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This article is part of a series:
→ Part 1 — How much fuel is in your body?
→ Part 2 — How do your muscles burn fuel?
→ Part 3 — How long can you go?
→ Part 4 — Carboloading
→ Part 5 — Race day carb availability
→ Part 6 — Putting it into practice
→ Part 1 — How much fuel is in your body?
→ Part 2 — How do your muscles burn fuel?
→ Part 3 — How long can you go?
→ Part 4 — Carboloading
→ Part 5 — Race day carb availability
→ Part 6 — Putting it into practice
Performance nutrition: Part 4 of 6:
How can you establish a high carbohydrate availability before a race?
Thomas Solomon PhD.
5th June 2021.
Now you know that your relatively tiny and finite glucose stores — muscle and liver glycogen — eventually get critically low during prolonged exercise. To prevent or delay fatigue, you need to maintain glucose availability to your working muscles. This means helping to prevent your liver getting “tired” of releasing glucose into the blood to maintain blood glucose levels within a healthy range while your muscles are taking massive gulps of the sugary goodness. In this 4th part of the series, I will delve into the nutritional strategies you can use to establish a high carbohydrate availability to help maximise race-day performance.
Reading time ~18-mins (3600-words).
Or listen to the Podcast version.
Or listen to the Podcast version.
Your blood acts like a “bathtub” that contains a small but stable amount of glucose. During exercise, the tub primarily drains into your muscles, which sip glucose from the tub at low exercise intensities and takes massive gulps at high intensities. While your muscles are being greedy, your liver adds glucose back into the tub to maintain a healthy blood glucose level. When you move faster, your muscles “burn” more glucose — they deplete their own glycogen at a faster rate and drain more glucose from the tub, forcing your liver to work harder to release more glucose.
That is integrative physiology in action.
But when your muscles’ demand for glucose exceeds the amount of glycogen it has available and exceeds your liver’s capacity to add glucose into the blood (which, during exercise, comes from liver glycogen breakdown), you are left with two options:
Allow blood glucose levels to drop, embrace the signs of hypoglycemia, and lower your power output until the imminent and inevitable fatigue occurs.
Or…
Consider how you can maintain a high carbohydrate availability.
So far in this series, you have learned how much fuel is stored in your body, how your muscles use that fuel, and how long your stored fuel can last. My goal in answering those questions was to help you arrive at the thought: “if my body’s carbohydrate stores are small and can be depleted quickly, are there nutritional strategies I can use to help make them last the distance?”.
I hope it has become clear that the purpose of such nutritional strategies is to delay glycogen depletion for as long as possible. As a result, the first obvious question emerges.
So, how do we know?
In the 1960s, Jonas Bergström and Eric Hultman completed a series of studies examining muscle glycogen resynthesis after exercise in untrained folks. They showed that exercise-induced glycogen depletion stimulates glycogen resynthesis and that glycogen-depleting exercise followed by a high carbohydrate diet (~8 grams of carbs per kg bodyweight per day) for 36 to 48-hours “supercompensates” muscle glycogen levels when compared to no- or low-carb refeeding. So, we've known since the 1960s that you need to deplete glycogen with exercise to expand your glycogen store when you refeed.
Bergström and Eric Hultman also found that a glycogen-depleting exercise followed by 3-days of a low-carb high-fat diet then 3-days of a high carb diet increased glycogen synthesis more than if the high-carb diet immediately followed the depletion bout. Further studies from Bergström and Bengt Saltin also showed that a high carb diet (70% of total kcals) increased pre-exercise muscle glycogen levels and prolonged exercise time-to-failure compared with either moderate (50%) or low-carb (30%) diets, plus longer exercise time-to-failure was correlated with a higher pre-exercise muscle glycogen content.
This collection of studies from Bergström combined with a 1971 in-the-field running race study from Bengt Saltin, is the basis for the classic 7-day pre-competition “depletion-supercompensation” carbo-loading protocol — exercise-induced glycogen depletion followed by a 3-day low carb diet followed by a 3-day high-carb diet leading into race day. The limitation was that all these observations were made in untrained regular folks. But, in the 1970s, Dave Costill’s work showed that a single 30km running race reduces muscle glycogen (and IMTG) and that muscle glycogen progressively declines each day when well-trained runners run 16 km at 80% VO2max on 3 consecutive days. This established the need for glycogen loading and embedded the classical pre-competition “depletion-supercompensation” carbo-loading protocol in athletes’ race prep recommendations during the 70 and 80s (e.g. Dave Costill (1980) and Ed Coyle & Andy Coggan (1984)).
Although adopted by many elite athletes then (and even today), a study in well-trained runners from William Sherman and Dave Costill in 1981 found that, when compared to continuous moderate carb intake, the depletion-supercompensation protocol is indeed superior for increasing muscle glycogen but not strictly necessary. By comparing a 7-day depletion-supercompensation protocol vs. a 7-day moderate carb diet (50% of kcals) vs. a “modified” depletion-supercompensation protocol — which included a 4-day moderate carb diet followed by a 3-day training taper with a high-carb (75% of kcals) diet — before a 21 km race, they found that the classic depletion-supercompensation protocol offered no advantage to muscle glycogen levels over the “modified” depletion-supercompensation protocol — simply eating high amounts of carbs for the last 3-days during the taper before the race is sufficient for glycogen supercompensation. Interestingly, however, there were no performance differences between groups suggesting that “carbo-loading” was not necessary for a race lasting only ~80-minutes.
The success of this “modified” supercompensation approach was supported by other work in the 1980s. For example, Blom and colleagues (1987) found that trained runners combining a low training load with a moderately high carbohydrate intake (~ 6 g/kg/day) for 2-days achieved a similar muscle glycogen level compared to the end of a “depletion-supercompensation” protocol, which included a glycogen-depleting run-to-exhaustion followed by 3-days of no training combined with ~9 g/kg/day of carbs.
It is important to note that when world-class endurance athletes are observed, they do not stop training in the week before racing, they tend to taper their volume of training. So, studies that use a training taper combined with manipulation of carbohydrate availability, such as the 1981 study from Sherman and Costill, are more ecologically valid. Recent studies in endurance-trained folks show that muscle glycogen supercompensation occurs within 1 to 2 days of a taper when combined with a high-carbohydrate diet (10 g/kg/day). However, when carbohydrate intake in the regular daily diet is high enough, supercompensation protocols are not required at all. For example, when endurance athletes are studied, one study has shown that a high-carb diet (10 g/kg/day) can maintain muscle glycogen during 7-days of intensive daily training, whereas 5 g/kg/day cannot. And, in another study, muscle glycogen was maintained when athletes consumed high amounts of carbs each day — either ~8, ~10, or ~12 g/kg/d (58%, 68%, 88% of total kcals) — during 3-weeks of training, riding 2-hours per day.
What does all this mean?
The final 2 to 3-days of a high-carb diet before your race is what will maximise your muscle glycogen store — your carbohydrate availability — and supercompensation is likely best achieved within 2-days of your last session if you rest and consume a daily high-carb diet. The old school “depletion-supercompensation” approach is risky — if you don’t know what you are doing, your glycogen stores can easily get eff dup — and it is unnecessary for ensuring carbohydrate availability.
To conceptualise this 60-year epic of data into a clear picture, José Areta and Will Hopkins’ 2018 meta-regression complied all known clinical data and concluded that VO2max and dietary carbohydrate intake are the biggest predictors of resting muscle glycogen levels. Their meta-regression indicates that fitter athletes can store more muscle glycogen and that athletes consuming a high-carbohydrate diet (more than 6 g/kg/day for at least 3-days or 7 g/kg/d for at least 2-days) have greater muscle glycogen levels than athletes eating a moderate carb (less than 6 g/kg/day) or a low-carb ketogenic diet (less than 50 grams per day).
So, yes, you can increase your carbohydrate availability during the days before your race with 2- to 3-days of taper while munching a high-carb diet (8 to 10 g/kg/day)... during which you might notice a slight 1-2% increase in body weight because 1 g of muscle glycogen is stored with 3 to 5 g of water (see here and here).
In the 1970s, Nilsson & Hultman used liver biopsies to show that liver glycogen is rapidly restored with carbohydrate refeeding — one of the reasons that endurance athletes should consider eating a morning breakfast — and that liver glycogen is related to the amount of carbohydrate eaten in the diet.
In the 1980s, the labs of Ralph DeFronzo, John Wahren, and John Gerich used 3H- and 14C radioisotopes of glucose and arterio-venous catheterisation of multiple tissues to show that when ~50 to 90-grams of glucose water is drunk after an overnight fast, 30 to 50% of it is taken up by the liver and 50 to 70% by the muscles.
But drinking liquid glucose for breakfast is not a sane human endeavour. Fortunately, in the 1990s, some sensible folks in Gerald Shulman’s lab studied the fate of mixed-nutrient meal ingestion in the morning following an overnight fast, in healthy but untrained human folks. Using 13C-NMR methods, they found that a liquid breakfast containing ~140 grams of glucose with ~16-grams of fat and ~30-grams of protein, took ~4-hours to restore liver glycogen to prior evening levels, and that about 30% of a “real food” breakfast of cereals, juice, and eggs, containing 290 grams of carbohydrate, ends up stored as muscle glycogen.
Many moons ago, in 1939, Erik Christensen and Ove Hansen in Copenhagen found that drinking “sugar water” containing 200 grams of glucose on the morning of a ride-to-exhaustion at a moderate-intensity, prevented hypoglycemia and delayed exhaustion in untrained regular folks. They were certainly on to something — many moons too soon — it took some time for folks to take interest in their observation. The following 80-years of work were summarised in a 2018 systematic review and meta-analysis of all known data, concluding that pre-exercise carbohydrate ingestion (compared to fasting) generally enhances aerobic exercise performance during bouts lasting more than an hour, but not for shorter durations. However, several studies show that ingesting carbohydrates during exercise can overcome the limitation caused by fasting before exercise. This has been shown during a 30 km running time trial in trained runners, and during short (~16-mins in trained cyclists and 10 km in recreational cyclists) and long (50 km in trained cyclists) cycling time-trials. So, you don’t need to panic if your race day breakfast is suboptimal so long as you can feed during the race (more on that later).
Most studies comparing fed vs. fasted performance have examined steady-state exercise but similar outcomes are found when taking a S.H.I.T. (short high-intensity interval training) — performance is improved in the fed state during long-duration high-intensity efforts (e.g. 90 min of high-intensity running that includes 5 × 1-min sprints in recreational athletes; see here and here) but not during short-duration SHITs (e.g. 10 × 6-second sprints in untrained folks, 6-sets of 3 × 10-second sprints in trained cyclists, or 3 × 1-min efforts in recreational athletes).
This is all good to know but maybe you are wondering...
However, these experiments fed “sugar water” to their subjects for breakfast, which, having been a subject in some of these actual studies, is just horrible… And, as I said earlier, it is not a sane human endeavour to eat liquid glucose (or maltodextrin and sucrose) for breakfast. Once again, we are fortunate that some sensible folks use ecologically-valid (aka meaningful) study designs. For example, trained cyclists see no performance differences when they eat breakfast meals containing ~230 vs. ~90, or ~215 vs. ~50 or, ~258 vs. ~122 vs. ~15 grams of carbs either 4-hours or 90-minutes before performance tests (including a 2-hour ride at 60% VO2max followed by ride-to-failure at 80% VO2max, 90-mins at 70% VO2max followed by a 10-km time trial, or a 50-km time trial that included three 1-km and 4-km sprints). And the same is true for trained runners who see no performance differences when they eat a breakfast containing either ~180 or ~75 g of carbs 4-hours before an 80-min run at ~70% VO2max followed by a run-to-exhaustion at ~80% VO2max.
So, as it turns out, being fed rather than fasted is a good idea, but the studies that have compared low- vs high-carb mixed-nutrient breakfasts before exercise find no performance differences during cycling time trials or rides-/runs-to-exhaustion. In other words, some carbs are better than no carbs on race morning but don’t stress over the total amount. Other folks have even chipped away at the type of delivery of pre-exercise carb intake but also find no differences in performance between solid vs. liquid or solid vs. gel-based (here and here) carb intake before performance tests. However, it is very important to note that in all of these studies, subjects consumed a moderate to high carbohydrate diet for several days leading into the trials, which would maximise their muscle glycogen levels during all trials (although this was not measured).
That’s some more useful info but maybe you are wondering...
That said, from a performance perspective, different timings of a pre-exercise meal have no notable influence on power output during a S.H.I.T. or time trial performance in trained cyclists or swimmers when “sugar water” (glucose) or a sports bar is consumed either 15 vs. 45 vs. or 75-mins, or 15 vs. 60-mins, or 5 vs. 35-mins before exercise. However, it is important to note that some athletes develop hypoglycaemia during the early stages of exercise if they consume carbohydrates during the 45 minutes before exercise. Glucose ingestion prior to exercise can increase insulin secretion at the onset of moderate- to high-intensity exercise, causing a drop in blood glucose due to an insulin-stimulated and exercise-stimulated glucose uptake by muscle combined with inadequate liver glucose output (see Costill et al 1977, Koivisto et al 1981, and Moseley et al 2003, in which I was a subject).
This risk of hypoglycemia doesn’t always impair performance but its consequential signs don't feel great and it is never worth the risk on race day! Consequently, efforts were made to blunt the insulin secretory response of a pre-exercise feed by using low glycemic index (low GI) foods — foods that cause a slower release of glucose into the blood without an insulin spike. In 1991, Thomas and colleagues found that consuming a low GI meal 1-hour before a ride-to-exhaustion at a low to moderate intensity (65-70% VO2max) maintained blood glucose levels and allowed cyclists to ride for longer than when a high GI meal was eaten. This has been confirmed by others, including my old postdoc supervisor, John Kirwan, who showed in 2001 that a moderate GI meal fed 45-mins before exercise was superior to a high GI meal for improving time-to-exhaustion at 70% VO2max.
And, one last point related to the timing of your pre-race feeding if your race is in the evening… Evidence in cyclists shows that skipping breakfast slowed an evening 20 km time trial by about 38-seconds, on average, despite complete caloric compensation at lunch. So, as an athlete, it will likely be very foolish to skip carbs at breakfast on race day.
This is a lot of info to absorb but here is a bite-sized morsel to chew on: performing fed (rather than fasted) exercise will give a far larger boost to your performance than stressing over the precise amount of carbs or the precise timing of your meals. So, stay calm.
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. Note that when your race starts later in the day, eating carbohydrates as part of each meal during the day is sensible. Also note that a pre-race meal should seek to maximise carbohydrate absorption into the blood. Since other macronutrients (fat and protein) slow gastric emptying and delay absorption, it is not a good idea to eat them in large quantities on race morning because they will slow glucose delivery to the liver and muscles and will increase the likelihood of GI issues during a high-intensity race.
Now you have a deep understanding of why you should start a race with high carbohydrate availability. But, to complete the performance nutrition equation, you need to know, “is it possible to maintain high carbohydrate availability during your race?” and “will high carbohydrate availability improve your race day performance?”. Sit tight and find out in the next part of this series...
Until then, keep training smart.
That is integrative physiology in action.
But when your muscles’ demand for glucose exceeds the amount of glycogen it has available and exceeds your liver’s capacity to add glucose into the blood (which, during exercise, comes from liver glycogen breakdown), you are left with two options:
Allow blood glucose levels to drop, embrace the signs of hypoglycemia, and lower your power output until the imminent and inevitable fatigue occurs. Or…
Consider how you can maintain a high carbohydrate availability.
I hope it has become clear that the purpose of such nutritional strategies is to delay glycogen depletion for as long as possible. As a result, the first obvious question emerges.
Can you increase your carbohydrate availability during the days before your race?
I will save you the drama of wondering and just say, “Yes!”. But, if you follow my posts, you know I won’t stop there.So, how do we know?
In the 1960s, Jonas Bergström and Eric Hultman completed a series of studies examining muscle glycogen resynthesis after exercise in untrained folks. They showed that exercise-induced glycogen depletion stimulates glycogen resynthesis and that glycogen-depleting exercise followed by a high carbohydrate diet (~8 grams of carbs per kg bodyweight per day) for 36 to 48-hours “supercompensates” muscle glycogen levels when compared to no- or low-carb refeeding. So, we've known since the 1960s that you need to deplete glycogen with exercise to expand your glycogen store when you refeed.
Bergström and Eric Hultman also found that a glycogen-depleting exercise followed by 3-days of a low-carb high-fat diet then 3-days of a high carb diet increased glycogen synthesis more than if the high-carb diet immediately followed the depletion bout. Further studies from Bergström and Bengt Saltin also showed that a high carb diet (70% of total kcals) increased pre-exercise muscle glycogen levels and prolonged exercise time-to-failure compared with either moderate (50%) or low-carb (30%) diets, plus longer exercise time-to-failure was correlated with a higher pre-exercise muscle glycogen content.
This collection of studies from Bergström combined with a 1971 in-the-field running race study from Bengt Saltin, is the basis for the classic 7-day pre-competition “depletion-supercompensation” carbo-loading protocol — exercise-induced glycogen depletion followed by a 3-day low carb diet followed by a 3-day high-carb diet leading into race day. The limitation was that all these observations were made in untrained regular folks. But, in the 1970s, Dave Costill’s work showed that a single 30km running race reduces muscle glycogen (and IMTG) and that muscle glycogen progressively declines each day when well-trained runners run 16 km at 80% VO2max on 3 consecutive days. This established the need for glycogen loading and embedded the classical pre-competition “depletion-supercompensation” carbo-loading protocol in athletes’ race prep recommendations during the 70 and 80s (e.g. Dave Costill (1980) and Ed Coyle & Andy Coggan (1984)).
Although adopted by many elite athletes then (and even today), a study in well-trained runners from William Sherman and Dave Costill in 1981 found that, when compared to continuous moderate carb intake, the depletion-supercompensation protocol is indeed superior for increasing muscle glycogen but not strictly necessary. By comparing a 7-day depletion-supercompensation protocol vs. a 7-day moderate carb diet (50% of kcals) vs. a “modified” depletion-supercompensation protocol — which included a 4-day moderate carb diet followed by a 3-day training taper with a high-carb (75% of kcals) diet — before a 21 km race, they found that the classic depletion-supercompensation protocol offered no advantage to muscle glycogen levels over the “modified” depletion-supercompensation protocol — simply eating high amounts of carbs for the last 3-days during the taper before the race is sufficient for glycogen supercompensation. Interestingly, however, there were no performance differences between groups suggesting that “carbo-loading” was not necessary for a race lasting only ~80-minutes.
The success of this “modified” supercompensation approach was supported by other work in the 1980s. For example, Blom and colleagues (1987) found that trained runners combining a low training load with a moderately high carbohydrate intake (~ 6 g/kg/day) for 2-days achieved a similar muscle glycogen level compared to the end of a “depletion-supercompensation” protocol, which included a glycogen-depleting run-to-exhaustion followed by 3-days of no training combined with ~9 g/kg/day of carbs.
It is important to note that when world-class endurance athletes are observed, they do not stop training in the week before racing, they tend to taper their volume of training. So, studies that use a training taper combined with manipulation of carbohydrate availability, such as the 1981 study from Sherman and Costill, are more ecologically valid. Recent studies in endurance-trained folks show that muscle glycogen supercompensation occurs within 1 to 2 days of a taper when combined with a high-carbohydrate diet (10 g/kg/day). However, when carbohydrate intake in the regular daily diet is high enough, supercompensation protocols are not required at all. For example, when endurance athletes are studied, one study has shown that a high-carb diet (10 g/kg/day) can maintain muscle glycogen during 7-days of intensive daily training, whereas 5 g/kg/day cannot. And, in another study, muscle glycogen was maintained when athletes consumed high amounts of carbs each day — either ~8, ~10, or ~12 g/kg/d (58%, 68%, 88% of total kcals) — during 3-weeks of training, riding 2-hours per day.
What does all this mean?
The final 2 to 3-days of a high-carb diet before your race is what will maximise your muscle glycogen store — your carbohydrate availability — and supercompensation is likely best achieved within 2-days of your last session if you rest and consume a daily high-carb diet. The old school “depletion-supercompensation” approach is risky — if you don’t know what you are doing, your glycogen stores can easily get eff dup — and it is unnecessary for ensuring carbohydrate availability.
To conceptualise this 60-year epic of data into a clear picture, José Areta and Will Hopkins’ 2018 meta-regression complied all known clinical data and concluded that VO2max and dietary carbohydrate intake are the biggest predictors of resting muscle glycogen levels. Their meta-regression indicates that fitter athletes can store more muscle glycogen and that athletes consuming a high-carbohydrate diet (more than 6 g/kg/day for at least 3-days or 7 g/kg/d for at least 2-days) have greater muscle glycogen levels than athletes eating a moderate carb (less than 6 g/kg/day) or a low-carb ketogenic diet (less than 50 grams per day).
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In the first part of this series, you already learnt that us humans have very little stored carbohydrate — about 500 grams (400 g in muscle + 100 g in liver) with about 4 g circulating in blood. While it is debated whether liver glycogen increases much above 150 g, an endurance athlete eating a “glycogen-loading” high-carbohydrate diet containing ~10 grams of carbohydrate per kg body mass per day might increase their stored glucose in muscle glycogen to ~600-700 g. So, an athlete’s body, particularly a large and muscular athlete, might contain as much 800 grams of total carbohydrate. This indicates that a world-class athlete working at 5 grams per minute of carbohydrate oxidation to run at world record marathon pace, can theoretically increase their “carbohydrate time-to-failure” from ~100-minutes (1h40m) to ~160-minutes (2h40m). Noting, of course, that these times are over-estimates because not all of the stored glycogen is accessible during exercise, plus world-class marathoners habitually eat a high-carbohydrate diet and are not particularly large or muscular.
So, yes, you can increase your carbohydrate availability during the days before your race with 2- to 3-days of taper while munching a high-carb diet (8 to 10 g/kg/day)... during which you might notice a slight 1-2% increase in body weight because 1 g of muscle glycogen is stored with 3 to 5 g of water (see here and here).
×
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Sorted. But perhaps you are wondering, “I know what to do before race day but what do I do on race day?”. The next obvious thing to address...
Can you increase your carbohydrate availability in the hours before your race?
We’ve known since the 1960s that, in us human folks, an overnight fast massively depletes liver glycogen stores. To put that in perspective, since your liver releases glucose at ~0.15 grams per minute during the night and since about half of that comes from liver glycogen, after a 10-hour overnight fast your liver glycogen level will be reduced by ~50-grams.In the 1970s, Nilsson & Hultman used liver biopsies to show that liver glycogen is rapidly restored with carbohydrate refeeding — one of the reasons that endurance athletes should consider eating a morning breakfast — and that liver glycogen is related to the amount of carbohydrate eaten in the diet.
In the 1980s, the labs of Ralph DeFronzo, John Wahren, and John Gerich used 3H- and 14C radioisotopes of glucose and arterio-venous catheterisation of multiple tissues to show that when ~50 to 90-grams of glucose water is drunk after an overnight fast, 30 to 50% of it is taken up by the liver and 50 to 70% by the muscles.
But drinking liquid glucose for breakfast is not a sane human endeavour. Fortunately, in the 1990s, some sensible folks in Gerald Shulman’s lab studied the fate of mixed-nutrient meal ingestion in the morning following an overnight fast, in healthy but untrained human folks. Using 13C-NMR methods, they found that a liquid breakfast containing ~140 grams of glucose with ~16-grams of fat and ~30-grams of protein, took ~4-hours to restore liver glycogen to prior evening levels, and that about 30% of a “real food” breakfast of cereals, juice, and eggs, containing 290 grams of carbohydrate, ends up stored as muscle glycogen.
×
![]()
So, a morning breakfast replenishes liver glycogen and “tops up” muscle glycogen. But, what does this mean for exercise?
Many moons ago, in 1939, Erik Christensen and Ove Hansen in Copenhagen found that drinking “sugar water” containing 200 grams of glucose on the morning of a ride-to-exhaustion at a moderate-intensity, prevented hypoglycemia and delayed exhaustion in untrained regular folks. They were certainly on to something — many moons too soon — it took some time for folks to take interest in their observation. The following 80-years of work were summarised in a 2018 systematic review and meta-analysis of all known data, concluding that pre-exercise carbohydrate ingestion (compared to fasting) generally enhances aerobic exercise performance during bouts lasting more than an hour, but not for shorter durations. However, several studies show that ingesting carbohydrates during exercise can overcome the limitation caused by fasting before exercise. This has been shown during a 30 km running time trial in trained runners, and during short (~16-mins in trained cyclists and 10 km in recreational cyclists) and long (50 km in trained cyclists) cycling time-trials. So, you don’t need to panic if your race day breakfast is suboptimal so long as you can feed during the race (more on that later).
Most studies comparing fed vs. fasted performance have examined steady-state exercise but similar outcomes are found when taking a S.H.I.T. (short high-intensity interval training) — performance is improved in the fed state during long-duration high-intensity efforts (e.g. 90 min of high-intensity running that includes 5 × 1-min sprints in recreational athletes; see here and here) but not during short-duration SHITs (e.g. 10 × 6-second sprints in untrained folks, 6-sets of 3 × 10-second sprints in trained cyclists, or 3 × 1-min efforts in recreational athletes).
This is all good to know but maybe you are wondering...
Will the amount of carbohydrate you eat in the hours before your race influence your performance?
Many folks have studied this question and find that, if you are not fasted and have eaten carbs at breakfast, the total amount of carbohydrate eaten has little influence on performance. For example, consuming ~1 or ~2 grams of carbohydrate per kilogram body mass of glucose 1-hour before a cycling time trial similarly improves performance over no glucose and ingesting 1 or 3 g/kg 3-hours before a ~90-min mountain bike race has no differential effect on race performance. Furthermore, taking 25, 75, or 200 grams of glucose 45-mins before exercise has no differential effects on cycling time trial performance, and consuming ~50, ~150, or ~300 grams of maltodextrin (a glucose polymer) and sucrose (a disaccharide of glucose + fructose) 4-hours before a cycling time trial all similarly improve performance compared to water.However, these experiments fed “sugar water” to their subjects for breakfast, which, having been a subject in some of these actual studies, is just horrible… And, as I said earlier, it is not a sane human endeavour to eat liquid glucose (or maltodextrin and sucrose) for breakfast. Once again, we are fortunate that some sensible folks use ecologically-valid (aka meaningful) study designs. For example, trained cyclists see no performance differences when they eat breakfast meals containing ~230 vs. ~90, or ~215 vs. ~50 or, ~258 vs. ~122 vs. ~15 grams of carbs either 4-hours or 90-minutes before performance tests (including a 2-hour ride at 60% VO2max followed by ride-to-failure at 80% VO2max, 90-mins at 70% VO2max followed by a 10-km time trial, or a 50-km time trial that included three 1-km and 4-km sprints). And the same is true for trained runners who see no performance differences when they eat a breakfast containing either ~180 or ~75 g of carbs 4-hours before an 80-min run at ~70% VO2max followed by a run-to-exhaustion at ~80% VO2max.
So, as it turns out, being fed rather than fasted is a good idea, but the studies that have compared low- vs high-carb mixed-nutrient breakfasts before exercise find no performance differences during cycling time trials or rides-/runs-to-exhaustion. In other words, some carbs are better than no carbs on race morning but don’t stress over the total amount. Other folks have even chipped away at the type of delivery of pre-exercise carb intake but also find no differences in performance between solid vs. liquid or solid vs. gel-based (here and here) carb intake before performance tests. However, it is very important to note that in all of these studies, subjects consumed a moderate to high carbohydrate diet for several days leading into the trials, which would maximise their muscle glycogen levels during all trials (although this was not measured).
That’s some more useful info but maybe you are wondering...
Will the timing of carbohydrate intake in the hours before your race influence your performance?
Remember that the intention is to maintain carbohydrate availability to the working muscles during your race for as long as possible. Breakfast restores liver glycogen levels, which have been depleted after an overnight fast. This restoration is rapid when “sugar water” is drunk (~2-hours, as shown by the tracer studies of the 1980s) but slowed when fats and protein are eaten with carbs (~4-hours, as shown by the NMR studies of the 90s). Therefore, from a metabolic perspective, your pre-race meal should be eaten between 2 and 4-hours before gun time to ensure maximal liver glycogen replenishment.That said, from a performance perspective, different timings of a pre-exercise meal have no notable influence on power output during a S.H.I.T. or time trial performance in trained cyclists or swimmers when “sugar water” (glucose) or a sports bar is consumed either 15 vs. 45 vs. or 75-mins, or 15 vs. 60-mins, or 5 vs. 35-mins before exercise. However, it is important to note that some athletes develop hypoglycaemia during the early stages of exercise if they consume carbohydrates during the 45 minutes before exercise. Glucose ingestion prior to exercise can increase insulin secretion at the onset of moderate- to high-intensity exercise, causing a drop in blood glucose due to an insulin-stimulated and exercise-stimulated glucose uptake by muscle combined with inadequate liver glucose output (see Costill et al 1977, Koivisto et al 1981, and Moseley et al 2003, in which I was a subject).
This risk of hypoglycemia doesn’t always impair performance but its consequential signs don't feel great and it is never worth the risk on race day! Consequently, efforts were made to blunt the insulin secretory response of a pre-exercise feed by using low glycemic index (low GI) foods — foods that cause a slower release of glucose into the blood without an insulin spike. In 1991, Thomas and colleagues found that consuming a low GI meal 1-hour before a ride-to-exhaustion at a low to moderate intensity (65-70% VO2max) maintained blood glucose levels and allowed cyclists to ride for longer than when a high GI meal was eaten. This has been confirmed by others, including my old postdoc supervisor, John Kirwan, who showed in 2001 that a moderate GI meal fed 45-mins before exercise was superior to a high GI meal for improving time-to-exhaustion at 70% VO2max.
And, one last point related to the timing of your pre-race feeding if your race is in the evening… Evidence in cyclists shows that skipping breakfast slowed an evening 20 km time trial by about 38-seconds, on average, despite complete caloric compensation at lunch. So, as an athlete, it will likely be very foolish to skip carbs at breakfast on race day.
This is a lot of info to absorb but here is a bite-sized morsel to chew on: performing fed (rather than fasted) exercise will give a far larger boost to your performance than stressing over the precise amount of carbs or the precise timing of your meals. So, stay calm.
What can you put in your performance nutrition toolbox?
As I discussed in detail in my previous series on nutritional manipulations for your training and in part 3 of this series, it is important to remember that no matter what dietary habits you are accustomed to, only carbohydrate (glucose) can be “burned” at a rate fast enough to meet the ATP demands of racing at a high-intensity (i.e. racing at a high fraction of your VO2max). If you are a low-carb/keto diet adapted athlete, you are indeed able to “burn” more of your bucket load of fat for longer, but to fuel rapid muscle contractions with a high rate of force development, you will need a high carbohydrate availability on race day. To echo what I said at the beginning of this series: “no matter how “fat adapted” you are nor which kind of diet you typically eat, your maximal race-day performance will be achieved with a high carbohydrate availability.”.
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. Note that when your race starts later in the day, eating carbohydrates as part of each meal during the day is sensible. Also note that a pre-race meal should seek to maximise carbohydrate absorption into the blood. Since other macronutrients (fat and protein) slow gastric emptying and delay absorption, it is not a good idea to eat them in large quantities on race morning because they will slow glucose delivery to the liver and muscles and will increase the likelihood of GI issues during a high-intensity race.
Until then, keep training smart.
Disclaimer: I occasionally mention brands and products but it is important to know that I am not affiliated with, sponsored by, an ambassador for, or receiving advertisement royalties from any brands. 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, A.P. Møller Foundation, and Augustinus Foundation. I’ve also consulted for Boost Treadmills and Gu Energy on their research and innovation grant applications and I’ve provided research and science writing services for Examine — some of my articles contain links to information provided by Examine but I do not receive any royalties or bonuses from those links. 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. My recommendations have never and will never be influenced by affiliations, sponsorships, advertisement royalties, etc. 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 accurate and clear scientific information to the masses to help 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 exercise physiology, nutrition, biochemistry, and molecular medicine. 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, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, ride, ski, hike, lift, and climb as much as my ageing body will allow. To stay on top of scientific developments, I consult for scientists, participate in journal clubs, 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.
Copyright © Thomas Solomon. All rights reserved.
I am Thomas Solomon and I'm passionate about relaying accurate and clear scientific information to the masses to help 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 exercise physiology, nutrition, biochemistry, and molecular medicine. 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, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, ride, ski, hike, lift, and climb as much as my ageing body will allow. To stay on top of scientific developments, I consult for scientists, participate in journal clubs, 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.
Copyright © Thomas Solomon. All rights reserved.