Basics of energy metabolism
For energy intake, organic substances must be supplied so that the body can obtain usable energy from them (energy metabolism). Energy suppliers are the macronutrients carbohydrates, fats, and proteins. Alcohol also supplies energy (7 kcal/g). For energy production, the macronutrients are oxidized step by step in the body. Approximately 60% is converted to heat, which is used to maintain body temperature. The remaining energy is stored in the form of adenosine triphosphate (ATP) or provided as an energy source for numerous metabolic processes. Energy is released by cleavage of adenosine triphosphate into adenosine diphosphate (ADP) and free phosphate (P). Since the intracellular ATP supply is very limited, the body makes use of different ways of ATP resynthesis (synthesis = production). ATP resynthesis occurs through anaerobic and aerobic energy production. The human organism requires energy for:
- Synthesis and renewal of endogenous substances.
- Mechanical work, as well as the maintenance of body temperature.
- Chemical and osmotic gradients
Anaerobic energy production includes ATP resynthesis from creatine phosphate and adenosine diphosphate and (anaerobic) glycolysis (breakdown of glucose to ATP and lactate). Aerobic energy production includes oxidation of glucose (aerobic glycolysis), free fatty acids (beta oxidation), and amino acids (in exceptional cases). The breakdown of glucose, free fatty acids, and amino acids produces acetyl-CoA as an intermediate product, from which adenosine triphosphate is formed with the release of carbon dioxide and water (citrate cycle and respiratory chain).
Process energy consumption
The increased energy demand of skeletal muscle caused by physical activity is met in the short term by anaerobic energy production or glucose present in the blood. If more energy is required, glycogen is broken down into glucose and glucose-1-phosphate by glycogenolysis (breakdown of stored carbohydrates) and transported via the blood to the cells requiring energy. At the same time, fatty acids are broken down into glycerol and free fatty acids (FFS) (lipolysis/fat breakdown) and likewise transported via the blood pathway to the energy-demanding cells. The stimulation of lipolysis occurs through the increase of lypolytic hormones (including norepinephrine, cortisol) and through the decrease of antilypolytic insulin (a lowered insulin blood level leads to the breakdown of fat from the fat cells). During intensive muscle work or when the glycogen depots are largely empty, gluconeogenesis produces more glucose from non-carbohydrate precursors (amino acids, glycerol or lactate) and provides it as an energy source. Due to the complex biochemical process of energy production via oxidation, the aerobic metabolic processes run slowly and form less ATP per unit of time than the anaerobic processes. At rest, 80% fatty acids and 20% glucose are oxidized. At light load intensity, it is 70 % fatty acids and 30 % glucose. At heavier exercise intensity, the oxidation ratio is about 50%: 50%.
Energy content of nutrients
The physiological calorific value of foods corresponds to their energy content when metabolized (cellular respiration) in the body and is sometimes less than the calorific value when completely burned in a flame (physical calorific value). The calorie (cal) is used as the unit of measurement. 1 g fat = 9 kcal1 g carbohydrates = 4 kcal1 g protein = 4 kcal
Note1 g alcohol = 7 kcal
Energy requirements
The body’s energy requirement is composed of basal metabolic rate, food-induced thermogenesis, and physical activity. Basal metabolic rate describes the energy consumption at complete physical rest to maintain the body’s function. It is essentially determined by age, sex, body cell mass (muscle and organ mass), genetic prerequisites, state of health (fever) and by heat insulation through clothing or ambient temperature.Women have a lower basal metabolic rate (approx. 200 kcal less) than men. Muscle mass is the main determinant of basal metabolic rate. Basal metabolic rate accounts for 55-70% of total energy expenditure. Thermogenesis corresponds to the energy expenditure required for food intake as well as utilization – digestion, absorption, transport, breakdown and remodeling processes.The amount of thermogenesis depends on the composition and quantity of the ingested food: 2-4% of the energy ingested with fats, 4-7% of the energy ingested with carbohydrates, 18-25% of the energy ingested with proteins. Thus, food-induced thermogenesis lasts approximately twice as long after a protein-rich meal as after a carbohydrate- or fat-rich meal of the same energy content.Furthermore, thermogenesis also describes energy consumption due to exposure to cold and heat, muscle work, psychological stimuli (stress, anxiety), hormones, and drugs.Thermogenesis is independent of gender and age. Thermogenesis accounts for approximately 10% of total energy expenditure. Basal metabolic rate and thermogenesis can be influenced only slightly. Physical activity is divided into intentional and spontaneous activity. Intentional activity is activity that is undertaken consciously (e.g., occupational work, sports). Spontaneous activity is e.g. spontaneous muscle contractions, fidgeting, body tension while sitting. Spontaneous activity is largely genetically determined and can consume between 100 and 800 kcal/day. The proportion of physical activity in total energy expenditure is highly variable and can be 15-35%. In individuals with low levels of physical activity in occupation and leisure, the proportion of total energy consumption is 15-25%. Energy expenditure can be measured by direct calorimetry (heat output measurement), indirect calorimetry (gas exchange measurement), double-labeled water (gold standard), or approximated by biometric data (body cell mass = muscle and organ mass). Basal metabolic rate measurement must be performed under consistent, standardized conditions: Early morning after sufficient night’s rest; more than 12 hours after the last food intake; lying down, without physical movement, but awake; in a healthy condition; naked at 27-29 °C, room temperature or lightly clothed at 23-15 °C. If the measurement takes place under less standardized conditions – but without physical exercise and after a longer period of abstinence from food – it is called resting energy expenditure (REE). Today, the resting energy metabolic rate replaces the so-called basal metabolic rate, since the measurement conditions prescribed for the basal metabolic rate cannot be observed in practice. Calculation of resting energy expenditure (REE) according to WHO:
REE in men = 10 × weight[kg] + 6.25 × height[cm] – 5 × age[years] + 5
REE in women = 10 × weight [kg] + 6.25 × height [cm] – 5 × age [years] – 161
Calculation of resting energy expenditure (REE) according to Harris and Benedict:
REE in men [kcal/day] = 66.473 + (13.752 × body weight[kg]) + (5.003 × height[cm]) – (6.755 × age[years])
REE in women [kcal/day] = 655.096 + (9.563 × body weight [kg]) + (1.850 × height [cm]) – (4.676 × age [years])
Calculation of resting energy expenditure (REE) according to Müller et al:
REE = 0.05192 × fat-free mass [kg] + 0.04036 × fat mass [kg] + 0.89 × sex (W=0, M=1) – 0.01181 × age [years].
Fat-free mass and fat mass can be measured by electrical impedance analysis (BIA). The use of the formula according to Müller is recommended because it is based on current data of the German population. The standard error (sampling error) of the mean (SEM) of the formula is 0.70 and the coefficient of determination (R²) is 0.71. Physical activity can be represented by the metrics Metabolic Equivalent (MET) or Physical Activity Level (PAL) to calculate power and/or total energy expenditure. MET: 1 MET corresponds to resting energy expenditure of 3.5 ml O2/kg body weight/minute.PAL: 1 PAL corresponds to resting energy expenditure. The calculation is based on an activity or exercise protocol.PAL values
| Sleep | 0,95 | |
| Sitting activity | 1.2 to 1.3 | Frail person |
| Sitting activity with small walking distances | 1.4 to 1.5 | Office worker |
| Standing activity | 1.6 to 1.7 | Assembly line worker |
| Predominantly walking activity | 1.8 to 1.9 | Waiter, salesman, craftsman |
| Physically strenuous activity | 2.0 to 2.4 | Construction workers, farmers |
ExampleMan, 45 years, 90 kg, 185 cm, 8 h office work (1.4 PAL), 8 h leisure (1.4 PAL), 8 h sleep (0.95 PAL).
Resting energy expenditure = 66.47 + (13.7 × 90 kg) + (5 × 185 cm) – (6.8 × 45 years) = 1,918.47 kcal/day
Power consumption = (8 × 1.4 PAL) + (8 × 1.4 PAL) + (8 × 0.95 PAL) / 24 = 1.25 PAL
Total energy consumption = 1,918.47 kcal/day × 1.25 PAL = 2,398.08 kcal/day
Excess intake
Energy supplied to the body in excess of consumption is stored as depot fat. Thus, excessive energy intake (positive energy balance) is the main cause of the development of overweight or obesity with its secondary diseases.
Deficiency
In the case of an energy deficiency (negative energy balance), the body falls back on its own energy reserves. These are first the glycogen stores, which are exhausted after 1-2 days of low-carbohydrate diet. Subsequently, the depot fat – then the muscle protein – is broken down for energy.A negative energy balance is the prerequisite to reduce increased body weight.
Intake recommendations
Energy requirements are influenced by numerous factors. During pregnancy, infants, children, and adolescents require additional energy for growth. During lactation, additional energy is needed for milk production.Dietary energy requirements are given as a guideline by the German Nutrition Society (DGE).
| Age | Guideline values for energy intake in kcal/day | |||||
| m | w | |||||
| Infants | ||||||
| 0 to under 4 months | 550 | 500 | ||||
| 4 to under 12 months | 700 | 600 | ||||
| PAL value 1.4 | PAL value 1.6 | PAL value 1.8 | ||||
| m | w | m | w | m | w | |
| Children and teenagers | ||||||
| 1 to under 4 years | 1.200 | 1.100 | 1.300 | 1.200 | – – | – – |
| 4 to under 7 years | 1.400 | 1.300 | 1.600 | 1.500 | 1.800 | 1.700 |
| 7 to under 10 years | 1.700 | 1.500 | 1.900 | 1.800 | 2.100 | 2.000 |
| 10 to under 13 years | 1.900 | 1.700 | 2.200 | 2.000 | 2.400 | 2.200 |
| 13 to under 15 years | 2.300 | 1.900 | 2.600 | 2.200 | 2.900 | 2.500 |
| 15 to under 19 years | 2.600 | 2.000 | 3.000 | 2.300 | 3.400 | 2.600 |
| Adults | ||||||
| 19 to under 25 years | 2.400 | 1.900 | 2.800 | 2.200 | 3.100 | 2.500 |
| 25 to under 51 years | 2.300 | 1.800 | 2.700 | 2.100 | 3.000 | 2.400 |
| 51 to under 65 years | 2.200 | 1.700 | 2.500 | 2.000 | 2.800 | 2.200 |
| 65 years and older | 2.100 | 1.700 | 2.500 | 1.900 | 2.800 | 2.100 |
The figures refer to normal-weight individuals. Individual adjustments are necessary for deviations from the normal range, such as overweight. Pregnant and breastfeeding women are recommended to take additional energy.Guideline values for the additional energy intake for pregnant women:
The following information applies only to normal weight before pregnancy, desirable weight development during pregnancy (body weight gain of 12 kg by the end of pregnancy), and undiminished physical activity:
- 2nd trimester (third trimester of pregnancy): + 250 kcal / day.
- 3rd trimester: + 500 kcal / day.
Guideline for additional energy intake for breastfeeding women:
- If exclusively breastfeeding during the first 4-6 months: + 500 kcal / day.
Energy metabolism in competitive sports
During athletic activity, energy is consumed in the muscles, which must be returned to the body in the form of food calories. A working muscle has a circa 300-fold higher energy turnover compared to the resting state. Athletically active people therefore have a higher energy requirement. Irrespective of this, however, it is not only important to cover the energy requirements of the muscles, but also to maintain a balanced diet. During competitive sports, not only glucose and fatty acids are burned, but also vital substances such as vitamins and trace elements. It also requires a sufficient supply of all energy carriers, i.e. carbohydrates, fats and proteins. If the supply of the three energy carriers is imbalanced, this inevitably leads to a reduction in performance. If one compares the energy requirements of a competitive athlete with those of an untrained person, a significant increase in the energy requirements of the athlete can be observed. In order to compensate for the additional demand caused by stress and to be able to achieve top athletic performance, the athlete’s diet should be appropriate to the type of sport, varied and consist of a wholesome mixed diet. Carbohydrate requirements in competitive sports
- Looking at the metabolism of carbohydrates in the human organism, it is noticeable that especially the simple sugar glucose and the storage form of glucose, glycogen, are important for the immediate provision of energy. In addition to the brain, the muscles represent an organ system that is continuously dependent on the supply of carbohydrates.
- Depending on the level of training of the athlete, different amounts of glucose can be stored in the body and released when needed. The more optimized the endurance state of the athlete, the more glucose can be stored. A total of about 500 g of glucose can be stored, which is equivalent to 2000 kcal. The largest and most important storage for glucose in the human organism is the liver.
- However, before the liver is stimulated to release the glucose, the consumption of glycogen reserves in the muscle.
- Depending on the type of sport, the need and the provision time of energy-containing carbohydrates differ. In endurance sports, a permanent and constant supply of glucose is often required. Since a state of oxygen presence is present during endurance training, aerobic energy production mechanisms can be used. However, if a sudden high load is required by the organism, aerobic energy production is not an alternative because it is too sluggish. Instead, the body resorts to anaerobic energy production. Depending on the load intensity, the anaerobic alactacide or anaerobic lactacide energy production predominates.
- Comparing the energy production mechanisms, it is clear that the advantage of anaerobic energy provision is the rapid metabolism of glucose, but as a disadvantage it can be seen that the absolute energy release is to be classified as much lower.
- Carbohydrates play an important role in sports nutrition, as they represent the energy carrier for the muscles, brain and erythrocytes.
- One gram of carbohydrate provides 4 calories and per liter of oxygen about 9% more energy than fat. Insufficient carbohydrate intake reduces concentration and can cause nausea and vertigo (dizziness).
Energy supply in the muscles under load.
- The only compound that the organism can directly apply for energy production is ATP (adenosine triphosphate). However, due to the low concentration in the muscle, this is only enough for a few muscle twitches and is not sufficient for athletic loads.To meet the energy demand, the muscle helps itself by providing creatine phosphate, through which the muscle can be supplied for about 15 seconds.
- Important for understanding the energy supply of the muscle is the realization that no energy supply mechanism runs on its own, but rather all run side by side and simultaneously. Moreover, it is important to note that exercise intensity and duration are the most important variables used to determine which system of energy production dominates.
- Oxidative energy production is particularly important in physical exertion lasting approximately two to eight minutes. Examples include judo, boxing and middle-distance running.
- If the load lasts longer, up to 45 minutes, predominantly aerobic energy production mechanisms are required. If the load duration is even higher, fatty acids are additionally metabolized in large quantities.
- As a consequence for the athlete results in the need for an adequate carbohydrate-containing basic nutrition with additional carbohydrate supply during endurance loads. In addition, after an exertion should be carried out as quickly as possible to replenish the stores.
Fat requirement in competitive sports
- Fat intake should not exceed 30%. Fats are carriers of fat-soluble vitamins – vitamins A, E, D, K – which are absorbed only in combination with fat.
- Furthermore, fats are important for heat insulation (subcutaneous fat tissue). With 9.3 kcal in a gram of fat, they represent a concentrated source of energy and are therefore considered a long-term fuel of the muscles. Fat storage, unlike other energy storage, is almost unlimited. However, too much fat unfavorably affects carbohydrate metabolism and puts a strain on the metabolism, as it remains in the stomach for a longer period of time.
- Furthermore, too much fat in the diet reduces performance, especially in endurance sports. Accordingly, from a nutritional-medical and performance-physiological point of view, care should be taken not to consume too high amounts of fat in the athlete’s diet and preferably consume vegetable fats. Vegetable fats such as olive oil, sunflower and peanut oil are carriers of essential fatty acids, which have a positive effect on serum cholesterol levels.
- At rest and during long periods of medium-intensity exercise, the muscle cell obtains its energy primarily from fat burning. However, if the load intensity increases, carbohydrates are increasingly used to provide energy. A trained body can therefore be recognized by the fact that it can still rely on fat-consuming metabolic mechanisms despite an increase in performance.
Protein requirements in competitive sports
- Proteins are very important in the diet of athletes, as they are needed to build muscle, hormones, immune proteins and the formation of enzymes that regulate metabolism. Proteins should occupy a share of 10-20% in the diet. There are no specific stores, as with carbohydrates or fats. Rather, muscles and liver, but also protein components of the blood are protein carriers.
- Protein contributes only to a very small extent to the provision of energy. However, with insufficient carbohydrate intake or empty stores as a result of high as well as long load intensities, the protein reserves are needed to provide energy. If sporting activities last particularly long, between 5 and 15 % protein can be burned in the form of amino acids. The amino acids valine, leucine and isoleucine in particular are used for energy production. Hormonal changes in the body also contribute to the increased consumption of amino acids.
- The body is able to convert proteins into carbohydrates. If too small amounts of carbohydrates are consumed through the diet, it comes to the increased conversion of endogenous proteins into carbohydrates (gluconeogenesis of glucose from glucoplastic amino acids). However, protein deficiencies can develop as a result. Protein deficiencies reduce physical performance and diminish immune response. Protein losses occur just as increased when, in addition to high muscle stress, too little protein is supplied through the diet.
- Training causes catabolic processes in the body, making a constant supply of essential amino acids is important.The amino acids valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan and lysine can not be formed by the body, which makes the supply through food urgently necessary.
- Suitable sources of protein are low-fat dairy products, lean meat, fish, as well as legumes. Animal protein is in contrast to vegetable protein of higher quality and covers the protein needs of the human body better. The different biological value is due to the different amounts of essential amino acids contained. However, it is not necessary to do without vegetable protein. The essential amino acids of animal and plant foods can be supplemented in such a way that an equally high biological value can be achieved. Favorable combinations are potatoes with egg or dairy products and cereals with egg, dairy products or legumes.
- For intensive muscle building, no more than 0.2-0.3 grams of protein per kilogram of body weight is additionally necessary. However, muscle building can not be increased by excessive protein intake in the diet. Too much protein can promote the occurrence of metabolic diseases such as hyperuricemia (gout). Excessive protein intake puts considerable strain on the kidneys due to increased excretion of urea. Kidney damage can be the result.
Within the individual sports phases such as endurance loads, strength endurance sports, fast strength and speed endurance, strength sports and agility and coordination, there are different macronutrient needs. Endurance athletes, such as runners and swimmers, need high levels of carbohydrates to maintain their stores. Proteins, on the other hand, make up the least amount in the diet. If athletes prefer more of a strength component, such as weightlifting and shot put, protein should be as high as 20% in the diet to support muscle growth. Macronutrient distribution in sports nutrition.
| Vital nutrients | Endurance | Strength |
| Carbohydrates | 50-60 % | 38-46 % |
| Fat | 27-33 % | 32-40 % |
| Proteins | 14-16 % | 20-24 % |
Competitive sports and energy supply
Muscle activity requires energy, which is supplied by the endogenous compound adenosine triphosphate (ATP). To obtain ATP, ingested macronutrients (vital substances) such as carbohydrates, fats, and proteins must be converted. With the help of adenosine triphosphate, the body can use vital energy from macronutrients. Another energy-rich compound is creatine phosphate (KrP). In case of increased energy demand, KrP can be quickly converted into ATP. Consequently, creatine phosphate can store energy for longer periods of time, while adenosine triphosphate is a more short-term energy store. While an athlete is exercising and the muscles are working, ATP is being broken down to provide the energy needed for the muscle. Since the available amount of ATP in the muscles is limited, it must be continuously regenerated. ATP synthesis takes place in four different ways.Creatine phosphate cleavageSince muscular energy supply by means of oxygen is insufficient during high performance – short, very intense exertions, high force application – energy is produced antioxidatively and thus anaerobically. During short sprints, throws or jumps, there is an increased energy demand and the body provides ATP very quickly, but in very small quantities, as a result of KrP cleavage. The energy is thus only available for a limited time – seconds to a few minutes.Both short-term and long-term stresses reduce the amount of creatine phosphate available. Thus, it is necessary to increase the muscular store of creatine phosphate via sufficient food intake to prolong performance. In particular, fish – herring, salmon, tuna – and meat – pork, beef – should be consumed in sufficient quantities due to their high creatine content.Lactate formationMuscular energy supply takes place aerobically and thus by means of sufficient oxygen supply. The macro- and micronutrients (vital substances) are oxidatively utilized.During maximum, high-intensity loads – middle-distance runs – the carbohydrate store is drawn upon and glucose oxidation occurs.Glycogen, the storage form of glucose, is broken down under rapid ATP supply. The increased glycolysis leads to increased lactic acid production and thus to an increase in the amount of lactate in the muscle cell. This results in a pH shift within the cell – decreasing the pH in the blood – and acidification of the muscle (lactic acidosis). On the one hand, the lactic acid inhibits the contraction of the muscle and, on the other hand, the enzymes for muscular energy production. As a result, the muscle fatigues, resulting in a drop in performance. The physical exertion must ultimately be terminated.Complete combustionThe muscular energy supply also takes place aerobically and thus by means of a sufficient oxygen supply. During long, maximum and high-intensity exercise – long cross-country runs depending on the intensity – glycogen is completely burned to carbon dioxide and water. The energy carrier ATP is formed at a slow rate and in high quantities so that performance is kept as high as possible during the period of exertion. Glycogen stores are very limited and are available for only about 90 minutes of intense exercise.Once the glycogen reserves in the muscle are depleted, performance decreases. This energy supply runs faster than lipolysis and provides about 9% more energy than the breakdown of fatty acids in relation to the amount of oxygen taken in. Complete fat burningFor longer periods of low or medium intensity exercise – longer cross-country runs depending on the intensity – the organism covers more than 60% of its energy requirements through the complete burning of fatty acids to carbon dioxide and water. Due to a sufficient supply of oxygen, the energy supply is aerobic. As a result of prolonged low movements, ATP provision takes place at a moderate rate. The total amount of ATP formed as well as the available proportion of fats is almost unlimited, which means that performance is maintained for a long time. If the body is thus not overstrained and is loaded with low intensity over a longer period of time, this improves endurance, stabilizes the immune system and ensures a large proportion of fat burning. Fat can only be burned effectively if an adequate supply of oxygen is guaranteed. As a rule, all forms of ATP synthesis run in parallel, but with different proportions. Which new ATP formation has priority depends on the type, intensity and duration of the load.The more intense the load – for example, the faster an athlete runs – the fewer fatty acids and the more glycogen are burned. In addition to individual macronutrient distributions (needs) in different sports, additional energy expenditure also varies. Additional energy expenditure during different major forms of exercise.
| Main load form | Energy expenditure in calories per hour |
| Endurance – middle and long distance running, cycling, swimming, etc. | 300-800 |
| Agility, coordination – golf, gymnastics, yoga, etc. | 150-550 |
| Strength – bodybuilding, weightlifting, shot put, etc. | 500-700 |
| Strength endurance – ballet, cycling, rowing, etc. | 300-1.100 |
| Speed endurance – basketball, soccer, handball, etc. | 300-1.200 |
| Quickness – baseball, track and field, etc. | 500-1.000 |