If energy is the currency of our lives, then learning how energy is gained and then stored is critical to understanding how to keep your body’s balance sheet in optimum condition. Dr Kirsty Fairbairn reveals how our bodies store energy.
Have you ever driven down a road and seen billboard after billboard enticing you to eat foods that are really high in calories? And have you noticed that television advertisements are usually for the same foods? The modern food world makes it really easy to eat a lot of energy. So how do we get this energy thing right?
In our What Is Energy segment, we talked about where energy comes from and about how body fat is important for energy storage. Throughout human history, body fat has been a survival mechanism – way back in the days before supermarkets existed and food was harder to access. We call the nutrients that provide energy macronutrients: they include fat, carbohydrate and protein. Now we’re going to look at how we store energy from these macronutrients.
Why Fat Storage Can Be A Good Thing
Did you know that an 80 kg (176 lb) healthy male can store approximately 460,000 kilojoules (110,000 calories) in 12 kg (26 lb) of body fat? An 80 kg female would store slightly more (do not worry, there is a good reason women are better at it). Fat is stored without water, making it a really dense and efficient energy store. Every gram of fat provides us with 37 kilojoules (9 calories) of energy. This makes fat stores ideal for humans faced with a poor food supply. Body fat is also useful to support a pregnancy in the face of a poor food supply. However, for many of us in the Western world especially, our food supply is not sporadic, but excessive instead.
Our body really does need some fat stores to function well. If our body fat levels get too low, we will have some serious physical and mental health problems. It is concerning for us to see social media posts or websites encouraging body fat loss in young people. You should seek specialist advice from a doctor, physician or dietitian if you think you need to lose body fat.
How Well Do We Store Carbohydrates?
The answer is: not as well as we store fat. We just don’t have nearly the same ability to store carbohydrate and yet carbohydrate is an important fuel for exercise. An 80 kg male can store just 500 g (1.1 lb) of carbohydrate, compared to that 12 kg (26 lb) of body fat. Carbohydrate is stored in mainly the liver and skeletal muscle. We can store around 100 g (3.5 oz) in the liver and around 400 g (14 oz) in skeletal muscle. In total, these carbohydrate stores could provide approximately 8,400 kilojoules (2,000 calories). That sounds somewhat puny compared to the 460,000 kilojoules (110,000 calories) that same 80 kg male can store in body fat.
Every gram of carbohydrate we store can provide 16 kilojoules (4 calories) – that’s just under half the kilojoules a gram of fat can provide. Carbohydrate is stored with water as glycogen in our body, meaning it is not as space efficient to carry around compared to energy stored as fat. This means that we really do need to eat carbohydrate regularly in our diet to perform exercise well. In the modern day of highly processed food though, it has become necessary to turn the microscope onto the types of food we get our carbohydrates from. See some of our other articles on carbohydrates to help you understand that more.
Can We Store Protein?
Ideally, protein would not be used for energy. Only when we are not eating enough calories from fat or carbohydrates, like starvation, would our body start to burn protein for fuel. If we do, one gram of protein provides 16 kilojoules (4 calories) of energy – similar to carbohydrates and just under half the energy we can get from one gram of fat.
So we can burn protein for energy, but guess where our protein stores are? In our skeletal muscle. So if we eat so few calories that we need to burn protein to get fuel, we are breaking down our skeletal muscle to do that. Not ideal for anyone, let alone an athlete. To learn more about how protein works in the body, see our articles on protein.
From a human evolution perspective, body fat is actually our preferred and effective way to store energy, in anticipation of a shortage of food. The problem we are often faced with now is that we have an oversupply of high-energy food. We can also store carbohydrates, but not nearly as well. Because of that, active people should regularly top up their carbohydrate stores to ensure they can complete their next exercise or training session and do it well. Our protein store is our skeletal muscle and if we don’t eat enough energy we will burn the protein in our muscle, which is not a good idea.