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The science of food energy

May 21st, 2009 · No Comments

For fuel, the body converts the three body fuels (carbohydrates, proteins and fats) into ATP (adenosine
triphosphate). As the name suggests, adenosine triphosphate (ATP) is made up of adenosine linked to three
phosphate groups. When one of the phosphates is detached from the adenosine energy is created, leaving ADP (one adenosine and two phosphate groups). This process creates energy that is used for muscle
contractions but the majority of the energy is heat, hence the reason you feel warmer when you exercise.

There is enough ATP stored in the body for basic functions but when we start exercising this store
is used up in seconds. Therefore the body must produce more by breaking down the three body
fuels mentioned above.


As we have said, the main source of energy for our body is carbohydrate that is stored as Glycogen
in the muscle and liver cells. This said, the body can only store a small amount of glycogen at any one time and therefore needs to be replenished if we want to maintain our levels of energy. So we look to food and drink to gain our daily carbohydrate requirements.
Carbohydrates provide the fuel for our bodies and can be found in a variety of foods. There are two types – Complex and Simple. For endurance sports it is advisable to gain most energy from Complex carbohydrates such as brown rice, oatmeal and sweet potatoes. These are broken down more slowly therefore providing lasting energy for the body to utilize. Simple carbs provide a rapidly absorbed source of energy and can be great for providing extra energy during long sessions or as a way to replenish your energy stores after a session. Sports drinks are a convenient way to take in carbohydrates whilst maintain your fl uid levels.

How much Carbohydrate do I need?
You can work out your body’s daily carbohydrate requirement from level of exercise and your body
weight. For example if you are training for 2 hours a day on a regular basis and your body weight
is 75kg then your carbohydrate requirement would be: 450gram a day. This is a rough guide and
is worked out by multiplying your body weight (in kilograms) by your exercise level:

Exercise level Grams per body weight per day
1 hour of exercise a day 5
2 hours of exercise a day 6
3 hours of exercise a day 7
4 hours of exercise a day 8-10
Simple meals and snacks such as a banana sandwich, 7oz of cooked pasta and 2-3 slices of bread/
toast can provide 50grams of carbohydrates.
Start by looking at your current diet and work out the amount of carbohydrates you take in on a daily basis. You can then work out if you need to supplement this or change your diet. Remember to try and keep the calorie count low to maintain your current weight.
Plan your daily dietary requirements around your training. For example if you have a session at seven in the evening look to topping up your carbohydrate intake at 3-4pm. This can be as simple as a carbohydrate rich energy bar or a couple of bananas. Make sure your fl uid levels are up during the session and if it last more than an hour use an energy drinks (such as SIS or High5) to top up your spent glycogen (energy stored in your muscle and liver cells) These drinks are quickly absorbed and provide a good source of carbohydrate energy.

Why start a session with half reserves of energy?

Replacing your glycogen levels after each session will mean that you will be fully recharged for the next time you train. This is imperative on the days you have more than one session. As a rule of thumb there is a two-hour window after a session where your body is more effective at absorbing these nutrients. There are a great range of recovery drinks available that also contain micronutrients to reduce muscle and joint soreness as well as providing protein and carbohydrates.


As we have seen carbohydrate is our main source of energy and there protein is available for other bodily functions, namely muscle protection and repair.

How can protein help us?

Protein is needed by the body to promote the growth and repair of tissue. The intensity of our training will dictate the protein requirement. For example if you are working to increase your leg strength and are using weights in the gym and doing a lot of hill work on your bike then your protein intake needs to be increased to aid in the muscle recovery and repair. The more intense the session the more protein is broken down. If you are not supplementing your protein intake suffi ciently then this may result in tissue breakdown, reduced muscle size and strength, which in turn will lead to the dreaded “overtraining syndrome”
So the fi rst place to start is to work out your average daily protein intake. From this starting point
you can begin to gauge any additional requirements based on the intensity of your sessions.
Experts recommend that athletes should look to consumer 1.1 and 1.8 grams per kilo of body weight per day. For endurance sports you should be looking to the lower end of this scale (perhaps 1.2 – 1.3g) whereas for muscular, explosive sports you should be looking to the higher end. It has been shown that athletes who have a low glycogen level at the start of a session tend to burn more protein than those who have a high glycogen level.
So the key is balance.

Where should I get my protein?

With most sports lean muscle is the goal. Finding a way to pack on lean muscle without the added weight is one way of increasing the power available. But how can you achieve this? As an athlete you must look beyond a simple diet and look further into the types of foods and supplements that can support your training.
Keeping your body lean means you have less weight to carry around and less stress on your joints so helping to avoid injury. So when looking at protein it is important to focus on lean protein. You can fi nd a good source of lean protein from chicken, tuna and egg whites. These are low in fat and high in protein helping you maintain race weight.
If you are looking to supplement your lean protein intake then look at Whey Isolate. This provides a great source of protein whilst maintain a low fat content.
Vegetarians can look to Soy protein supplements as a way to support their requirements.


Start with health assessment. Doctors and professional sports people use the Body Mass Index (BMI) to classify body weight to assess health risks and performance. The BMI gives a range for a particular persons height and therefore does not assume that there is an optimal body weight for a set persons height.

The simple calculation of your BMI is as follows:

YOUR WEIGHT (Kg) Divided by YOUR HEIGHT(square metres)
For example my weight is 75kg and I am 1.83m high.
Therefore my BMI = 75 / (1.83 * 1.83)
BMI = 22.09
We then compare this to the BMI Classifi cation Table below:
Result Classifi cation Grade Health Risk
Less than 20 Underweight
20 to 24.9 Normal Grade 0
25 to 29.9 Overweight/plump Grade 1
30 to 40 Moderately Obese Grade 2
40 plus Severely Obese Grade 3

As you can see the BMI classifi cation is not an exact science and should be used as a rough guide. There are also certain limitations with the BMI. For instance it does not take into consideration that athletes have more muscle weight than those who are elderly where muscle deterioration may have taken place. But it’s a good starting point. Fat distribution has been highlighted as more important than total body fat measurement. For example fat stored around your stomach area carriers a bigger health risk that fat stored in your hips or thigh area.

A high majority of where we store fat is down to our genetic build, for example women tend to store fat around the hips, thighs and triceps whereas men’s bodies tend to favor the stomach for fat deposits. Your fat distribution can be measured using another simple calculation: The Waist Circumference Test. Simply divide your waist circumference by your hips circumference. The resulting fi gure should be less than 0.95 for men and on or below 0.8 for women.
If the resulting value is greater than these fi gures then this suggests excess fat in the abdomen and can result in a higher health risk.
As you can see, these tests provide a simple basis to assess your health risk. Further body fat tests can be used to more accurately measure the amount of body fat you may have for example underwater weighing, bio-electrical impedance and skinfold measurements but these will not be covered in this document.

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