Energy is simply defined as the capacity to perform work. Physical activity during exercise and sport involves a significant amount of work. The capacity to do this work ultimately comes from the food that we eat. Energy from food can be viewed as the currency that allows our bodies to pay for the physiological cost of exercise. The type, intensity and duration of exercise undertaken determines how much energy is required by the body. The energy requirements of athletes vary significantly from sport to sport and also at different times in a competitive season. They can even vary substantially from one day to the next with a fluctuating training schedule. Energy intake should, ideally, match the energy expenditure fluctuations associated with training and competition loads as well as the energy demands of other activities of daily living and the energy required for basic physiological functions including growth and development in younger athletes. Varying energy intake on a daily basis to meet the energy demands of each specific training day is an example of nutrition periodisation. Nutrition periodisation recognises that nutrient requirements are not static and nutrient intake can be manipulated at different times to achieve different training and performance objectives.
Energy availability is a concept used in sports nutrition to determine whether energy intake is likely to be insufficient to sustain healthy physiological functions during periods of high energy expenditure. Energy availability is calculated by subtracting the energy expended during an athlete’s exercise training from the total dietary energy intake. It is usually expressed in kilojoules or calories relative to an individual’s fat free mass and it provides an indication of the amount of energy remaining for normal physiological function after the demands of exercise have been met. Low energy availability in athletes can lead to a condition known as relative energy deficiency in sport (RED-S) which is associated with the following negative consequences for athletes.
Knowledge Check – Energy
Both endogenous (stored in the body) and exogenous (consumed in foods and beverages) carbohydrate can be used as fuels during exercise. The contribution of carbohydrate breakdown to total energy expenditure increases as the intensity of exercise increases. Carbohydrate is also used by the central nervous system and may influence pacing strategies, concentration and perceptions of fatigue during exercise. Given these important roles of carbohydrate during exercise, it was once thought that all athletes and exercising individuals should consume a high carbohydrate diet in order to maximise their performance, recovery and adaption to training. However, most athletes undertake periodised training programs in which the intensity, duration and frequency of training is manipulated at different times of the season or year in order to develop different characteristics that underpin successful performance. Consequently, carbohydrate requirements are not static and more recent guidelines suggest that carbohydrate intake should be manipulated to suit the goals of individual athletes at any given point in their training schedule.
There is approximately 500g of glycogen stored in the body with about 400g in the muscles and 100g in the liver. These stores can vary significantly between individuals with trained athletes able to store up to 700g of muscle glycogen. Glycogen stored in the muscle can be used directly for energy during exercise. Liver glycogen must be converted to glucose and then transported in the blood to the working muscles for use during exercise. Endogenous glycogen can fuel high intensity endurance exercise for around 2-3 hours before stores become depleted. Exogenous carbohydrate can also be used to provide energy during exercise when it is ingested in solid, liquid or gel forms.
Carbohydrate stores in the muscle are unlikely to limit performance in a single bout of short duration, high intensity exercise such as sprinting or weightlifting. However, fatigue during repeated high intensity exercise or multiple sets of resistance exercise may occur earlier if individuals have not consumed adequate carbohydrate in the days before exercise. A moderate daily carbohydrate intake of 4-7g/kg body mass per day has been recommended for athletes involved in strength and power sports, but it is likely that this requirement may vary during different phases of the training cycle, particularly if other training goals such as changes in body composition are desired. Athletes involved in endurance based sports with a higher training load may require carbohydrate intakes of up to 8-10g/kg body mass per day, however, this also has the potential to vary significantly depending on the specific goals and objects at any given point in the training cycle. Carbohydrate requirements for athletes are expressed in g/kg of body mass, rather than a percentage of total daily energy intake, in order to reflect the fuel needs of exercising muscles. Therefore it is also important to consider the body fat levels of different athletes to avoid overestimates of carbohydrate requirements based on a g/kg total body mass guideline.
Muscle glycogen depletion and reduced blood glucose concentration is associated with fatigue during long duration endurance exercise. A high carbohydrate diet in the days before a single bout of endurance exercise is associated with increased muscle glycogen storage and enhanced performance during exercise compared with the consumption of a low carbohydrate diet. Since these early discoveries in the 1960s, much research has been conducted on strategies to augment muscle glycogen stores before endurance events. A protocol involving a few days of carbohydrate depletion through a restricted diet and a high training load, followed by a few days of ‘supercompensation’ in which training load was restricted and a very high carbohydrate diet was consumed was originally thought to be optimal. However, subsequent studies have shown that ‘supercompensation’ of glycogen stores can still occur without a preceding depletion phase. Therefore, current recommendations suggest that a reduced training volume and a high carbohydrate intake of approximately 8-10g/kg body mass for 3-4 days before the event is likely to result in optimal muscle glycogen storage. This form of ‘carbohydrate loading’ has been used for many decades by athletes competing in endurance and ultra-endurance sports.
The pre-event meal is often considered to be just as important for an athlete’s comfort and confidence as it is for enhancing the metabolic capacity of the muscle immediately before exercise. Ideally, the foods consumed immediately before exercise should be sufficient to keep muscle and liver glycogen stores high, prevent hunger and avoid any gastrointestinal discomfort, while also enabling appropriate practices that facilitate psychological preparation and allow for any of the athlete’s superstitions. Carbohydrate intake of 1-4g/kg in the pre-event meal may be able to achieve all of these goals, however, the pre-event meal guidelines are quite broad and can vary significantly for different athletes. Interestingly, the carbohydrate content of the pre-event meal is less important when sufficient carbohydrate is consumed during the subsequent exercise task. It is important for athletes to choose foods that are familiar and readily available in the pre-event meal.
Carbohydrate During Exercise
Consuming carbohydrate during exercise is associated with improved endurance exercise performance. Carbohydrate ingestion is likely to provide additional fuel for exercise and delay the depletion of muscle glycogen stores. Interestingly, improved performance occurs even when carbohydrate is consumed during exercise tasks lasting approximately one hour, which would not be long enough to fully deplete muscle glycogen stores. Indeed, some studies have shown that rinsing a carbohydrate solution in the mouth without ingestion can enhance performance during exercise lasting approximately one hour. This suggests that carbohydrate consumption during exercise may have the capacity to improve performance through multiple mechanisms. The presence of carbohydrate in the mouth induces a signalling response to the central nervous system which stimulates brain regions related to reward and motor control. Carbohydrate consumption during these shorter duration endurance exercise tasks is more likely to improve performance in studies where participants begin exercise after an overnight fast. Other studies that have provided carbohydrate rich meals or periods of carbohydrate loading before exercise have typically not shown that carbohydrate can enhance performance during exercise tasks lasting approximately one hour. These findings have practical applications for endurance athletes competing in events lasting approximately one hour or less because many athletes find it uncomfortable to consume foods and beverages before and/or during exercise. If an athlete prefers to train or compete on an empty stomach, they may still be able to have enhanced performance if a small amount of carbohydrate is consumed during exercise. Whereas other athletes might prefer to load up on carbohydrate before the event and not be distracted during the event by the need to consume foods or beverages.
While it is clear that very small amounts of carbohydrate, even when just rinsed in the mouth and not ingested, can improve performance during events lasting approximately one hour, the potential for carbohydrate to enhance performance during longer events is likely to be determined by the amount and type of carbohydrate ingested . Studies using metabolic isotope tracers have enabled the determination of the rate at which ingested carbohydrate is used to produce energy during exercise. These studies have shown that maximal rates of exogenous glucose oxidation occur at approximately 1g/min, which is much higher than other types of carbohydrate such as fructose and galactose which are oxidised at a rate of 0.6g/min. These findings have led to recommendations that 30-60g per hour of carbohydrate should be ingested during exercise lasting longer than one hour. Interestingly, studies using a mix of different types of carbohydrate during prolonged exercise lasting 2-3 hours have reported even higher exogenous carbohydrate oxidation rates of up to 1.8g/min. This is likely to occur because the rate limiting step for exogenous carbohydrate oxidation appears to be the rate of absorption in the gut. Different types of carbohydrate are absorbed via different intestinal transport mechanisms and a mix of multiple transportable carbohydrates will produce a higher rate of exogenous carbohydrate oxidation compared with a single carbohydrate source. It is recommended that athletes consume 60-90g per hour of multiple transportable carbohydrates (e.g. a mix of glucose and fructose) during exercise lasting 2-3 hours or longer.
The recommendations for carbohydrate intake before and during exercise are often based on findings from laboratory studies usually conducted on stationary cycle ergometers. Exercise in this context is probably least likely to result in gastrointestinal discomfort compared with other modes of exercise, such as running, which are usually performed outdoors in variable environmental conditions. Therefore, it is possible that optimal intake of carbohydrate during long duration exercise may be extremely difficult to achieve for endurance athletes in training or competition due to the potential for gastrointestinal discomfort associated with such a high intake of carbohydrate during exercise. Interestingly, some recent studies have suggested that it is possible to train the gut to tolerate large carbohydrate intakes through repeated exposure during exercise. Liquid, semi-liquid and solid forms of carbohydrate all appear to have similar benefits when consumed during exercise which means that athletes should choose the form of carbohydrate that they prefer and feel most comfortable consuming during exercise. It is important for athletes and exercising individuals to use carbohydrate recommendations as a guide or starting point, but they should be also encouraged to trial different carbohydrate intake strategies to identify which one best suits their individual requirements.
Carbohydrate and Training Adaptation
Despite the clear benefit of carbohydrate ingestion in enhancing performance during an acute bout of endurance exercise, it may also be possible for athletes to derive benefits from training with low carbohydrate availability. Training after an overnight fast, or training while consuming a diet low in carbohydrate, or even just restricting carbohydrate intake after the previous training session all enable athletes to perform a training session with low carbohydrate availability. Training with low carbohydrate availability appears to result in a greater transcriptional activation of enzymes involved in carbohydrate metabolism and actually enhances the metabolic adaptations to training. However, many of the studies conducted in this area use a design that involves multiple training sessions performed in close proximity so that there is not enough time to restore muscle glycogen between training sessions. It has been argued however, that the enhanced adaptation in these studies may simply be due to the proximity of training sessions rather than performing exercise with low carbohydrate availability. Although there is still some debate among sports nutrition researchers in this area about the exact mechanisms, training with low carbohydrate availability may be a strategy that can enhance muscle adaptations to exercise.
The disadvantage of training with low carbohydrate availability is that performance during training may be reduced which may compromise other non-metabolic training goals such as enhanced neuromuscular coordination. Many athletes now use a periodised approach to training where some training sessions are performed with low carbohydrate availability and some are performed with high carbohydrate availability depending on the goals of the individual session and the specific phase of the training cycle. This is a good example of how athletes can use an understanding of sports nutrition to add value to their training programs and achieve the specific adaptations that maximise performance.
Carbohydrate for Recovery
Restoring muscle and liver glycogen to ensure adequate carbohydrate substrate availability for future training or competitive requirements is an important recovery nutrition goal. Muscle glycogen stores can be usually be fully restored within 24 hours with adequate provision of exogenous carbohydrate (8-10g/kg body mass) and no additional exercise. In practice, consideration needs to be given to the training schedule, as multiple training sessions within a day are common and reduce the timeframe for recovery. It is important that an individual’s specific energy needs, training demands and performance feedback should be used to tailor carbohydrate intake to optimise recovery from training and competition.
While the total amount of carbohydrate consumed is the most important consideration for restoring muscle and liver glycogen after training, other factors such as the glycaemic index of the foods consumed and the amount of protein consumed may also influence muscle glycogen resynthesis. High glycaemic index foods and the coingestion of protein can accelerate muscle glycogen resynthesis after exercise when carbohydrate intake is less than optimal. However, these factors have little influence when more than 1.0-1.2g/kg body mass of carbohydrate is consumed each hour after exercise. Caffeine may also accelerate muscle glycogen resynthesis, but the amount of caffeine required is very high and may disrupt other aspects of recovery such as sleep.
Knowledge Check – Carbohydrate
Fats, available in subcutaneous adipose tissue, intramuscular triglyceride, and exogenous fat from food, can all be broken down to produce energy for muscle contraction. Triglycerides consist of 3 fatty acids attached to a glycerol backbone and form the largest reservoir of fuel in the body. While aerobic metabolism of fat molecules produces significantly more ATP than carbohydrate, the rate of energy production is slower. The contribution of fat oxidation to total energy production decreases as exercise intensity increases, whereas the contribution of carbohydrate oxidation increases at higher exercise intensities. The maximal rate of fat oxidation occurs at approximately 45-65% of VO2max.
Fat metabolism produces approximately 19.5kJ per litre of oxygen, which makes it slightly less efficient that carbohydrate which produces 21.1kJ per litre of oxygen. However, fat stores in the body are much greater than carbohydrate stores and more energy is produced per gram of fat than per gram of carbohydrate. The energy storage from fat in a 70kg man is approximately 350-550MJ, compared with only 7.5-9.0MJ from carbohydrate. Unlike carbohydrate stores, fat stores in the body are very unlikely to become depleted during exercise.
Fat Oxidation During Exercise
The type and intensity of exercise can influence the rate of fat oxidation during exercise. Fat oxidation is greater when the type of exercise is one that recruits a large muscle mass. For example, fat oxidation during running is slightly higher than during cycling at a given exercise intensity because of the greater muscle mass recruited. However, this likely to be in proportion with the increase in total energy production. The contribution of fat oxidation to total energy production declines as exercise intensity increases with carbohydrate providing most energy for muscle contraction when exercise intensity is very high. The maximal rate of fat oxidation occurs at an exercise intensity of approximately 45-65% VO2max, but this can vary between individuals. Fat oxidation increases slightly as the duration of exercise increases, particularly after 90-120 minutes of submaximal exercise. This is likely to be due to the greater mobilisation of free fatty acids from adipose tissue stores as the duration of exercise increases.
One of the key metabolic adaptations to endurance training is an increase in the ability to oxidise fat during exercise. The maximal rate of fat oxidation is higher in trained compared with untrained individuals. The use of fat at a given submaximal exercise intensity is also higher in trained individuals. Fat storage inside muscle cells is also increased after a period of endurance exercise training. Endurance training, regardless of any concurrent dietary interventions, appears to result in a greater metabolic capacity for fat oxidation during exercise. However, this is just one of many physiological changes that occurs after a period of endurance training and some of the other changes in the muscular and cardiorespiratory system are likely to have a much greater influence on the improved exercise performance that results from endurance training.
Women have greater capacity for fat oxidation than men due to higher oestrogen levels, which upregulate enzymes and transport proteins involved in fat oxidation. Higher oestrogen levels of the menstrual cycle’s follicular phase might be expected to elicit higher fat oxidation rates. Training performed at different times of the menstrual cycle can potentially cause slightly different metabolic responses to training, however, the longer term effects on training adaptation are yet to be systematically investigated by controlled research studies.
The ingestion of carbohydrate before the commencement of exercise can reduce the rate of fat oxidation during exercise. In contrast, fasting for six hours or more can significantly increase fat oxidation during exercise. Several weeks of a very high fat, low carbohydrate diet upregulates key enzymes in the beta oxidation cycle and results in a significant increase in the ability to oxidise fat during exercise.
As discussed above, the type, intensity and duration of exercise, training status, sex and certain nutrition interventions can have a profound influence on the capacity to oxidise fat during exercise. While it is tempting to assume that increased fat oxidation during exercise will automatically result in improved performance, this is often not observed after interventions designed to increase fat oxidation during exercise. This is possibly because other physiological changes have a more substantial effect on exercise performance. Similarly, it is often assumed that increasing fat oxidation during exercise will automatically result in more desirable changes in body composition over time. However, energy balance over a long period of time is likely to have a much more profound effect on body fat loss than the rate of fat oxidation during a single exercise session.
High Fat Diets
Interest in the benefit of high fat diets for endurance athletes has experienced a resurgence in recent years after these diets were previously discounted by many sports nutrition researchers and practitioners. Early studies on high fat diets may have been limited by the duration of the dietary intervention, low participant numbers, the degree of carbohydrate restriction achieved during these studies and/or the duration of the exercise performance test used to assess the benefits of the intervention. One of the key limitations of research in this area is the lack of a universal definition of a ‘high fat diet’. This makes it very difficult to compare the results of different studies and also causes confusion when the results of studies are translated into practice. These diets also receive a lot of coverage in the popular media, where the format of communication makes it difficult to carefully debate and discuss the nuances of individual studies to provide guidance for athletes and exercising individuals wanting to use this dietary approach to improve performance. Clear definitions of non-ketogenic low carbohydrate, high fat diets (NK-LCHF) and ketogenic low carbohydrate, high fat diets (K-LCHF) have recently been published. These definitions are described below and provide a framework for future studies and communication about high fat diets and exercise performance.
Non-ketogenic low carbohydrate, high fat (NK-LCHF) dietary manipulations have been implemented in athletic and recreational populations alike in pursuit of bioenergetic adaptations and ultimately performance improvements. Carbohydrate intake is kept below known muscular requirements (<2.5g/kg/d) but sufficient to avoid a state of sustained ketosis, typically providing approximately 15-20% of daily energy intake. Protein provides a similar contribution to energy intake while fat provides 60-65% of total daily energy intake. Evidence of adaptation exists after just 5 days, although consistent performance enhancements have not been seen despite increases in rates of fat oxidation during exercise of up to 100%. The key adaptations being targeted via this method are increased availability of intramuscular triglycerides and increased capacity for lipid breakdown, transport and oxidation, including upregulation of enzymes associated with fat metabolism. Dietary manipulation to achieve a 50-70% contribution to total energy intake from fat can stimulate a 50-80% increase in intramuscular triglyceride levels. This effect is similarly reversed with drastic reduction in energy intake from dietary fat. While it is recognised that a high fat diet increases intramuscular lipid content and reduces muscle glycogen utilisation and total carbohydrate oxidation, it is thought that glucose uptake remains unaffected. Highly trained athletes achieve metabolic adaption to high fat diets in as little as 5 days, and interestingly, have been able to retain the glycogen-sparing effect even after acute restoration of glycogen stores. These findings provided some of the earliest evidence that a periodised approach to carbohydrate and fat intake could potentially take advantage of both systems.
In comparison with NK-LCHF, ketogenic low carbohydrate, high fat (K-LCHF) diets have a similar protein intake (15-20% energy intake), however fat intake increases to 75-80% and carbohydrate is held below 5% to maintain a state of ketosis. While evidence of adaptation occurs after 5 days, it can be 2-3 weeks before the associated fatigue and lethargy dissipate. Keto-adaptation is when the body is adapted to use predominantly fat for activities requiring energy and is also able to tolerate relatively low levels of blood glucose due to the availability of ketones for the central nervous system. This state of keto-adaptation facilitates very high rates of fat oxidation during exercise which can be an advantage to performance in events lasting several hours such as ironman triathlons. Athletes in these events that are not in a state of keto-adaptation need to rely more on carbohydrate for energy which may be unavailable in the latter stages due to muscle glycogen depletion and/or an inability to tolerate very high intakes of carbohydrate in foods and beverages during exercise. Keto-adaption essentially provides athletes with an internal motor with a far greater capacity to produce energy for very long periods than their non-keto-adapted counterparts.
Despite the plausible physiological reason for why K-LCHF diets might enhance endurance performance, there have been very few studies conducted that investigate the effects of these diets in elite athletes. Burke et al (2017) investigated the impact of a K-LCHF diet compared with two high carbohydrate diets (a high carbohydrate availability (HCHO) diet and a periodised carbohydrate (PCHO) diet) on performance of elite race walkers during an intense 3-week training period. Participants all consumed the same amount of energy relative to their body weight and protein intake was also controlled. The K-LCHF group experienced greater perception of effort throughout the study to the extent that some participants weren’t able to complete some training sessions – this was not experienced among HCHO and PCHO participants. The K-LCHF diet group significantly increased their capacity to use fat during exercise, but also had greater oxygen use at a set exercise intensity than the other groups. This reduced economy of exercise was attributed to the fact that there is slightly less energy produced per litre of oxygen when fat is used compared with carbohydrate. The HCHO and PCHO groups had significantly faster race times following the three-week diet and training intervention, but this did not occur in the K-LCHF group. The study provides evidence that K-LCHF diets can significantly change substrate metabolism during exercise, but this does not translate to a performance benefit in elite endurance athletes. However, the study was conducted during a short-term, intense training period and the findings may only be applicable to that context. Clearly, more research is required in this area.
One area in which K-LCHF may well have benefit for athletes is in ultra-endurance events that last several hours such as Ironman triathlons, particularly in recreational, rather than elite, athletes. Recreational athletes competing in ultra-endurance events are likely to be exercising for a much longer time during competition due to a lower capacity for high intensity endurance exercise. Therefore, increasing the capacity to oxidise fat during exercise may result in these athletes being able to meet the energy demands of exercise almost exclusively with fat. Consequently a K-LCHF diet might be more beneficial for recreational athletes due to the lower intensity and longer duration of exercise. It has recently been suggested that nutrition guidelines for ultra-endurance event such as an Ironman triathlon should be different for athletes performing at different levels. Elite athletes are likely to benefit from a periodised carbohydrate intake during the training season to increase capacity for fat oxidation, but have a high intake of carbohydrate before and during competition to delay muscle glycogen depletion and maximise exogenous carbohydrate oxidation to meet high energy demands during the event. Whereas, recreational athletes may be better suited to adapting to a K-LCHF diet and developing the capacity for fat oxidation to meet the energy demands while avoiding potential risks of gastrointestinal discomfort associated with large intakes of carbohydrate before and during the event.
Knowledge Check – Fat
Protein is an important nutrient that plays a role in tissue growth, repair and regeneration as well being involved in cell signalling, transport, enzyme action, and the function of the endocrine and immune systems. Protein is often considered in the context of strength and power-based sports because high levels of muscle mass can often predict success in these sports. However, the protein needs of endurance athletes are also increased compared with sedentary individuals.
Protein for Endurance Athletes
Protein oxidation is minimal during very low intensity exercise, but it can contribute up to 5-10% of the total energy produced during higher intensity, prolonged endurance exercise. Protein oxidation is increased further when glycogen stores become depleted. Given that the energy expenditure during training for some endurance athletes could easily be 6000-7000kJ each day, this means that approximately 40g of protein could be oxidised during training. The extra protein oxidised during training will increase the need for dietary protein in endurance athletes.
Although endurance training doesn’t provide the same stimulus for increased muscle protein synthesis and subsequent increases in lean body mass over time like there is with resistance training, there are still a number of adaptations to endurance training that rely on increased protein synthesis. For example, plasma volume is expanded after endurance training and greater plasma proteins must be synthesised to facilitate this expansion. Endurance training also causes an increase in the number of red blood cells and extra protein is required for the synthesis of these new cells.
Athletes undertaking endurance training clearly require more dietary protein than sedentary individuals because of an increased oxidation of protein during exercise, physiological adaptation to training and also for repair and regeneration of tissues structures after exercise. Daily protein recommendations for endurance athletes range from 1.2-1.7g/kg body mass and this should ideally be evenly distributed throughout the day and include include a dose of 0.3g/kg body mass consumed immediately after training to ensure amino acids are available for tissue repair and resynthesis.
Protein consumption during prolonged endurance exercise has the potential to enhance performance by providing additional substrate for energy production. Although carbohydrate is often considered to be the preferred energy substrate for high intensity endurance exercise, adding protein to a carbohydrate-based beverage has been shown in some studies to enhance exercise time to exhaustion. However, the effects of a beverage containing both protein and carbohydrate during exercise appear to be similar to the effects seen when a carbohydrate-only beverage is consumed if the carbohydrate-only beverage has a similar total energy content. Designing studies in this area can be difficult because it is challenging to differentiate between the effects of adding protein and the effects of consuming a beverage with a higher total energy content. Despite the promise of early studies in this area, it is generally accepted now that protein ingestion during prolonged endurance exercise is unlikely to have significant benefits for performance unless access to carbohydrate is restricted.
Protein consumption during endurance exercise can have a positive effect on some biochemical markers of muscle damage after intense and prolonged endurance exercise. Therefore, protein consumption during endurance exercise may allow endurance athletes to recover better before their next training session, even if performance during the session is not affected. This is potentially a nutrition strategy that can be used during intensive training periods.
Protein consumption is important for endurance athletes after exercise to make amino acids available for tissue repair and regeneration processes. The guidelines for protein consumption after exercise are quite similar for endurance athletes as well as strength/power athletes. However, endurance athletes use these amino acids for tissue repair and regeneration rather than protein synthesis that results in a significant increase in muscle mass, which would be more likely to occur when protein is consumed after resistance training. Around 0.3g/kg of protein that is quickly digested and contains all essential amino acids is recommended for endurance athletes as soon as possible after exercise.
Protein for Strength and Power Athletes
Protein is particularly important for strength and power athletes who require high muscle force production for optimal performance. Muscle protein synthesis is the process in which new contractile elements of the muscle are made, therefore enhancing the muscle’s ability to generate force. Resistance training provides the stimulus for increasing muscle protein synthesis and this response can be increased when protein is consumed after exercise. Maximising muscle protein synthesis after resistance training is therefore important for driving training adaptations.
Dietary protein is derived from both animal and plant-based sources. Proteins originating from animal sources such as meat, milk and eggs are generally classified as high quality than protein from plant-based foods. High quality proteins are better to consume after exercise because they can deliver the all amino acids in the best possible ratio for incorporation into body proteins. However, specific combinations of different plant-based protein sources may also be able to deliver all amino acids.
There are some studies that have specifically compared the effects of different protein sources on muscle protein synthesis after resistance exercise. Plant-based proteins generally result in lower muscle protein synthesis after exercise compared with proteins from animal foods. The rate of protein digestion and amino acid absorption into the blood can also influence muscle protein synthesis after exercise. Whey and casein proteins are both found in milk, however their digestion and absorption rates and leucine content are quite different. Leucine is an important amino acid because it can provide a stimulatory signal for protein synthesis. Ingestion of the more quickly digested and leucine-rich whey protein provides a rapid and greater rise in plasma amino acids, but over a shorter time period. The more slowly digested casein with its lower leucine content, provides a more prolonged post-prandial elevation in plasma amino acid concentration. Understanding these characteristics of the different proteins allows for the strategic use of whey protein in the acute post-exercise period and casein-based proteins overnight, when there is a longer period before the next meal to capitalise on the slower release of amino acids.
Many studies investigating muscle protein synthesis after resistance exercise often compare isolated proteins rather than whole foods with different protein composition. Therefore, translating the findings of these studies into guidelines for food choices after exercise can be difficult. Dietary protein is typically consumed as mixed meals rather than isolated food components or dietary supplements. Other nutrients in the meal have the potential to influence the rate of digestion and absorption of protein in addition to the rate of amino acid uptake by the muscle. The appearance of amino acids in the blood is generally slower after the consumption of a mixed meal compared with the consumption of isolated proteins, however, this will vary significantly depending on the constituents of the meal. In fact, some compounds in the whole food matrix may actually augment protein synthesis through signalling pathways or more direct nutrient interactions. The figure below shows how components of the whole food matrix can potentially have synergistic effects on muscle protein synthesis.
Mixed meals can contain multiple protein sources with different amino acid profiles, carbohydrates, fibre, fat and many different micronutrients. Each of these components can impact on recovery and adaptation to exercise and should also be considered when providing guidelines about the most appropriate food choices after exercise. Nevertheless, it is generally accepted that protein sources that are rapidly digested and absorbed, contain all of the essential amino acids and are high in leucine will be the most effective at stimulating muscle protein synthesis after exercise.
The amount of protein required to maximally stimulate muscle protein synthesis after exercise is approximately 0.3g/kg body mass which is about 20-40g for most athletes. Muscle protein synthesis is thought to be maximally stimulated when a threshold amount of leucine is present in the blood. This is achieved with consumption of approximately 1-3g of leucine. Protein synthesis is elevated for 24 hours after resistance exercise, so it is important to consume additional protein every 3-4 hours in this period. Studies have shown that smaller amounts consumed more frequently, as well as larger amounts consumed less frequently, are not as effective at stimulating muscle protein synthesis. The amount of essential amino acids in the protein source consumed after exercise also has a significant impact on muscle protein synthesis. Approximately 12-15g of essential amino acids appears to maximally stimulate muscle protein synthesis after exercise.
Increasing daily protein intake above the recommendations for the general population can result in increased muscle size, strength and total lean body mass when combined with resistance training. A total daily protein intake of 1.5-2.0g/kg body mass along with a positive energy balance appears to be sufficient for most people to achieve increased lean body mass and augment strength gains from resistance training. However, this is likely to vary between individuals depending on the protein sources in the diet and the timing and distribution of protein intake throughout the day.
A meta-analysis of 23 studies including 525 participants has found that the total amount of protein consumed each day had the most effect on gains in muscle size and strength after resistance training rather than the amount or type of protein consumed after exercise. This is important because a lot of attention is given to protein intake in the short window immediately after exercise, but protein synthesis and breakdown is occurring throughout the day and it is important to have a regular supply of amino acids available to ensure the best opportunity for overall gains in strength and lean body mass. Very high intakes of protein, perhaps even higher than 3.0g/kg body mass per day may be warranted when athletes are trying to lose body fat with restricted energy intake, but also want to maintain or even increase lean body mass at the same time. These high protein intakes, that are three or four times what is recommended for the general population, don’t appear to cause any adverse effects in young, fit and healthy individuals.
Knowledge Check – Protein