Why are carbohydrate diets important to improve your sports performance?

Interest in the influence of food on physical activity capacity is as old as mankind. From the earliest times, certain foods were considered essential preparation for strenuous physical activity . At a recent consensus conference on food, nutrition and sports performance, carbohydrate-containing foods were found to have the greatest influence on exercise performance. ‍

Most studies on the influence of carbohydrate intake on sports performance have been conducted in laboratories using cycling or treadmill running. Performance is usually assessed as the time to exhaustion (endurance capacity) during constant intensity exercise, or the time to run a predetermined distance or complete a prescribed workload (throughput) in the shortest possible time.

In some studies, researchers have combined the elements of endurance capacity and performance in a protocol to try to simulate a habitual pattern of activity in sports. For example, running at a constant submaximal pace for an hour or more and then completing a set distance in the fastest possible time; or cycling at a constant submaximal workload and after an hour pedaling as fast as possible to complete a set workload as quickly as possible.

The division between endurance capacity and endurance performance is artificial, since in any real-life endurance race or event, both endurance capacity and pace are required for success.

However,by better understanding simple endurance capacity, we can get a clearer picture of the essential determinants of endurance performance.

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Carbohydrate diets and endurance capacity

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An early study exploring the relationship between diet and exercise capacity found that, after a period on a high-carbohydrate diet, endurance capacity on a bicycle ergometer doubled compared to exercise times achieved after consuming a normal mixed diet. In contrast, a fat and protein diet reduced exercise capacity to almost half that achieved after a normal mixed diet. This clearly demonstrated the benefits of consuming a high-carbohydrate diet prior to prolonged exercise and was the first to establish the importance of carbohydrate content in the diets of athletes preparing for competition.

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The benefits of carbohydrate loading prior to prolonged submaximal exercise have been demonstrated primarily during cycling. A relationship was demonstrated between endurance performance during cycling ergometry and pre-exercise muscle glycogen concentration. The importance of muscle glycogen during prolonged exercise was confirmed by subsequent studies showing that fatigue occurs when muscle glycogen concentrations are reduced to low values.

Therefore, it is not surprising that attempts were made to find methods to increase muscle glycogen stores in preparation for prolonged exercise. One study examined the influence of different nutritional states on glycogen resynthesis during recovery from prolonged exhaustive exercise. It found that a diet low in carbohydrate and high in fat and protein for 2 to 3 days after prolonged submaximal exercise produced a delay in muscle glycogen resynthesis, but when this was followed by a diet high in carbohydrate for the same period of time, glycogen supercompensation occurred.

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This dietary manipulation not only increased pre-exercise muscle glycogen concentration, but also resulted in a significant improvement in endurance capacity. Although this original method of carbohydrate loading was recommended as part of the preparation for endurance competitions, the low-carbohydrate, high-fat, high-protein diet phase for some athletes is an unpleasant experience. Therefore, alternative ways to increase glycogen stores prior to exercise were explored without including a period on a high-fat, high-protein diet. It was found that a high-carbohydrate diet consumed for 3 days prior to competition, accompanied by a decrease in training intensity, resulted in an increase in muscle glycogen concentrations of the same magnitude as that achieved with the traditional carbohydrate loading procedure.

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Carbohydrate diets and resistance training

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In a recent study, the influence of carbohydrate loading on running performance was studied during a 30 km laboratory treadmill simulation. One of the objectives of this study was to determine at what point during the run run runners began to show signs of fatigue and how this was modified by dietary manipulation. The treadmill was instrumented so that subjects controlled their own speed by means of a slight manual switch.

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Changes in speed, time and distance run were displayed on a computer screen in view of the subjects. The runners were divided into two groups after the first 30 km treadmill time trial. One group increased their carbohydrate intake during the 7-day recovery period, while the other group ate additional protein and fat to match the increased energy intake of the carbohydrate group. Although there was no overall improvement in the performance times of the two groups, the carbohydrate group ran faster during the last 10 km of the simulated race. In addition, eight of the nine runners in the carbohydrate group had faster times during the 30 km than during their first attempt, and better times than the control group. Although the carbohydrate group ran faster than the control group, after carbohydrate loading they had lower adrenaline concentrations. This was attributed to the carbohydrate loading and subsequent maintenance of normal blood glucose concentrations throughout the run. Noradrenaline concentrations increased, as expected, during the 30-km simulated runs after normal dietary conditions and after carbohydrate loading.

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Carbohydrate diets and high-intensity exercise

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More people participate recreationally in "multiple sprint" sports (such as soccer, field hockey, tennis, basketball and rugby) than endurance sports (cycling, swimming or running). These multiple sprint sports involve a mixture of brief periods of maximal intensity exercise followed by recovery periods of rest or light activity, and last up to 90 minutes. However, only a limited amount of information is available on the influence of diet on short duration maximal intensity exercise. One of the reasons for the limited research on this topic has been the lack of adequate laboratory methods to study metabolic and physiological responses to maximal exercise. Microcomputers are now available and are used to record rapid changes in energy production during short-duration maximal exercise.

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Although there is rapid utilization of muscle glycogen during several brief periods of maximal exercise, the rate of glycogenolysis decreases as exercise continues. For example, in a series of 10 maximal sprints of 6 seconds duration and 30 seconds recovery on a cycloergometer, glycogen degradation was halved during the last sprint. This glycogen sparing is probably the consequence of increased aerobic metabolism of glycogen and free fatty acids.

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Performance is impaired when we have athletes whose practice requires a combination of submaximal running and brief periods of sprinting, so carbohydrate loading would likely be beneficial for participants in sports of this style.

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Composition of pre-exercise meals

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The type of carbohydrate in pre-exercise meals and its influence on subsequent endurance capacity has, until recently, received very little attention. The description of carbohydrates as simple or complex is an inadequate way of classifying them. A metabolically more informative way of describing carbohydrates is the degree to which they increase blood glucose concentrations. Carbohydrates that produce a large increase in blood glucose concentration in response to a standard amount of carbohydrate (50 g) are classified as having a high glycemic index. The metabolic response during exercise is different as a consequence of the glycemic indices of carbohydrates consumed before exercise, so the choice of carbohydrates in pre-competition meals could have an effect on performance.

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In a study on the influence of high and low glycemic index carbohydrate foods on exercise capacity, the low glycemic index carbohydrate appeared to improve endurance capacity to a greater extent than the high glycemic index food. This study used lentils as the low glycemic index food, and potatoes and glucose as the high glycemic index food, and compared the responses to these after drinking a glucose solution or plain water.

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Eating a high-fat meal three to four hours before exercise is not recommended as nutritional preparation for endurance competitions, as these meals take longer to digest. There is some evidence from animal studies suggesting that increased fat intake will result in lower than normal carbohydrate oxidation during exercise. If this glycogen sparing were to occur after a high-fat meal, it would be expected to delay the onset of fatigue in a manner similar to the consumption of carbohydrate-rich meals prior to exercise. A recent study attempted to answer this question by comparing the endurance performance of subjects after isocaloric meals rich in either fat or carbohydrate four hours before submaximal exercise. The pre-exercise meals contained approximately 280 g of carbohydrate in the high-carbohydrate meal and 84 g in the high-fat meal. There were no statistically significant differences between the endurance times of the high-carbohydrate and high-fat (low-carbohydrate) meals.

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Exercise recovery

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Recovery from exercise is not a passive process. Tissues repair and reproduce, fluid balance is restored and substrate reserves are replenished.

Carbohydrate replenishment is one of the most important events during recovery. 

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When several days separate periods of exercise or sport participation, a normal mixed diet containing about 4 to 5 g/kg body weight (BW) of carbohydrate is sufficient to replenish muscle glycogen stores. However, daily training or competition places high demands on the body's carbohydrate stores. Therefore, the normally high carbohydrate intake of athletes may not be sufficient to prevent a gradual depletion of this important fuel reserve.

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For example, even when daily carbohydrate intake is 5 g/kg body weight, cycling or running for one hour each day gradually delays the daily restoration of muscle glycogen stores. Increasing carbohydrate intake to 8 g/kg body weight per day may not be sufficient to prevent a significant reduction in muscle glycogen concentrations after 5 consecutive days of hard training. These studies underscore the importance of prescribing adequate amounts of carbohydrate to athletes in training and justify the need for more frequent recovery days between periods of intense training.

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The clear message from more than half a century of research on the links between food, nutrition and exercise capacity is that, along with natural talent and proper training, a diet rich in carbohydrates and adequate fluid intake to avoid dehydration are the two most important elements of the formula for successful participation in sport. Of course, there is an underlying assumption that athletes normally consume a well-balanced diet, composed of a wide variety of foods, and containing sufficient energy to meet their needs.

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It is also essential that we talk about athletes who are keto-adapted, whose metabolism is able to extract the necessary energy from fats to perform the desired activity, from here we could extract the importance of staying healthy with an optimal metabolism that is able to adapt to different scenarios and that has the facility to continue functioning without problem. 

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Bibliographic References 

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