Nutrition and Bioenergetics. The Integration of Nutrition, Metabolism and Performance

Dr. Iñigo San Millán, PhD, University of Colorado School of Medicine

Nutrition is a crucial field in any athlete’s performance as without a proper nutrition it is impossible to obtain good performances. Every athlete is born with a genetic makeup and a defined physiology. The main goal of training is to alter and improve the physiology an athlete is born with. However it doesn’t matter how well we train or how good a coach a cyclist works with is, we will never be able to improve that physiology correctly and therefore improve performance if we don’t eat properly.
I have personally seen many situations where a talented athlete is leaving the exercise physiology laboratory with defined goals, training zones and information to improve performance and everything is ruined by a wrong nutrition. Unfortunately there is too much misleading information on nutrition, magic diets, urban legends and myths that seem to keep hunting not only cyclists but their coaches year after year.
By being part of the academics I am a firm believer of the importance of education in order to have a better understanding about nutrition and its interactions with physiological and metabolic responses and the impact on performance. It is the intention of this article to present a scientific review of the basics of bioenergetics, carbohydrate, fat and protein metabolism and the integration of nutrition, metabolism, physiology and performance.

BASIC EXERCISE BIOENERGETICS - Where do we get the energy and why nutrition is so important?

The capacity of an athlete to exercise ultimately depends on the ability to transform chemical energy into mechanical energy.  For this, skeletal muscle needs to synthesize Adenosine-5’-Triphosphate (ATP) for muscle contraction. ATP is responsible for all energy processes in human cells ATP needs to be synthesized constantly during exercise as it is indispensable for muscle contraction. ATP generation is achieved by two mechanisms: anaerobic and aerobic metabolism as well as through fats and carbohydrates (CHO) mainly, with some contribution from protein. Fats and CHO are stored in skeletal muscle and in the case of fat it is stored primarily in the adipose tissue but also in skeletal muscle and in the liver. Each energy system and substrate will be activated depending on the metabolic and physiological stress, contractile necessities for ATP generation of the muscle (exercise intensity) and the fiber recruitment pattern. A large majority and range of exercise intensities can generate ATP through Fat and CHO generate ATP through aerobic metabolism, also called oxidative phosphorilation. Depending on the level of fitness of an individual fat can sustain the biggest part of ATP generation aerobically up to 55-75% of VO2max intensity although CHO is also used at small rates during low and moderate exercise intensities. Beyond this point ATP generation needs to be faster in order to keep up with a higher frequency and power in muscle contraction and CHO will become the major energy used by skeletal muscle up to 100% of VO2max. When exercise intensities are very high and maximal or close to maximal and therefore above 100% of VO2max, ATP cannot be generated by the aerobic mechanism so the ATP needs to be generated through the anaerobic mechanism also called substrate phosphorilation.

CARBOHYDRATE - Metabolism during Exercise

Carbohydrate (CHO) metabolism is of great importance during exercise, especially during high exercise intensity where it is the predominant energy subtracts for skeletal muscle. Glycogen is the storage form of glucose and carbohydrates (CHO) in animals and humans. Carbohydrates are a very limited source of energy accounting for only about 1-2% of total bodily energy stores (Goodman, 1988). Furthermore, about 80% of total CHO is stored in skeletal muscle, about 14% is stored in the liver and about 6% in blood in the form of glucose, so this would represent about 300-400g of glycogen stored in muscle and about 70-100g stored in the liver (Sherman, 1995). Glycogen cannot be utilized for energy purposes by muscle so it needs to be broken down to Glucose 1-Phosphate by an enzyme called enzyme phosphorylase. This is the process of called glycogenolysis. The process of glucose breakdown in muscle for fuel utilization is called glycolysis which at rest, accounts for 15-20% of peripheral glucose utilization in skeletal muscle. At an exercise intensity of 55-60% VO2 max, glucose utilization by skeletal muscle increases to about 80-85% of wholebody disposal (Kjaer et al., 1991). Since muscle glycogen is crucial for ATP synthesis during exercise, proper glycogen storages are of great importance for athletic performance. Multiple studies show that glycogen depletion is associated with fatigue and decrease in performance and that athletes who have low carbohydrate diets or low glycogen storages will decrease exercise capacity ( Coyle et al., 1983; 1986; Coggan & Coyle, 1991; Sahlin et al., 1990; Maughan et al., 1997; McConell et al., 1999) as well as an increase risk for overtraining (Sherman & Wimer, 1991; Sherman, 1995; Snyder et al., 1995).

Glycolysis occurs mainly in the cytosol and this process can be aerobic through the complete oxidation of Pyruvate (oxidative phosphorilation in the Mitochondria) or anaerobic (substrate phosphorilation in the cytosol). Exercise intensity determines the substrate demands of skeletal muscle to generate ATP. During exercise skeletal muscles use primarily Fat and CHO for energy purposes and at low exercise intensities fat is the preferred substrate although there is always some glucose oxidation. At higher exercise intensities of about 50-60% of VO2max, ATP synthesis demand increases and fat cannot entirely meet the rate of ATP synthesis so glucose oxidation increases. Although the oxidation of fat yields a much higher amount of ATP, glucose utilization is much faster and therefore necessary for ATP synthesis during higher exercise intensities.

Exercise intensity is the main regulator for skeletal muscle CHO utilization and the mechanisms responsible for CHO utilization during exercise involve hormonal and local factors as well as glycogen availability. Epinephrine (Adrenaline) is the main hormone involved in CHO metabolism during exercise. B-adrenergic activity increases with exercise intensity and Phosphorilase is the enzyme responsible for glycogen breakdown to glucose and it is regulated by epinephrine. The release of epinephrine from the adrenal medulla is directly proportional to exercise intensity. Epinephrine stimulates muscle glycogenolysis by increasing phosphorilase activity thus it is a major regulator of CHO metabolism during exercise. Availability of free fatty acids (FFA) during exercise is also closely regulated by epinephrine. During high exercise intensities epinephrine reduces the blood flow to adipose tissue eliciting a constricting effect on adipose tissue therefore reducing plasma FFA availability to the muscles during high intensity exercise (Romijn et al., 1993; Roberts et al., 1996). Muscle fiber composition and activation patterns play an important role in substrate utilization. During high exercise intensities, contraction time is lower, shortening velocity is higher and power production is higher as well so Type II muscle fibers are recruited at these exercise intensities (Gollnick et al., 1974). Since Type II muscle fibers have a higher glycogenolyic capacity and lower mitochondrial density, glucose utilization in these fibers will prevail over fat.

Exercise duration also plays an important role in CHO metabolism during exercise. Since glycogen storage capacity is about 500g in muscle and liver the length of the exercise activity will be very important for the regulation of CHO metabolism. Glucose uptake in skeletal muscle is dependent on glycogen content (Hargreaves et al., 1992) and hypoglycemia during exercise can be prevented by the sufficient intake of CHO (Coggan & Coyle, 1991). Exercise duration is intimately related to glycogen storages as low glycogen storages during endurance events are associated with hypoglycemia, fatigue and decrease of performance (Hermansen et al., 1967; Coggan & Coyle, 1987; Coyle et al., 1983; 1986; Sahlin et al., 1990; Maughan et al., 1997; McConell et al., 1999).


Lipids are a very important energy source for endurance exercise. Although ATP generation for muscle contraction from lipids is slower than carbohydrates, the amount of ATP produced by lipids is far higher than that from CHO which makes lipids the fuel of choice by skeletal muscle during endurance exercise as well as it will have a glycogen sparing effect. The main source for lipid metabolism is subcutaneous adipose tissue. Even the leanest athletes have more than 100,000 Kcal of potential energy in their adipose tissue. Lipid metabolism during exercise is a highly coordinated and integrated process starting at the adipose tissue and ending at the mitochondria in skeletal muscle. This process involves mobilization or breakdown of adipose tissue, circulation from adipose tissue to skeletal muscle, uptake and final mitochondrial oxidation in skeletal muscle. Lipolysis is the first step in lipid metabolism and it is the breakdown of adipose tissue as well as intramuscular triglyceride. Triglycerides in the adipose tissue and muscle are broken down into free fatty acids (FFA) and glycerol by hormone sensitive lipase (HSL). Hormonal control of lypolysis is tightly regulated by several hormones, especially catecholamines (Epinephrine and Norepinephrime) which are probably the major hormones regulating lipolysis. Catecholamines bind to both β-adrenergic and α2- adrenergic receptors on the fat cell (adipocyte) membrane.
This activates a cascade of cellular signals which starts by the activation of adenylate cyclase (AC) which increases cyclic adenosine monophosphate (cAMP) which activates cAMP-dependent protein kinase which ultimately phosphorilates HSL which finally elicits lipolysis. At rest low level of plasma catecholamines bind to α2 receptors eliciting an inhibitory effect on lypolysis while during exercise plasma catecholamines increase and stimulate β-adrenergic receptors stimulating lypolysis (Arner et al., 1990). However, during very high exercise intensities, catecholamines have an inhibitory effect on lypolysis probably by causing a constriction in capillarization and blood flow to adipose tissue (Roberts et al., 1996; Romijn et al., 1993) and eliciting a potent glycogenolytic effect. Although it seems to be different catecholamine thresholds for both lipolysis and glycogenolysis (Galster et al., 1981) the exact mechanisms and catecholamine concentrations to elicit either lipolytic or glycogenolytic effects is not entirely understood.

Insulin also regulates lipolysis although its effects during exercise are not as profound as when at rest or as powerful as catecholamines during exercise. At rest insulin inhibits lipolysis (Jensen et al., 1989; Campbell et al., 1992; Galbo, 1992) but during exercise, insulin secretion decreases allowing higher a lipolytic activity (Wasserman et al., 1989).

Upon adipose tissue lipolysis, FFA must be transported to skeletal muscle. . Once inside the muscles FFA are attached to coenzyme A (CoA) which forms fatty acyl-CoA which then is transported across the outer mitochondrial membrane by carnitine palmitoyl transferase I (CPT-I), and finally transported to the mitochondrial matrix by carnitine. Once inside the mitochondrial matrix, fatty acids undergo β-oxidation where fatty acyl-CoA is degraded to Acetyl CoA which can then enter the citric acid cycle.

Skeletal muscle also contains small lipid droplets called intramuscular triglycerides (IMTG) which are stored in the cytoplasm of skeletal muscle cells close to the mitochondria. Depending on different circumstances such as endurance exercise and low glycogen content IMTG can play an important role in the contribution to lipid metabolism during exercise (Gollnick & Saltin, 1988) which depending on the exercise duration and glycogen availability can contribute to a great extent to lipid metabolism during exercise.


Although not considered a major contributor to energy during exercise, the metabolism of proteins during exercise can be important, especially depending on the exercise intensity, type, duration and nutritional status of the athlete. Proteins are made up of amino acids and there are over 20 amino acids and are divided in two groups: Non-essential, which are those that can be synthesized in the body and essential, those ones that need to be obtained from the diet. Amino acid metabolism is a sum of very complex and different mechanisms. Amino acid metabolism although accounting for a small percentage of total ATP synthesis during exercise may play an important role in the intermediate metabolism and performance as well as recovering after training/competition.

There are several amino acids that play an active role during exercise activity. Amino acids may provide between 3% to 10% of the total energy during exercise depending on exercise intensity and duration (Felig, 1973; Wahren et al., 1973; White & Brooks, 1983; Philips et al., 1993; Tarnopolsky et al., 1995). Although these percentages may not be very high but may play a very important role in exercise performance especially when glycogen levels are low and in this case the contributions to energy from amino acids will be greater (Lemon & Mullin, 1980). There are several amino acids that are quite important during exercise. Alanine is an important glucogenic amino acid, especially during endurance exercise and it is synthesized in the muscle and then exported to the liver to be converted to glucose through what is called the glucose-alamine cycle (Felig, 1973). Leucine, isoleucine and valine make up the branched-chain amino acids (BCAA) and may also play an important part during exercise. Leucine is a ketogenic amino acid, isoleucine is both ketogenic and glucogenic whereas valine is a glucogenic aminoacid. BCAA seem to be the kind of amino acids most used by the muscle during exercise. Since these aminoacids are building blocks of the muscle, an excessive utilization of aminoacids as what happens during intense and long exercise along with decrease glycogen content, may lead to an excessive muscle breakdown and a catabolic situation for the muscles which causes muscle damage and would be detrimental for performance. Therefore a BCAA supplementation during endurance exercise may have some sparing effects on endogenous muscle BCAA utilization and therefore decrease the possibilities of muscle damage (MacLean et al., 1994).


After discussing general bioenergetics and substrate metabolism we can clearly see that nutrition is a key part of the training regime of any athlete. Ingesting insufficient amounts of calories (Kcalories-Kcal) can result in a lack of important macro and micro nutrients. This is especially true when it comes to carbohydrates. Unfortunately many societies “demonize” CHO and there are multiple books and diets out there claiming that high protein and or high fat diets along with an important CHO restriction are the appropriate way for an athlete to lose weight , have a healthy diet and even improve performance. However, most of these books and diets lack of scientific evidence. This is especially true for athletes who restrict CHO as there is massive amount of scientific evidence that clearly shows that a good CHO diet is crucial to maintain performance. As previously discussed, multiple studies show that fatigue and decrease in performance is associated with low carbohydrate diets causing glycogen depletion (Hermansen et al., 1967; Coyle et al., 1983; 1986; Coggan et al., 1987; Sahlin et al., 1990; Maughan et al., 1997, McConell et al., 1999) and how low glycogen levels may cause overtraining (Sherman, 1995; Sherman & Wimer, 1991; Snyder et al., 1995, Achten et al, 2004). Since glycogen storage capacity is very limited many high performance athletes may find it difficult to even keep up with CHO intake and therefore end up with some patterns of glycogen depletion (Costill et al., 1988, Kirwan et al., 1988; San Millan et al., 2011).

The potential problem many cyclists with a low CHO diet face is that if glycogen levels are low or there is glycogen depletion, muscles increases the utilization of protein and amino acid utilization as a gluconeogenic precursor increases (Tarnopolski et al., 1995; Lemon & Mullin, 1980) and since protein and amino acids are the building blocks of muscle, the latter one may enter a catabolic situation (muscle breakdown) as the muscle may “eat itself to feed itself” by increasing the amount of protein and amino acids used for energy purposes. This situation may lead to muscle damage and furthermore this may lead to chronic overtraining as it has been shown that muscle damage limits and interferes with glycogen storage and synthesis (O´Reilly et al., 1987; Costill et al., 1990) so even with a high CHO diet it would be difficult to maintain glycogen storages and therefore enter a vicious circle which may lead to overtraining and decrease in performance.

Endurance athletes elicit a great deal of physiological stress to their bodies activating so many physiologic and metabolic responses. Both macro and micro nutrients are of great importance for the regulation of these responses and therefore for performance. By having a well balanced diet we will assure that we can supply the body with the necessary macro and micronutrients important for all physiological functions during exercise as well as during recovery. Of all macronutrients carbohydrates are of crucial importance for cyclists due to the high rate of utilization on a daily basis and the very small storage capacity in our body (500g). Our body can handle a dietary deficiency of many macro and micronutrients for a few days, but a deficiency of just 1-2 days of carbohydrates for a competitive cyclist may have a strong negative impact on performance. A competitive cyclist should have a good CHO diet with up to 7-12g/Kg/day of CHO both on long and intense training days as well as on competition days. (Costil et al, 1988; Achten et al, 2004, Halson et al 2004) It is important to have a proper CHO intake throughout the entire day and especially during training and competition.

Regarding the necessary daily protein for an endurance athlete, current research indicates that a daily protein intake of 1.2– 1.4 g/d for endurance athletes should be sufficient (Lemon 2004). high quality protein foods like dairy products, eggs, meat, fish and soy products should be chosen.

As a summary, it is important to understand the metabolic responses to exercise and the different substrate utilization patterns in order to properly integrate nutrition, metabolism and performance in competitive cyclists.

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