Energy Systems

Today’s blog post is going to focus on energy and the different types of energy systems our bodies use when exercising. Hopefully this will help you to understand the science of how our bodies work and how elements such as nutrition can be applied to your training.

Often heard in a phrase related to health/fitness such as “I’ve got no energy to go to the gym” or “I’ll eat X to give me energy for my run.” So, what exactly do we mean by “energy”?

Scientifically, energy is defined as the ability to do work. Within the human body, this work can be chemical, mechanical or transportation. All of this “work” requires energy. Energy cannot be created or destroyed, only transferred from one form to another. For example, chemical energy from our food is then transferred to mechanical work when we use our muscles to perform movement like a bicep curl.

Energy in this form is measured in calories (we’ve all heard of this one haven’t we?!) or kilocalories. A calorie is the amount of energy needed to raise the temperature of 1g of water by 1°c.

And where do we get our energy or calories from? FOOD. The 3 main macronutrients carbohydrates, fat and protein provide us with our daily energy. As you may recall from previous blog posts, carbs and protein provide 4 calories per gram and fat is 9 calories. Alcohol can also be used as energy, providing 7 calories per gram; although it is definitely NOT recommended to use alcohol as an energy source for exercise!

Our bodies mainly use carbohydrates and fat as energy for activity, with protein being used by as little as 5-10%, which would only be in extreme cases of carbohydrate depletion. Carbohydrates are stored in the muscles and liver as glycogen or circulating in the blood as glucose, ready to be used. Fats are stored as triglycerides in adipose tissue all over the body or as free fatty acids in muscle cells (intramuscular fat) or in the blood.

Interestingly, although our bodies preferred energy source is carbohydrates, the potential of energy coming from fat is higher. The average human body has enough stored glycogen to give around 2000 calories (400g in the muscles and 100g in the liver= 500g. 500x 4= 2000), which would fuel around 2 hours of exercise. However, an average man of say 70kg with 15% body fat would have enough energy stored for 94,500 calories! (70/100 x 15x 1000= 94.5 x1000 as a calorie is actually very small so a kilocalorie or Kcal is used).

Before the body can actually use the energy within our stores, it needs to be converted to a usable form; this form is called ATP or adenosine triphosphate. This is the “energy currency” we use to power activity and is constantly generated, used and then regenerated again in our cells. The molecule is made up of one adenosine molecule attached to 3 phosphate groups. The bonds between the phosphate groups and adenosine contain the energy we need, so when a bond is broken this energy is released and used by our bodies. After the bond is broken the molecule left is called ADP (adenosine diphosphate) plus a free phosphate group.

ATP can then be regenerated by joining a free phosphate with an ADP molecule, and energy is then stored. However, our cells only contain a very small supply of ATP for a few seconds work, so in order to sustained prolonged activity, ATP must be regenerated. This is done via 3 different metabolic pathways; the phosphocreatine system, anaerobic glycolysis and the aerobic system. You will probably be familiar with the terms “aerobic” and “anaerobic” simply meaning with and without oxygen, respectively.

Starting with the first and shortest process; the phosphocreatine system. Phosphocreatine (PCr) is a molecule formed when a protein called creatine, found naturally in our cells, joins to free phosphate. When it is broken down, the free phosphate joins the ADP to form a new ATP molecule. However, the supply of PCr in our body is very small meaning activity is limited to short bursts of high intensity activity, for example a tennis serve, a high jump or a snatch in power lifting.

After this, the body then needs to use other fuel sources to produce ATP; this is where our carbohydrates or fat come in.

For activity lasting around 30 seconds to a few minutes, anaerobic glycolysis is used. This term refers to the breakdown of glucose in the absence of oxygen. As we begin to exercise, our bodies can’t deliver oxygen to our cells quick enough so stored muscle glycogen is used a fuel source. Glycogen is broken into glucose and is converted into a substance called pyruvic acid; 2 of these molecules are produced per glucose molecule. When no oxygen is present, the pyruvic acid is converted into lactic acid; this is the compound responsible for the burning sensation we get during repeated muscle contractions. Because of this effect, we cannot sustain activity of this kind for very long, meaning ATP production is limited to just 2 molecules per glucose.

This is where the next system comes into play; aerobic metabolism.

The aerobic system includes 3 parts;

1. Glycolysis or beta oxidation

2. Krebs cycle

3. Electron transport chain

Let’s look at the first one. After a few minutes of exercise, oxygen has now been delivered to our cells and the pyruvic acid formed from the breakdown of glycogen is converted into a compound called Acetyl Coenzyme A. This complicated sounding compound enters a clever little process called the Krebs cycle; a number of metabolic pathways which produce 1 ATP per cycle. The by-products of the cycle then enter the next stage; the Electron Transport Chain. More reactions occur which result in a further 17 molecules of ATP being produced. As there are 2 molecules of pyruvic acid per glucose this gives 36 molecules of ATP. Add the initial 2 from glycolysis and there is a total of 36-38 ATP produced per glucose as there is some ATP used to fuel to system. Pretty efficient compared to anaerobic respiration which only produces 2!

The other system is beta oxidation, whereby fat is utilised for energy. Triglycerides are broken down to release fatty acids and glycerol with the fatty acids then being converted to Acetyl CoA. This then enters the Krebs cycle and ETC as above to produce ATP. Due to the chemical structure of fatty acids, the potential energy production is higher than that from carbs; it has the ability to produce up to 200 ATP! However, this is a longer pathway, therefore our bodies prefer to use carbohydrates initially for fuel. Once this is used, fats can be used providing a slower release of energy.

After all that complicated science I’ll recap briefly on a time scale:

During the first few seconds of activity, ATP already present in cells is used and broken into ADP. ATP is then regenerated by the Phosphocreatine system but this is limited by the supply of creatine in our cells. Between 30-60 seconds of activity anaerobic glycolysis occurs. Glycogen from our muscles and liver is then broken into glucose to be used as the main fuel. At around 2 minutes, oxygen can be delivered to our cells and aerobic respiration kicks in. Energy (ATP) is then produced via the Krebs cycle and Electron Transport Chain via glycolysis, using glucose, or beta oxidation, using fat, as fuel.

It is important to note that different types of activity use different energy systems. They are used to varying degrees, swapping between them as the demand is required, with one eventually dominating.

Understanding how our energy systems work can help food choices for sports performance. For example, high intensity activities use carbohydrates as fuel, but a longer duration of activity will utilise fat as fuel. Athletes and regularly active people need to ensure they have enough carbohydrates for optimum performance. If this is limited, the body will gradually begin to use fat. However, the danger is if both are low the body will use protein, which is a big no no as it can lead to muscle breakdown, impacting the strength of associated joints and increasing the risk of injuries. And this is something we don’t want! So, remember food is fuel, quite literally!

If you need help with nutrition for sport and exercise, work with me and I will help to optimise your performance!

#energy #exercise #sportsperformance #nutrition #exercisescience