Optomising Training & Racing with Carbohydrates
Supplied by: Andrew N Bosch, PhD; Associate Professor; Division of Exercise Science & Sports Medicine; Department of Human Biology; University of Cape Town; Sports Science Institute of South Africa.
Introduction
During prolonged running events such as the marathon and ultra-marathon, three fuels are used by the muscle that are of importance: muscle glycogen, blood glucose, and fat. As exercise duration increases beyond, say, the half marathon, the muscle glycogen stores become depleted and fat provides an increased proportion of the energy needs. While the fat supply is large enough even in the leanest of athletes to provide fuel for the body for days, the limitation lies in the rate at which energy can be released from fat. Thus a depletion of muscle glycogen means a decline in performance and for this reason increasing muscle glycogen stores to the highest possible concentration prior to the commencement of a marathon is critical for optimal performance.
It is not only muscle glycogen that becomes depleted, however. Liver glycogen stores also become increasingly low during long runs, and when this happens, blood glucose concentration drops, and as a result one will feel weak, light headed, and the legs will feel like you’re running through water, or will perhaps have a strange “floating” feeling. These are symptoms of hypoglycaemia. Thus it is important to have high muscle glycogen at the start of long races such as marathons, but also to prevent the development of hypoglycaemia.
While running a race demands a large amount of glycogen, the amount of glycogen used in daily training of an athlete who is training at an elite level can be so high that unless care is taken to eat sufficient carbohydrate each day, the muscle glycogen reserves can become depleted just as a result of training. When this happens, fast sessions will be almost impossible to do (or will be greatly reduced in quality) and the legs will feel heavy and fatigued.
Thus it is not only during racing that attention should be given to ensuring sufficient muscle glycogen, but also during daily training. Therefore, maintaining an adequate daily intake of carbohydrate is important when one is including hard training sessions in the schedule.
In this article I will explain carbohydrate loading before a marathon, the importance of maintaining adequate muscle glycogen stores for optimal training, procedures to follow on the morning of a race, and what you should do to optimize your race from the point of view of fluid and carbohydrate ingestion during marathon and ultra-marathon races.
Carbohydrate Loading before racing a marathon
Carbohydrate loading is a dietary technique used prior to an endurance race such as the marathon to enhance performance. To carbohydrate load, high carbohydrate foods or drinks, or both, are ingested on the days before an event to increase the stores of carbohydrate (muscle glycogen) in the muscles.
The carbohydrate-loading diet was first popularised in the late 1960’s after it had been found by researchers at that time that muscle glycogen stores could be increased by ingesting a high carbohydrate diet for three or more days, subsequent to depletion of the muscle glycogen stores by prior exercise. Specifically, glycogen depletion was deliberately achieved by a prolonged bout of exercise and then ingesting a diet low in carbohydrate for a number of days. With these low muscle glycogen concentrations at the start of exercise the average exercise time before exhaustion in a bike test was only 57 minutes. After this the diet was switched to a high carbohydrate one for a number of days (~600 g of carbohydrate daily), after which muscle glycogen concentrations increased to very high levels and exercise time increased to 167 minutes before exhaustion occurred. This technique to increase glycogen became popular amongst marathon runners and was known as “carbo-loading”.
After some time, after more research work was done, it was found that while the “loading” diet worked, it was not necessary to follow such an extreme protocol to get the high muscle glycogen concentrations. The new discovery was that things were different in well trained people. The original research was done with relatively untrained people: The subsequent research on trained people showed that the depletion phase (consisting of the prolonged bout of exercise followed by the low carbohydrate diet for a number of days) is unnecessary in trained athletes. Specifically, it was found that trained people need only to eat a high carbohydrate diet for three days (500-600g of carbohydrate/ day; or 7-10g/kg body weight/day) which, combined with a reduction in training, results in the same glycogen concentrations as when the original loading regimen is followed. The reason for the difference between trained and untrained people lies in an enzyme involved in the storage of muscle glycogen (glycogen synthase) which is activated in untrained individuals by the depletion phase of the carbohydrate loading regimen. In trained people this enzyme is already maximally activated as a result of daily training partially depleting the muscle glycogen reserves each day. No further activation occurs following a period of low carbohydrate intake and thus the depletion phase is unnecessary in people who regularly engage in prolonged training.
Most recently, in a further refinement, it has been shown that in highly trained athletes even three days of carbohydrate loading as described above is longer than needed to maximise muscle glycogen stores. By ingesting 10 g/kg body weight/day of carbohydrate, maximal muscle glycogen concentrations can be attained within just 24 – 36 hours in trained athletes. I think the 24hrs is a little on the short side, and certainly in some of the studies in which I have been involved with, 24hrs has not been sufficient to get to really high glycogen concentrations. A safer bet is 36 to 48 hrs of high carbohydrate. Having said that, those runners who struggle to ingest large volumes (to supply 10g carbohydrate/ kg body mass), can eat less each day for a longer period of time.
Maintaining carbohydrate stores (muscle glycogen) during training
As described above, it was some 45 years ago that it was first established that performance was enhanced by high muscle glycogen concentrations and impaired by low muscle glycogen concentration, and hence the need to carbo-load as described in the previous paragraph. In 1980 it was suggested by one of the foremost running physiologists of the time, Prof David Costill of the USA, that hard training on a daily basis, itself, could result in a substantial lowering of muscle glycogen (which would affect the ability to do hard training sessions at a good speed), but that a high carbohydrate diet consumed on a daily basis could maintain muscle glycogen concentration at optimal levels. This led to a study by Costill in 1981 that proved this point and showed that muscle glycogen could indeed be depleted after successive days of high level training, but that it could effectively be replenished after each day of training provided that sufficient carbohydrate was ingested. Specifically, runners in the study ingested either a low carbohydrate diet, a mixed diet, or a high carbohydrate diet while maintaining a high training load. When the high carbohydrate diet was ingested (650g every 24 hours), muscle glycogen concentrations were kept at normal levels, but this was not achieved when the low carbohydrate and mixed diets were consumed. Here the muscle glycogen stores gradually got lower and lower and as they did so the runners struggled to maintain the training load. The high carbohydrate consumption successfully restored muscle glycogen every 24 hours and training load could be maintained. Interestingly, Kenyan and Ethiopian runners eat a diet that contains 10g/kg body mass as their normal diet; exactly what the science says is optimal for hard training.
Glycaemic Index of Carbohydrate for Optimising Muscle Glycogen Storage
Various studies have been conducted to determine whether the glycaemic index of the carbohydrate used when attempting to optimise muscle glycogen stores has any effect on the rate of muscle glycogen formation. A number of studies have shown improved muscle glycogen storage when the carbohydrate ingested has been of a high glycaemic index. Importantly, it should be noted that in the study referred to previously in which it was shown that maximal muscle glycogen stores could be attained within 24 hours of ingesting a high carbohydrate diet, a high glycaemic index carbohydrate was used. Nevertheless, the effect of glycaemic index on rate of muscle glycogen storage remains to be fully resolved. There appears to be sufficient evidence, however, to suggest that inclusion of at least some high glycaemic index carbohydrates as part of the carbohydrate intake when attempting to optimise the rate of muscle glycogen synthesis, is justified.
Types of Carbohydrate for Optimal Muscle Glycogen Restoration
Besides consideration of the glycaemic index of the carbohydrate foods used in attempting to optimise the rate of muscle glycogen synthesis when carbohydrate loading before a race, or maintaining a high carbohydrate intake on a daily basis to optimise training, any high carbohydrate, low fat food or drink can be used as part of a high carbohydrate diet. Amongst suitable food items are bread, rice, pasta and potatoes. Potatoes have a high glycaemic index, making them an ideal food-type to include in a diet aimed at rapidly restoring muscle glycogen used in daily training, as this would take advantage of the apparent higher muscle glycogen concentrations achieved from ingestion of high glycaemic index foods. It is of course difficult or impossible to eat such large amounts of food on a daily basis, and even more so when this has to be increased even more when carbo-loading for a race. Bear in mind that a 60kg athlete will be looking at ingesting around 600g of carbohydrate per day, if trying to reach an intake of 10g/kg body mass. And that is 600 g of carbohydrate, not 600 g of carbohydrate foods. One hundred grams of potato for example, will supply little more than 20g of carbohydrate. When one considers these volumes, it is little surprise that cyclists competing in the Tour de France drink and eat huge amounts of carbohydrate-containing fluid and foods every day of the race. One way around the problem is to ingest a lot of the carbohydrate in the form of a drink. I have athletes that I work with do this as a matter of routine. Tanith Maxwell, for example, a Boxer Athletics Club athlete, drinks a carbohydrate drink consisting of a particular type of carbohydrate called a glucose polymer, after each training session and at various times during the day to meet her carbohydrate requirements. It’s a lot easier to drink 50g of this carbohydrate a few times each day to replenish muscle glycogen than to eat the equivalent amount of carbohydrate food. Of course, there are some athletes who will not struggle to eat the necessary amount of carbohydrate as solid food, and they may well be able to do without the glucose polymer supplement.
Race morning
A small breakfast eaten a few hours before a race has been shown to be beneficial. However, it is important that the athlete experiments with the time at which the breakfast is ingested. Although some studies have shown no ill effect as a result of ingesting carbohydrate a short while before the start of exercise, others have shown what is called a “reactive hypoglycaemia”. I have seen this in many elite athletes; indeed, the better trained the athlete is, the longer the time that seems to be necessary. Most elite athletes need about 3 hrs from time of eating breakfast till the start of the race.
A pre-race meal should be relatively low in fat and fibre to facilitate gastric emptying and minimizes gastrointestinal distress; be high in carbohydrate to maintain blood glucose and maximize (liver) glycogen stores; contain a moderate amount of protein and be familiar to the athlete.
Fluid and carbohydrate ingestion during long races
A suitable drink to ingest during a marathon or ultra is one that will supply a large amount of carbohydrate that can be rapidly absorbed. However, we have a conflict here, because while wanting to provide large amounts of carbohydrate, the presence of carbohydrate in a drink results in a slower rate of gastric emptying compared to water alone, since the carbohydrate raises what is called the “osmotic pressure” of the drink. This occurs to a greater extent if the carbohydrate in question is one of the simple carbohydrates, such as glucose, and when the concentration of the carbohydrate in the drink gets too high. Other factors that influence the rate of emptying besides the type of carbohydrate and the concentration of carbohydrate in the drink is the volume of fluid in the stomach: increased volume in the stomach increases gastric emptying rate, but in he case of runners the volume cannot be too high as otherwise discomfort will be felt and this will hardly be beneficial to performance, overall!. Thus the drink must contain the right type of carbohydrate, be of the correct concentration, and be ingested in the correct amounts and at the right time. In this way sufficient carbohydrate can be provided to prevent hypoglycaemia and provide fuel to the muscle, as well as provide sufficient fluid so that body fluid levels do not drop so low as to potentially affect performance. Most importantly though, we want a high rate of gastric emptying with a fairly high concentration carbohydrate drink so that delivery of carbohydrate to maintain blood glucose levels and fuel to the muscle is optimised. This is important, as there have been hundreds of research studies, including our own at the Division of Exercise Science & Sports Medicine of the Department of Human Biology of UCT that have shown that ingestion of carbohydrate during prolonged exercise enhances performance.
How do we achieve this balance between supply of adequate fluid and optimal carbohydrate delivery? The answer is actually quite simple and the plan that follows has taken into account the International standard of refreshment tables being available every 5km during marathons. Runners should aim to ingest 500 – 600 ml of a 10% carbohydrate solution (to which has been added ~1 g of salt if the supplement to be ingested does not already contain sodium) per hour, in “doses” of approximately 150 ml, approximately every 15 mins. The type of carbohydrate to drink may be forced on you if you are not an elite runner and have to make use of what is available at the tables. This could be “Coke”, “Powerade”, “Energade”, etc. These certainly work, although the sweetness of these drinks may become an issue later in the race, and they might not empty optimally, and the concentration (amount of carbohydrate) may also be a little less than optimal. If you are in the position where you have access to personal seconds, or are an elite level runner competing in International races, then you will have the option of having your own drinks provided at the second tables. A better option then is to mix up your own drink consisting of a 10% glucose polymer i.e. 100g of glucose polymer in 1L water. I have had Tanith Maxwell, who I have worked with for many years, routinely use this mixture in all her races, including the race in which she set her marathon PB(2.32.00) (Berlin Marathon 2010) and the 2015 Two Oceans(3rd) and Two Oceans 2016(2nd) . Because this is slightly more concentrated than the commercial sports drinks despite being less sweet, it provides more carbohydrate for the same volume ingested. And we know from many experiments that it empties well from the stomach (even at 10% concentration) and is readily oxidised (used as a fuel) by the muscles. Undiluted “Coke” is also a 10%, carbohydrate drink (sucrose) but it is often perceived as being too sweet for most runners to tolerate undiluted and it also empties slower than the glucose polymer option, and thus less carbohydrate is “delivered”.
If you are in a position where you have to rely entirely on what is available at the seconding tables, then one option is to use the “Coke “, “Energade”, “Powerade”, etc that may be provided and to make up the shortfall in carbohydrate by supplementing with one of the commercially available sachets that contain a carbohydrate gel. If these are used, depending on how much of the provided carbohydrate-containing drinks are used, it will be necessary to ingest one or two sachets per hour (~ 25-50 g carbohydrate), making sure that a decent quantity of water is ingested with each so that you don’t have a highly concentrated gel of some description in the stomach that will soon lead to a slowing of gastric emptying rate and the possible development of nausea. A final option is to drink only water during the race and to supplement with more of the sachets, in this case two to three per hour. Should symptoms of hypoglycaemia develop (weakness, extremely sluggish legs as if running in water, dizziness, loss of interest in the race) then some form of additional sugar should be ingested immediately, such as taking the contents of two or three sachets.
It is important that each runner experiments with the advice given here before the race, as what works well for one runner might not work quite as well for another. Nevertheless, the guidelines given here are such that it works “straight-off” for most runners. Having said that, I do find that there are a small number of runners for whom various aspects of the protocol have to be modified in some way. It is also very important to “practice” your chosen regimen in faster-paced long training runs, first to establish that it “works” for you (for example, do you get any feelings of fullness, or nausea, or find the drink too sweet after 2hrs of running) and secondly to “train the gut”. With training (ingesting carbohydrate-containing fluid while running), it will become a lot easier to tolerate larger volumes of fluid without feelings of fullness, and the stomach will accommodate to empty faster during running).
Summary
Athletes who are training hard need to keep their glycogen stores from becoming too low as this will negatively affect high intensity training runs. To adequately replenish glycogen stores, sufficient amount of carbohydrate must be ingested each day. Any carbohydrate-containing food is suitable, and can also be ingested in the form of a drink.
Prior to long races such as the marathon, it is important to carbo-load prior to the event by ingesting large amounts of carbohydrate (8 to 10g/ kg body mass) for 2 days. As with maintenance of muscle glycogen on a daily basis, this can be achieved by either eating solid food or by using a carbohydrate-containing drink.
On the morning of a race, a small breakfast eaten approximately 3 hrs before the start is beneficial. After this, no further carbohydrate food or drink should be taken before the start.
During a marathon or ultra race, carbohydrate should be ingested from early in the race. The aim should be to ingest 500 to 600 ml per hour of a 10% carbohydrate drink, achieved by drinking approximately 150 ml at each seconding table. On very hot days, or if feeling thirsty, it is quite permissible to drink more.