delivered more than 50 conference presentations & invited talks at universities and medical societies. I have coached and provided training plans for truck-loads of athletes and regular folk, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, lift, and climb as often as possible. To stay on top of scientific developments, I participate in journal clubs and peer-review papers for medical journals, and I invest every Friday in reading what new delights have spawned onto PubMed. In my spare time, I hunt for phenomenal mountain views to capture through the lens, boulder problems to solve, and for new craft beers to drink with the goal of sending my gustatory system into a hullabaloo.