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What is Lost When a Calorie is Burned?

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What is lost when a calorie is burned?

Bart Loews:

According to the laws of thermodynamics, nothing. In terms of your body, it’s not terribly helpful to think in terms of calories, because they’re just a measure of heat.

Let’s drill down to your energy systems and how your muscles work. Your muscles are composed of two strands of proteins: myosin and actin. The myosin has little hooks that grab on to the actin and then pull it in to create a contraction. It does this using ATP.


The myosin head takes on the ATP and shears one of the phosphate molecules off. The remaining ADP binds to the actin and then rotates pulling the actin down. The ADP molecule is then released and a new ATP molecule is taken on. The ADP molecule is then recycled.

This is what is actually happening when your muscles contract. The shearing of the phosphate from the ATP does generate heat and creates a little bit of excess waste, which is where the calorie theory started from, how you heat up when you perform activities.

Okay, so what does that have to do with loss? I’m getting there …

ATP cannot be stored long term. It’s a comparatively large molecule and it is water soluble, so it will break down inside the cells if left unused. Your body has to have a way of creating it on the fly using fuel sources that can be stored long term.

These fuel sources are:

  • Glucose (sugar)
  • Adipose (fat)

Both sugar and fat can be broken down into acetyl CoA in the cell and then turned into ATP using the Krebs (or Citric Acid) cycle.


During this cycle the primary yield is ATP, while the primary byproducts are water and CO2. The CO2 is exhaled while the water is either repurposed in the body or urinated out. This is one of the major reasons why you lose weight: the conversion of sugars and fats into energy.

The other reason is that your body requires a certain amount of glucose to be stored in your liver, kidneys, and muscles for optimal performance. If you don’t have enough glucose through food, your body will synthesize glucose from fats and proteins for storage as well.

This post originally appeared on Quora. Click here to view.

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Big Questions
Why Does Turkey Make You Tired?
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Why do people have such a hard time staying awake after Thanksgiving dinner? Most people blame tryptophan, but that's not really the main culprit. And what is tryptophan, anyway?

Tryptophan is an amino acid that the body uses in the processes of making vitamin B3 and serotonin, a neurotransmitter that helps regulate sleep. It can't be produced by our bodies, so we need to get it through our diet. From which foods, exactly? Turkey, of course, but also other meats, chocolate, bananas, mangoes, dairy products, eggs, chickpeas, peanuts, and a slew of other foods. Some of these foods, like cheddar cheese, have more tryptophan per gram than turkey. Tryptophan doesn't have much of an impact unless it's taken on an empty stomach and in an amount larger than what we're getting from our drumstick. So why does turkey get the rap as a one-way ticket to a nap?

The urge to snooze is more the fault of the average Thanksgiving meal and all the food and booze that go with it. Here are a few things that play into the nap factor:

Fats: That turkey skin is delicious, but fats take a lot of energy to digest, so the body redirects blood to the digestive system. Reduced blood flow in the rest of the body means reduced energy.

Alcohol: What Homer Simpson called the cause of—and solution to—all of life's problems is also a central nervous system depressant.

Overeating: Same deal as fats. It takes a lot of energy to digest a big feast (the average Thanksgiving meal contains 3000 calories and 229 grams of fat), so blood is sent to the digestive process system, leaving the brain a little tired.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
How Are Balloons Chosen for the Macy's Thanksgiving Day Parade?
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The balloons for this year's Macy’s Thanksgiving Day Parade range from the classics like Charlie Brown to more modern characters who have debuted in the past few years, including The Elf On The Shelf. New to the parade this year are Olaf from Disney's Frozen and Chase from Paw Patrol. But how does the retail giant choose which characters will appear in the lineup?

Balloon characters are chosen in different ways. For example, in 2011, Macy’s requested B. Boy after parade organizers saw the Tim Burton retrospective at the Museum of Modern Art. (The company had been adding a series of art balloons to the parade lineup since 2005, which it called the Blue Sky Gallery.) When it comes to commercial balloons, though, it appears to be all about the Benjamins.

First-time balloons cost at least $190,000—this covers admission into the parade and the cost of balloon construction. After the initial year, companies can expect to pay Macy’s about $90,000 to get a character into the parade lineup. If you consider that the balloons are out for only an hour or so, that’s about $1500 a minute.

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