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If you're like us, you love the sound of a brunch buffet. But not everything you eat at that glorious buffet is going to be turned into energy. Your body has to work with different forms of food in different ways. In this episode of Crash Course Anatomy & Physiology, Hank takes us through more about our metabolism including cellular respiration, ATP, glycogenesis, and how insulin regulates our blood sugar levels.

Looking for a refresher on ATP and Celluar Respiration? Check out this video from Crash Course Biology:

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Introduction: Brunch Buffets 00:00
Cellular Respiration 2:03
Absorptive State 3:26
Basal Metabolic Rate 4:18
Insulin Regulates Blood Glucose Levels 4:40
Lipoproteins: LDL and HDL Cholesterol 6:17
Postabsorptive State 7:04
Insulin & Diabetes 8:10
Review 9:05
Credits 9:30


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CC Kids:
Here's a couple of words you might find useful in this study of human metabolism. They also happen to be two words that I enjoy in general: brunch buffet. I'd throw in all-you-can-eat, but that's kind of implied and it would be six words, unless you hyphenate them. But still, brunch buffet. You've got eggs, and bacon, and french toast, biscuits, and fruit, and some kind of, weird sort of Jello thing in there.

And once you've paid your $12.99, you know there's a pretty good chance that it's all going to eaten. But here's the thing. Not everything that you fit into your face during brunch time is going to be handled the same way by your body. 

After you're done digesting, some nutrients will go straight to your body's pile of stuff to burn right away, but others will have to be converted into something else. For example, the carbs and fats in your buttered toast can be directly oxidized into usable energy, but the amino acids in the bacon have to be converted into molecules that get broken down like carbs if you want to get energy out of them. 

And while excess carbs and fats can be stored in larger, polymer versions of their original forms, any extra amino acids can't be. They have to be converted and then stored as fat or glycogen. So as I've mentioned before, the molecules in your body are constantly changing shape, and renewing and rearranging themselves to either build things or to use energy. And eating food replenishes these nutrients, especially glucose.

Then, depending on what your body needs, and when you last ate, and certain hormones, like insulin, will help decide what to burn and what to store for later. This is, of course, an important function, which is why things can go badly if this process doesn't work properly because of a metabolic disorder like diabetes. In that case, your body can't properly use and store nutritional energy, which makes the very acts of eating and metabolizing more difficult, and possibly dangerous.

Exactly how glucose levels can spike or plummet, how we convert nutrients into energy, and how all of that relates to eating, and hunger, and weight, and metabolism, and your health in general, is guess what, complicated. But as you are a living thing that must eat in order to stay alive, I'd say it's worth learning how it works, and brunch is as good a place to start as any.


Waiting in line for the all-you-can-eat brunch buffet is a good time to recall the law of conservation of energy. The law simply states that energy can never be created or destroyed; it can only change forms. And the process of metabolism involves conversions of energy, either through catabolizing reactions that release the energy stored in your food, or anabolizing ones that use or store that energy. 

So, you can enjoy your short stack of flapjacks and biscuits and gravy, but remember, that energy has to either get released, or stored, somewhere inside you. Now, the conversion of food energy is part of the great balancing act of maintaining homeostasis, and one of its greatest tricks is cellular respiration.

Cellular respiration is how we derive energy from the food that we eat, specifically glucose, which is where most of our food ends up. In order for glucose to become energy we can use, we have to give it some oxygen, and convert each sugar molecule into six molecules of CO2 and six molecules of water, which usually produces about thirty-two molecules of ATP.

Cellular respiration occurs in three simultaneous phases where glucose is broken down by glycolysis, and other catabolic reactions, and then turned into pyruvic acid and acetyl-CoA, and finally ushered into the Krebs cycle. It's all pretty complicated, but we've covered it extensively in Crash Course Biology, so rather than getting into the blow by blow here, I recommend you check out that video for a refresher.

For now, just remember that cellular respiration breaks down nutrient molecules to generate ATP, and in doing so it captures some of the chemical energy that was in those bombs for use in your body.

Now when it comes to how that energy is converted and where, that depends largely on when you ate your last meal. We all switch back and forth between two nutritional states--the absorptive, or fed state, which is during or after eating, and the post-absorptive, or fasting state, when the GI tract is empty and the body is running off of stored supplies.

So, say you're just post-brunch buffet, and you're still in the absorptive state, unbuckling your belt as your digestive system breaks up the eggs and bacon and French toast and syrup into a bunch of mostly glucose molecules that pass into your blood stream.

The first bit of glucose gets delivered throughout the body and is tapped to generate ATP on the spot through cellular respiration. But since the meal was big and rich, there's more glucose floating around than your cells need at the moment. And remember, ATP is too unstable to be used for storage, which means all the extra glucose is going to get stored as fat or glycogen.

And that storage is part of how you can end up gaining weight, because how much energy gets stored depends partly on your basal metabolic rate, that's the number of daily calories your body needs to do business as usual. And that rate can be influenced by your age, your sex, and body size and composition.

A young body builder is going to burn way more calories than that tiny grandma, but generally if you're absorbing more nutrients than you are burning, you will gain weight. So, yeah, for that you can blame the law of conservation of energy.

Now, of course, all nutrients are important and your body is basically made of protein, so that's kind of key. But in terms of immediate fuel, glucose is the easiest source of food for cells like your neurons. And it's always a good policy to keep your brain well fed, so it doesn't get hangry, or dead.

So your body likes to maintain a blood glucose level of 70 to 100 milligrams per deciliter. But let's say, that thanks to the buffet, you've now got 140 mg/dL. This means that too much sugar is swimming around in your blood and your body isn't happy about it.

If your blood sugar levels get too high, it can damage blood vessels, especially those in the nervous system, heart, kidneys, eyes, and extremities. That's why diabetes is often associated with a higher risk of heart and kidney disease, loss of vision, and foot amputation.

But typically, rising blood sugar sets off a series of events that trim them back down. Specifically, they trigger special beta cells in the pancreas to start secreting more of the hormone that regulates everything that happens when you're in the absorptive state. And this hormone is the all powerful insulin.

Insulin's job in this instance is to move glucose out of the blood and into storage, and to do that it triggers a shift from catabolic reactions to anabolic ones. For example, it puts a stop to glycogenolysis, or breaking down glycogen in your liver and muscles for glucose and energy, and instead ramps up the process of glycogensis, where extra glucose is linked together to form glycogen. It also activates lipogenesis, where very cool chemical reactions in the liver can convert glucose to triglycerides and then ship them off to your adipose tissue for storage.

A similar thing happens when the extra fatty acids that you got from the eggs and bacon aren't immediately needed for energy. They get processed through lipogenesis too, and are built back into triglycerides, and then tucked away for a rainy day.

The thing is, if you're thinking about this, all these lipids hate water, right? Fat and water don't mix. So you should be asking yourself, how can they be transported in the blood when they're so hydrophobic?

The answer is lipoproteins, four special proteins made by the liver that surrounds fats and allow them to move in the blood stream as an emulsion. You've probably heard of some of these. One is low density lipoprotein, also know as LDL cholesterol. It's what delivers some of the glucose and fat that you just ate to your body's fat deposits.

Another is high density lipoprotein, or HDL cholesterol, which is the good cholesterol. It starts out as an empty protein packet sent out by the liver to gather other cholesterols from the blood and artery walls and other cells. HDL then delivers cholesterol back to the liver, or to places like the ovaries, testes and adrenal glands, which use it to make steroid hormones.

Alright, so now your body has put all the glucose and lipids and proteins where they need to go, and you can just coast into the post-absorptive state. So, several hours later, even though your small intestine is still working on what's left of the buffet, your cells have been helping themselves to the remaining glucose in the blood, and eventually your blood sugar level will start to drop.

But remember, your neurons are on an exclusive glucose meal plan, so they need sugar at the ready. And if your body senses an imbalance in blood sugar levels, it sounds the alarm. A decrease in glucose stimulates alpha cells in the pancreas to release insulin's nemesis--the hormone glucagon. It starts raising the blood sugar level by triggering the liver and adipose tissues to metabolize their fat and glycogen stores, thereby releasing fatty acids, glycerol, and glucose back into the blood.

But if it's been a day or two since you've eaten, for some reason, and you've burned through both your blood glucose and your glycogen stores, and you don't have any sugars to feed your brain; in that case, your body will launch into gluconeogenesis and convert fats and amino acids into glucose, so ATP synthesis can continue in your brain cells. This process is a sort of last ditch effort by your body to protect the nervous system from the damaging effects of low blood sugar.

Now, of course, this whole system has its flaws. One of them is that the whole set up is almost entirely dependent on the proper release and reception of insulin. People with diabetes either don't produce enough insulin, or have abnormal receptors for it, which is why they often have to inject a pharmaceutical version after eating. Otherwise, their blood sugar levels will remain too high and they'll start peeing out large amounts of glucose to try and balance the levels in the blood.

The problem is because glucose is being excreted and not stored, it's not available when blood sugar starts to drop. So, the body has to draw on fat and protein tissues for energy, which is one reason why sudden weight loss can be an early sign of diabetes.

Whether you're diabetic or not, I think that we can all agree that it is important to respect your blood sugar levels and remember to continue feeding the beast. But at brunch, you know, use small plates and try to keep the trips to the buffet line to under five, or so.

Today, you learned how your body uses energy from food, including from cellular respiration, which converts glucose into ATP, glycogenesis which converts it to glycogen, and lipogenesis which converts it to triglycerides. And you also learned how insulin regulates your sugar levels.

Thank you to our headmaster of learning, Linea Boyev and thank you to all of our Patreon patrons, whose monthly contributions help make Crash Course possible, not only for themselves, but for everyone, everywhere. If you like Crash Course, and want to help us make videos like this one, then go to

This episode was filmed in the Dr. Cheryl C. Kinney Crash Course Studio. It was written by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson. It was directed by Nicholas Jenkins, edited by Nicole Sweeney, our sound designer is Michael Aranda, and the graphics team is Thought Café.