Meeting energy needs is the first nutritional priority for athletes and one of the most frequently asked questions is “How much should I eat to stay fit and healthy ?” A healthy diet contains the right proportions of carbohydrate (CHO), fat, protein, vitamins and minerals. Carbohydrates and fats are the major sources of energy although energy can be obtained from protein. Energy is measured in Kilocalories (kcal) or Kilojoules (kJ): 1kcal = 4.2 kJ.
1g of Carbohydrates contains 3.75 kcal (16kJ)
1g of Fat contains 9 kcal (37kJ)
1g of Protein contains 4 kcal (17 kJ)
The amount of food we eat (energy intake) should match energy expenditure. If this is the case then we should neither gain nor lose weight as we are in energy balance. If there is inadequate energy intake relative to energy expenditure, the body will use fat and lean tissue mass for fuel. Loss of muscle results in reductions in strength and endurance capacity.
Total daily energy expenditure is influenced by heredity, age, sex, body size, fat-free mass, environmental factors, pregnancy and the intensity, frequency and duration of exercise. It is generally made up of three components:
1/ Resting Metabolic Rate. This is divided into Basal Metabolic rate (BMR) which is the minimum energy requirement to sustain the body’s functions in the waking state and Sleeping Metabolic Rate.
2/ The thermic effect of food intake (TEF) which is the energy expended on digestion, absorption, and storage of foods – amounts to about 10% of daily energy intake.
3/ The energy (thermic) cost of exercise (TEE) includes normal everyday activities as well as the energy cost of soccer training and matches.
The BMR demands around two-thirds of our daily energy intake required for normal everyday activities and is the largest part of our daily energy expenditure. Body mass, statue and age or estimates of fat-free body mass provide accurate estimates of BMR. The following equation can be used to calculate BMR for people aged between 18-30. Energy intake and expenditure are expressed in Kilocalories (kcal) and Body Weight in Kilograms is represented by W.
Males: BMR (kcal or kJ/24 h) = 17.5 X W + 651
Females: BMR (kcal or kJ/24 h) = 14.7 X W + 496
According to various studies, a man around 25 years of age with a sedentary lifestyle is recommended to consume around 2500 kcal whereas a man of a similar age who is very active should consume about 3333 kcal. Older people are recommended to have a lower intake than younger people and males generally have a higher intake than females.
These equations can be used to calculate daily energy expenditure of soccer players when their physical activity level (PAL) is taken into account. PAL is a multiple of BMP and has been classified for different activities. The physical activity ratio (PAR) is another classification system often used. Soccer has a PAL value of 7.5 whereas a relatively sedentary lifestyle would have a PAL of 1.3. However, if we average out the energy spent from playing soccer over the whole day (as players will probably only have moderate amounts of extra activities), a PAL of around 1.7 is more relevant (1.6 for females). The following calculates the energy intake required by a 75 KG soccer player using BMR:
17.5 X 75 KG + 651 = BMR of 1963 X 1.7 (PAL) + 10 % (for TEF) = 3671 kcal or 15421 kJ per day.
Studies on elite soccer players show that the energy intake is likely to be 2033 – 4000 kcal (8500 – 16500 kJ) per day, although this depends on factors such as intensive training where intake may need to be increased or periods of inactivity such as injury where the player should reduce intake. Variations in energy intake is likely to lead to weight loss or gain. For more information on changing dietary habits, click here. Elite players have been found to have diets which are too high in fat and too low in CHO intake. They should be encouraged to consume more unsaturated fat (e.g. margarine) instead of saturated fat (e.g. butter) as well as complex carbohydrates such as pasta and rice. For more information on CHO, fat and proteins, click here to go to the nutrition advice section menu.
So what are the respective amounts of CHO, fat, and protein required for a player with a daily intake of 16000 kJ or 3809 kcal. Firstly, for a standard healthy diet, general dietary recommendations of 55-60% CHO, 25-30% Fat and 10-15% protein are advised. A simple calculation (see below) shows that the player should be consuming 550-600g of CHO, 108-130g of fat and 94-141g of protein.
CHO Intake = 55-60% of 16000 kJ (Energy intake) / 16 kJ (energy in 1g of CHO) = 550-600g
Fat Intake = 25-30% of 16000 kJ (Energy intake) / 37kJ (energy in 1g of Fat) = 108-130g
Protein Intake = 10-15% of 16000 kJ (Energy intake) / 17 kJ (energy in 1g of Protein) = 94-141g
The energy cost of football has been estimated as being 16.4 kcal per minute amounting to 1480 kcal or 6210 kJ for a ninety minute match. This highlights the need for adequate post-match fluid and food intake in order to replace lost energy.
Players can evaluate their energy balance through recording their own weight. Any changes in weight may indicate a diet that is either lacking or too excessive. However, players can positively gain some weight due to increased muscle mass from weight training and an examination of body composition (such as skinfold measurements) may be a better indicator the stability of body weight. Players may also weigh and record all the food and drink they consume and food labels supply valuable information on what food contains. Although this is tedious, it can provide an idea on energy intake and the composition of the food they eat.
THE EFFECTS OF TEMPERATURE ON PERFORMANCE
A human being can only tolerate a variation of around 4°C in internal body temperature without physical and mental performances being impaired. It is well known that soccer players like in many other sports are at some time or other subjected to extreme weather conditions. The temperature of the climate the game is played in is all-important and will affect body temperature and thus the ability to perform.
The control of body temperatures depends on the balance between overcooling and overheating. The normal temperature of the body is 37°C. When playing football heat production is increased and has to be removed or else performance will be affected. Temperature must stay within a range of 37° C to 40° C. Body and the outside air temperature/humidity can be measured relatively accurately through the use of thermometers.
Most of the heat is conducted in the bloodstream to the skin where it is released into the environment in four different ways:
Conduction: excess heat is transferred through direct contact with an object (e.g. when swimming in cold water).
Convection: circulating air allows heat to be transferred to air passing over the skin (depends on the air temperature).
Radiation: Involves heat being lost from the body through radiation (e.g. loss of heat within a cold environment).
Evaporation: excess heat is transferred to sweat which is then evaporated. Sweating is the most important means of dissipating heat when exercising under hot conditions.
However, even with the large heat loss through sweat evaporation, body temperature does rise during a match as not all of the heat produced can be released. Evidence also shows that due to the intermittent nature of soccer, the general increase in temperature is greater than for continuous exercise and also the higher the exercise intensity the greater the heat production.
The harmful effects of cold exposure are mediated by the balance between heat production and heat loss. There is a close relationship between muscle performance and muscle temperature – as temperature decreases, so does performance. For example, muscle strength is impaired and slower reaction times have been shown under cold conditions. Vasoconstriction where the blood flow to muscles or the outer parts of the body is reduced takes place.
Under normal conditions, body fluid loss can be around 2 litres resulting in a negative effect on soccer performance (a loss of only 2% in body mass affects performance). Even in winter conditions, there is still a danger of dehydration if correct fluid replacement is not implemented. Studies under cold conditions have shown that if players do not drink sufficiently, then significant dehydration can still take place.
The risk of injury is also increased when exercising in the cold. Whether this be due to a poor playing surface or incorrect warm-up procedures, athletes should take the necessary precautions. For more information read our article on injury prevention.
To reduce the effects of cold weather, players should be correctly hydrated, undertake a good warm-up session and wear warm appropriate clothing. The coach must play a major part by making sure that players are correctly prepared and checking if the climatic conditions are suitable.
When playing football in hot weather, the heat lost through convection and radiation is minimal thus requiring heat to be dissipated through the evaporation of sweat. In humid conditions the bodies ability to reduce its temperature is further reduced as sweat evaporation is inhibited. The active muscles and the skin are in direct competition for the limited blood supply as the muscles need blood to provide the necessary oxygen and the skin needs blood to facilitate heat loss. Thus, exercising in hot and humid conditions can be dangerous or at least lead to poor performance.
The distances run by players has been shown to be markedly reduced when playing in hot conditions. Evidence also shows that the loss in body water can be more than 3.5 litres (compared to 2 litres in normal conditions). As mentioned earlier a loss of 2% in body mass affects match performance and a 5% loss will decrease the ability to exercise by as much as 30%.
Players can help reduce the effects of a hot climate through:
Fluid intake: adequate fluid intake before, during and after the game/training will avoid dehydration. Players should be correctly rehydrated before competition and take additional drinks when possible. Before exercise, the player’s weight should be at it’s norm. Drink absorption is at its best when mixed to a weak solution of 2.5 % carbohydrate and rehydration drinks should contain sodium. Thirst is a poor indicator of hydration levels and probably indicates that the player is already dehydrated.
Judging their efforts: Many tournaments are played in hot and humid temperatures which is likely to affect playing performance. Players should try to judge carefully their efforts over the whole match without exhausting themselves from too much high-intensity exercise too soon in the game.
Planning: The coach can judge or measure whether the temperature is too hot for training. Training should not take place in temperatures above 40°C and can be moved to cooler periods (morning/early evening). Plenty of drinks must be made available and exercise intensity levels adjusted accordingly. Training should as well include regular rest periods. Special attention to the conditions must be taken with children when training or playing matches.
Acclimatisation: Players can be acclimatised to hot conditions by spending a defined period of time in the designated place beforehand (10-14 days is recommended as a good adaptation period). However, training sessions should be shorter and easier at the beginning in hot weather. It seems as well that the higher the training level of individuals, the quicker their adaptation to hot conditions with one of the reasons being a greater ability to sweat.
Clothing/Skin Protection: Clothing should be light coloured if possible and made of lightweight, breathable material so that sweat can evaporate. Football kit soaked in sweat prevents heat loss so should be frequently changed. Sun-block can be used to prevent sunburn.
This article has described the effects of temperature on playing performance. Both the coach and player must take the necessary measures to prevent problems arising when playing soccer in adverse weather conditions. Correct clothing, fluid intake, hydration levels, adaptation, planning and rest periods all play a part in maintaining playing performance. If temperature related illness is suspected, consult a qualified doctor.