Sports Nutrition

 

The importance of Nutrition in sport


All physical activity requires energy, the intensity level of the activity dictates how much energy is required. In order to ensure the body has sufficient energy to perform physical activity, the body must be fuelled by breaking down the food we eat.

In sport at all levels, an understanding of nutrition and how it effects sports performance is vital in order to get the most out of your sport. If you are looking to improve your performance in any sport, whether it be football, rugby, basketball, weight lifting, sprinting, cycling, long distance running, motor sport etc, it is important to know how to ensure your body has the right energy for your sporting activity and sufficient levels of energy to maintain performance. In order to do this you must first understand the basics of how the body produces energy for different types of physical activity.

 

Energy and the body

Why does my body produce energy?

When you exercise, your body must start producing energy much faster than it does when it is at rest. The muscles start to contract more strenuously , the heart beats faster to pump blood around the body more rapidly, and the lungs work harder. All these processes require extra energy.

The body has three main energy systems it can use for different types of physical activity. These are called:

 

How does my body produce energy?

Energy is produced by splitting a chemical bond in a substance called ATP (adenosine triphosphate). ATP is produced in every cell of the body from the breakdown of carbohydrate, fat, protein and alcohol - four fuels that are transported and transformed by various biochemical processes into the same end product.

The body only stores enough ATP to keep up basic energy requirements while at rest ( sufficient to keep the body ticking over). When you start exercising, energy demand suddenly increases, and the supply of ATP is used up within a few seconds. As more ATP must be produced to continue exercising, more fuel must be broken down.

How do I ensure I have enough energy for my sport

In order to ensure you are fuelled with energy ready for your sport or training it is important to understand that different foods provide different forms of energy and your type of sport or training intensity will dictate how much of each form of energy you will need.

Foods are made up of different amounts of carbohydrates, fats, proteins and alcohol. These nutrients provide us with a certain quantity of energy when broken down in the body. The typical energy values of the different food components is shown below.

  • 1g of Carbohydrates = 4kcal (17kj) of energy
  • 1g of fat = 9kcal (38kj) of energy
  • 1g of protein = 4 kcal (17kj) of energy
  • 1g of alcohol = 7 kcal (29 kj) of energy

As you can see, fat is the most concentrated form of energy, however this does not make it the 'best' form of energy for exercise. This is due to the way the body stores fat and uses it for energy. Below is a simplified breakdown of the four nutrients the body uses for energy, how they are stored by the body and how important each one is for exercise.

Carbohydrate - carbohydrate is stored as glycogen in the muscles and liver, along with about three times of its weight in water. Altogether there is about three times more glycogen stored in the muscles than in the liver. There is no endless supply of glycogen as the body can only store a certain amount, like the petrol tank in a car. The average body stores enough to last you one day if you were to eat nothing (1600-2000 kcal). This is why low-carb diets tend to make people lose quite a lot of weight in the first few days, due to the loss of glycogen and water. Endurance training helps to improve the body's glycogen storage capacity, increasing muscle mass will also increase your storage capacity for glycogen.

Liver Glycogen maintains blood glucose levels during prolonged exercise and at rest. It is therefore important to ensure the body's muscle glycogen stores aren't fully depleted during intensive exercise as the body would then start to use liver glycogen stores and break down muscle protein for extra energy.

Carbohydrates are essential for energy, the production of ATP during most forms of exercise comes from broken down carbohydrates and fats.

Fats - Fat is stored as adipose in almost every region of the body, the majority is stored around the organs and beneath the skin, a small amount (intramuscular fat) is stored in muscles. The amount stored in different parts of the body depends on genetic make-up and individual hormone balance, in men it tends to be around the middle (abdomen) and women around their hips and thighs. Although there's little you can do about how your body distributes fat, you can definitely change the amount of fat stored.

Fat is important for energy as it is broken down during the production of ATP and during prolonged intensive exercise the body breaks down fat stored in the body for energy. However consuming the good fats, such as Omega 3 fatty acids are the most beneficial. See Nutrition (Fats).

Protein - protein makes up our muscles, it is not usually known as energy source but may play a role after prolonged exercise, such as in marathons or during very intense bouts of exercise. When glycogen stores are exhausted, the protein in muscles (and organs) may make upto 10% of the body's fuel mixture.

For a person following a low-carb diet, or during a period of semi starvation, glycogen levels become low, so more proteins would be broken down to provide the body with fuel. This is part of the reason why someone following a low-carb diet can lose significant amounts of weight as half of the weight lost is muscle.

Alcohol - alcohol cannot be used directly by the muscles for energy during exercise. It first has to be broken down by the liver which has the specific enzymes in order to carry out this function. The liver will carry out this job at a fixed speed therefore if you thought exercising harder helps to break down alcohol then you're wrong.

Alcohol is not an important nutrient for the body!

The ATP-PC (Phosphagen) system

ATP is a molecule stored in small amounts in cells, enough to maintain basic energy needs. This basic energy need is just enough to allow your muscles to exercise at maximal intensity for about 1 second. After this, ATP must be regenerated from one of the body's three main energy systems. Each system generates ATP at different rates due to the different biochemicals used in each system, the systems used depends on the exercise intensity and duration.

PC (Phophocreatine) is made up of Phosphate and Creatine and is made when Protein and Creatine are linked to a Phosphate molecule in the body. The high energy compound Phosphocreatine can then be used to regenerate ATP rapidly when supplies are low, therefore provide backup for ATP.

The ATP-PC system is what generates energy for maximal bursts of strength and speed that last for upto 6 seconds. This system can release energy very quickly but is unfortunately in limited supply and provides only 3-4 kcal of energy. Once this supply is exhausted, ATP must then be produced from other fules such as fat or glycogen via one of the other energy systems (anaerobic or aerobic systems).

 

The Anaerobic glycolytic,or lactic acid System

Also known as Glycolosis, this particular energy system refers to the anaerobic breakdown of glucose or glycogen for energy. The anaerobic system activates as soon as you begin high-intensity activity and is the main energy system used in events lasting upto 90 seconds, such as a weight training set in the gym or a 400-800m sprint. This energy system is very fast, producing quick energy for exercise. When a sudden large demand for energy exists,glucose bypasses the energy producing pathways that would normally use oxygen, and follows a different route that does not use oxygen. As this saves time, the sudden energy demands are quickly met, after 30 seconds of high-intensity exercise this system contributes upto 60% of your energy output; however, this contrubution falls to only 35% after 2 minutes as the body's glycogen stores quickly dwindle. One of the by-products of this fast-delivery service is a metabolite called pyruvate. Under the right conditions, pyruvate will break down into more energy, these conditions exist only in the Aerobic system not in the Anaerobic system therefore in this instance pyruvate doesn't breakdown and turns into lacate (lactic acid). If lactate is not removed from the muscles a problem develops with muscle contraction thus causing ther athlete to slow down or stop.

There's a number of reasons why pyruvate doesn't breakdown, causing the build up of lactate (lactic acid) in the muscles. One of these reasons could be a lack of oxygen getting to a particular cell, or the part of the cell that breaks down pyruvate may not be able to handle the sudden influx of pyruvate. Another reason could be that the particluar enzymes required for breaking down pyruvate are in short supply. Any one of these reasons can contribute to lactate build up in the muscles during intense-exercise. These areas can be developed with training and nutrition ensuring increased fitness and energy levels.

 

The Aerobic system - comprising of the glycolytic (carbohydrate) and lipolytic (fat) systems

In the aerobic system ATP is generated by breaking down fat ( by lypolysis) and Carbohydrate (by glycolysis) with the use of oxygen. The by - product from glycolysis, pyruvate can be used as a source of energy instead of fat, this will produce energy at a slightly faster rate than if the body was using only fat as a fuel source. Breathing heavily (releasing carbon dioxide) and sweating (cooling down the body) are a result of pyruvate breakdown.

This particular energy system cannot produce energy as fast as the other two systems (the atp-pc system and the anaerobic system), however it can produce larger amounts. When you start to exercise, you initially use the ATP-PC and anaerobic glycolytic systems, but after a few minutes your energy supply gradually shifts to the aerobic system.

Aerobic glycolysis is fuelled by the muscle glycogen stores (carbohydrate storage from the diet), when exercise lasts longer than 1 hour and glycogen stores become depleted, additional glucose from the bloodstream plays an important role in supplying the body with energy. Typically after 2 hours of high intensity exercise (at a rate greater than 70% of your VO2 max), almost all of your muscle glycogen stores will be depleted. At this stage glucose from the bloodstream and increasing amounts of fat are used to fuel your muscles. This therefore emphesises the importance of ensuring high glycogen stores through the consumption of sufficient amounts of carbohydrate in the diet and sports supplementation before during and after exercise.

Muscle Fibres

The body's muscle fibres use all three of the energy systems however certain systems are used more than others depending on the muscle fibre type. The muscle fibre types are classified as follows:

  • Fast-twitch (FT), also known as type II muscle fibres - mainly use the ATP-PC and anaerobic glycolytic systems
  • Slow-twitch (ST), also known as type I muscle fibres - mainly use the aerobic system.

Everyone is born with a different proportion of FT fibres to ST fibres and the this can vary quite considerably between individuals. People with a higher proportion of FT-fibres are more likely to be good at sports involving explosive power and speed such as sprinters. Whereas people with higher proportions of ST fibres, such as distance runners are better able to develop aerobic power and endurance.

 

The energy supply of the muscles during aerobic and anearobic exercise

Carbohydrate (glycogen when stored) and fat play a vital roll in providing the energy to fuel your muscles during aerobic exercise, however the use of these two energy sources vary according to a number of factors. The most important of which are:

  • the intensity of the exercise - during high intensity exercise such as sprinting and heavy weight training (anaerobic exercise) the muscles rely on glycogen stored in the muscles for its major fuel source as opposed to fat. However, during aerobic exercise a mixture of fat and muscle glycogen is used and at low intensity (less than 50% VO2 max) fat is the main fuel source

 

  • the duration of exercise - the longer you exercise the lower glycogen stores become and as the supply of muscle glycogen drops, the contribution that blood glucose makes to your enregy needs increases. The proportion of fat used also increases but fat cannot be burned without the presence of carbohydrate. At this stage fatigue will be setting in, on average you have enough muscle glycogen to last between 90-180 minutes of endurance activity. If you increase your intensity then the rate at which muscle glycogen stores are depleted will also increase. When doing a mixture of aerobic and anaerobic activity (interval training), muslce glycogen stores will become depleted after 45-90 minutes. When doing mainly anaerobic activities such as heavy weight training, muscle glycogen will deplete within 30-45 minutes. Once this occurs Protein will make an increasing contribution to energy needs. This come from the breakdown of muscle protein and can result in loss of muscle mass

 

  • your fitness level - after aerobic training your muscles adapt in order to improve your performance, and your body becomes more efficient at using fat as a fuel. As glycogen is in shorter supply than fat, the more fat the body uses as you exercise, the longer glycogen stores will last, thus improving your endurance levels meaning you can exercise for longer

 

  • your pre-exercise diet - a diet low on carbohydrate will result in low muscle glycogen and liver glycogen stores. This will effect performance aerobically and anaerobically, reducing your ability to sustain exercise at 70% VO2 max for longer than an hour and effect your ability to perform short bursts of power. When your muscle glycogen stores are low your body will become more relient on protein and fat. This doesn't provide an ideal fat loss strategy as lean muscle tissue will also be lost.

Energy systems used in Sport

Each sport is made up of different types of exercise, therefore relies on certain energy systems more than others.

Below is a table of popular sports and an estimated percentage breakdown of the energy systems used.

Sport
Percentage of Energy Systems Used
ATP-PC (Phosphagen)
Anaerobic (Glycolytic)
Aerobic (Glycolytic and Lipolytic)
Football (soccer)
50
20
30
Rugby
10
70
20
Basketball
60
20
20
Field events
90
10
0
Running (distance)
10
20
70
Sprinting
90
10
0
Swimming (1.5km)
10
20
70
Swimming (50m freestyle)
40
55
5
Tennis
70
20
10
Fencing
90
10
0
Skiing
33
33
33
Volleyball
80
15
5
Rowing
20
30
50
Golf swing
95
5
0
Gymnastics
80
15
5
Hockey
50
20
30

 

For the next article in sports nutrition see fatigue.

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