The Metabolism of Fats, Proteins & Carbohydrates

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  • 0:01 Macronutrients
  • 0:41 Carbohydrates
  • 2:01 Fats
  • 2:55 Proteins
  • 4:11 Lesson Summary
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Lesson Transcript
Instructor: Kevin Newton

Kevin has edited encyclopedias, taught history, and has an MA in Islamic law/finance. He has since founded his own financial advice firm, Newton Analytical.

Everybody eats. But what happens to that food once it is eaten? Cellular respiration explains carbohydrates, but what about fat and protein? This lesson explores how all three macronutrients are used by the body.


Let's say that you're eating a nice balanced meal of a juicy steak and extra buttery mashed potatoes. I know, every cardiologist must be frustrated with me after that. Still, it's a perfect example of what happens to macronutrients as they enter the body. Macronutrients are the building blocks of what we eat and appear as carbohydrates, proteins, and fats. If you thought that little bits of steak and butter were carried through the body, you're going to be disappointed. Instead, the body breaks down each in a different way. In this lesson, we are going to learn more about each of those macronutrients, as well as how they impact the body.


You often hear people coming down on carbohydrates pretty hard. After all, how many diets and clean eating plans have the rule of avoiding carbs on page one? Still, the reason for this is pretty telling. Your body can break down most carbohydrates pretty efficiently compared to other macronutrients. But first, what is a carbohydrate? On a molecular level, a carbohydrate is a string of carbon atoms surrounded by oxygens and hydrogens. Since the ratio of oxygens to hydrogens is the same as it is in water, it gets the suffix -hydrates. Most carbohydrates are pretty easy for the body to synthesize into pure energy. Through the process of cellular respiration, carbohydrates come to the cells as glucose molecules, which are quickly turned into energy.

So, what's all this talk of complex versus simple carbs, then? Simple carbs are very easy for the body to turn into glucose. These are simple carbohydrates. Likewise, the ones that take much more effort are called complex carbohydrates. Finally, there are some carbohydrates that the body can't break down into glucose. These are called fiber. Insoluble fiber just passes through the digestive tract, while soluble fiber grabs waste products in the bloodstream as it passes through.


All those carbohydrates from the potatoes wouldn't be as good without the fat from the butter. Neither, for that matter, would the steak be the same without all that fatty goodness. And there's a reason that they just taste so good - fat is a fast source of energy. Whereas every gram of protein or carbohydrate is only worth 4 calories of energy, each gram of fat is a whopping 9 calories. Now all the low-fat claims of those diet products make sense, don't they?

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