Carbohydrates, in the form of glucose, are your body's preferred source of energy. But when you don't consume enough carbs, your body may turn to fat or protein for energy. Fat is incredibly rich in energy and can fuel your body even if you're fasting. Fat can be converted to glucose, but the process is so inefficient that you lose energy.
When fat is broken down into glycerol or odd carbon-chain fatty acids, it can be converted to glucose.
Using Macronutrients for Energy
Food gives you the nutrients you need to survive. There are micronutrients in food, including vitamins and minerals that your body needs. While micronutrients are important, the bulk of food is made of macronutrients like protein, carbs or fat. They all serve different roles, but can all be used for energy. That's one reason why humans are so adaptable, dietwise.
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When you eat food, you're giving your body energy. By digesting food, you break it down into smaller and smaller parts. Carbohydrates turn into glucose. Protein becomes amino acids. Fat breaks down to triglycerides and fatty acids. All these smaller parts of food can be used for energy.
Energy seems like a broad term, but it really refers to adenosine triphosphate, or ATP, which gives energy to most living things. Plants, animals and humans alike all depend on ATP to power cells. Your body simply wouldn't function without ATP.
It's easy to think of ATP as a battery, as described in an article from the University of Utah. It carries energy around your cells. It gives muscles the energy to contract, and it gives neurons the energy to fire.
Read more: Food Sources of ATP
Breaking Down Glucose to Pyruvate
You need resources to make ATP, which you get from food. Carbohydrates are the easiest source of fuel for your body. The process to extract energy from carbs is much simpler and faster than getting it from protein or fat.
Your digestive system breaks down carbohydrates into glucose, which is the simplest of all sugars. Your body breaks the glucose down further using different chemical reactions. The process of breaking down glucose is called glycolysis.
In glycolysis, your body creates two molecules called pyruvates per unit of glucose. These are molecules your body uses to create ATP. These pyruvates can be used in two different ways, depending on how hard your body is working.
Lactic Acid Cycle
If your body needs energy immediately, it turns pyruvate into lactic acid. When you do an intense exercise like lifting heavy weights or sprinting, your body uses lactic acid to make energy.
Intense exercise is called anaerobic exercise because you don't need oxygen to make energy. That's why it's faster to turn pyruvate into lactic acid, because no oxygen is required.
There are two major drawbacks to anaerobic energy. The first is that as lactic acid builds up it slows down and stops the chemical process that makes energy. That's why you can't sustain a sprint and your muscles fail during heavy weightlifting.
The other drawback is that you can't make much ATP from this process. It's fast but inefficient. For each molecule of glucose you use, you can create four ATPs. That's far less than when your body uses oxygen.
At rest or during an endurance workout, your body uses aerobic energy. That means it's using oxygen to create fuel. More specifically, it means you're using oxygen to extract a lot of energy from pyruvate.
To do this, your body converts pyruvate into something called acetyl CoA. Once it has acetyl CoA ready to go, the Krebs cycle can begin.
The Krebs cycle, also called the TCA cycle or citric acid cycle, is a chain of chemical reactions that extracts ATP from acetyl CoA using oxygen. It's much more efficient than anaerobic metabolism. You end up gaining 30 ATP molecules per molecule of glucose.
Using Fat for Energy
The bodies of most living species use this process to extract energy from glucose. Many animals can also use fat to create acetyl CoA, which means they can extract energy from fat if they run out of carbohydrates.
To extract energy from fat, your body breaks down fat cells or fat from the food you eat into triglycerides. This process is called lipolysis. Triglycerides are the type of fat that goes into your bloodstream, which makes it available for the rest of your body to use.
Once it's in your bloodstream, your body can turn the triglycerides into fatty acid and glycerol. Fatty acids are then turned into acetyl CoA, which enters the Krebs cycle in the same way that glucose did.
Fat metabolism yields about twice the amount of energy per weight as glucose, which makes it a rich source of energy.
With all the flexibility and nuance in your metabolism, it seems like there's nothing the body can't do. In fact, it can even create glucose from amino acids, which are the building blocks of protein. However, it has a tough time creating glucose from fat.
There's no real need to create glucose from fat. Your body can create an incredible amount of ATP from fat already. Even if your body is lacking in blood glucose, it can break down muscle and turn it into glucose using amino acids.
Creating New Glucose
Gluconeogenesis is a term that describes the synthesis of new glucose by other molecules. It can use amino acids, lactic acid and glycerol, which is extracted from fat. The problem with gluconeogensis is that you actually use energy. Six ATPs are required throughout the process.
This process happens mostly in the liver but also in the kidneys. Your body takes these molecules and converts them into pyruvate. Glucose can be broken down into pyruvate to use for energy, but now the reverse happens. Your body turns pyruvate to glucose by reversing the chemical reactions that created pyruvate.
From a survival standpoint, this is extremely useful. It allows humans to eat high fat, low carbohydrate diets without starving. It's even helpful between meals to keep your energy levels constant.
Read more: Do Fat and Protein Turn into Glucose?
Converting Fatty Acids to Glucose
There's one other way fat is converted to glucose: using fatty acids. However, it's even less efficient than converting glycerol to glucose. The catch is that you can only use fatty acids with an odd number of carbons. Every fatty acid has carbons, but most have an even number of carbons.
With odd carbon fatty acids your body can create something called propionyl CoA, which is similar to acetyl CoA. Your body takes this molecule and converts it to succinyl CoA. ATP is required to convert the molecule, which means you're losing energy. With succinyl CoA, your body can make pyruvate, which it converts back to glucose.
- GSU Physics: Adenosine Triphosphate
- University of Utah: ATP
- GSU Physics: Glycolysis
- The Medical Biochemistry Page: Gluconeogenesis: Endogenous Glucose Synthesis
- University of Waterloo: Gluconeogensis
- JAC Online: Anaerobic Glycolysis System
- Lumen Learning: Glycolysis
- ResearchGate: How Many Molecules of ATP Are Produced by the Anaerobic Fermentation of Amino Acids?
- Chemistry: Gluconeogenesis