The human body bases the order in which it burns the fats, carbohydrates and protein you give it on three factors: How many cells require it, how much effort it takes to convert it to usable energy and how much it can store. Your body burns carbohydrates before fat because they score well in all three categories.
When you consume carbohydrates, your body converts them to glucose, then to glycogen. In glucose form, it stays in your blood to fuel your activity and metabolism. If there's any left over, it goes to your liver and muscles, where it's stored as glycogen. Your body can tap into these reserves if it needs to, if you stop giving it carbs. But your muscles and liver can only hold so much, about 500 g in a healthy man. Your body can store just about all the fat you eat, however, up to 98 percent of it. It sends fat to your adipose tissue, or fat cells. Your body can hold onto it indefinitely there, so there's no need for your body to use it first.
Ease of Metabolism
If you eat too many carbs, there will still be some left over after your body uses them for energy, and after it stores as much glycogen as it can. Your body has the ability to convert the balance into fat for use as energy in the event of starvation, then ultimately convert it back into carbohydrates if you need them. But your metabolism has to work harder to achieve this. So your body burns carbs immediately instead, because this is a far easier task.
The Brain’s Needs
Your brain requires a significant amount of energy to function, twice as much as any other cell in your body. Although it can use fat for energy, it's not an efficient process. Fat stores must first be broken down into ketones for your brain to use them, and it doesn't use ketones as well as it uses glucose. The harder you think and the more you concentrate, the more glucose your brain needs. Your body can supply this demand by metabolizing carbohydrates first.
Other Tissue Needs
All your cells can use glucose for energy, and your central nervous system must have glucose in order to operate. It can't take its energy from fat. Carbohydrates fuel your entire body, and fat does not. Therefore, your body uses the carbs you eat first, because they alone can keep your entire body running efficiently. Fat's use is narrower; there's not as much need for it.
- ABC News/Health; Studies Show Glucose and Oxygen Help Brain; Amanda Onion; 2011
- Partnership For Essential Nutrition: Low Carbohydrate Diets --Do They Work and Are They Healthy?
- Franklin Institute; Nourish – Carbohydrates Fuel Your Brain; 2004
- FAO Corporate Document Repository: Carbohydrate Food Intake and Energy Balance
- Elmhurst College; Overview of Metabolism