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.

Why Are Sloths So Slow?

Sloths have little problem holding still for nature photographers.
Sloths have little problem holding still for nature photographers.
Geoview/iStock via Getty Images

When it comes to physical activity, few animals have as maligned a reputation as the sloth. The six sloth species, which call Brazil and Panama home, move with no urgency, having seemingly adapted to an existence that allows for a life lived in slow motion. But what makes sloths so sedate? And what horrible, poop-related price must they pay in order to maintain life in the slow lane?

According to HowStuffWorks, the sloth’s limited movements are primarily the result of their diet. Residing mainly in the canopy vines of Central and South American forests, sloths dine out on leaves, fruits, and buds. With virtually no fat or protein, sloths conserve energy by taking a leisurely approach to life. On average, a sloth will climb or travel roughly 125 feet per day. On land, it takes them roughly one minute to move just one foot.

A sloth’s digestive system matches their locomotion. After munching leaves using their lips—they have no incisors—it can take up to a month for their meals to be fully digested. And a sloth's metabolic rate is 40 to 45 percent slower than most mammals' to help compensate for their low caloric intake. With so little fuel to burn, a sloth makes the most of it.

Deliberate movement shouldn’t be confused for weakness, however. Sloths can hang from branches for hours, showing off some impressive stamina. And because they spend most of their time high up in trees, they have no need for rapid movement to evade predators.

There is, however, one major downside to the sloth's leisurely lifestyle. Owing to their meager diet, they typically only have to poop once per week. Like going in a public bathroom, this can be a stressful event, as it means going to the ground and risking detection by predators—which puts their lives on the line. Worse, that slow bowel motility means they’re trying to push out nearly one-third of their body weight in feces at a time. It's something to consider the next time you feel envious of their chill lifestyle.

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Are Any of the Scientific Instruments Left on the Moon By the Apollo Astronauts Still Functional?

Apollo 11 astronaut Neil Armstrong left the first footprint on the Moon on July 20, 1969.
Apollo 11 astronaut Neil Armstrong left the first footprint on the Moon on July 20, 1969.
Heritage Space/Heritage Images/Getty Images

C Stuart Hardwick:

The retroreflectors left as part of the Apollo Lunar Ranging Experiment are still fully functional, though their reflective efficiency has diminished over the years.

This deterioration is actually now delivering valuable data. The deterioration has multiple causes including micrometeorite impacts and dust deposition on the reflector surface, and chemical degradation of the mirror surface on the underside—among other things.

As technology has advanced, ground station sensitivity has been repeatedly upgraded faster than the reflectors have deteriorated. As a result, measurements have gotten better, not worse, and measurements of the degradation itself have, among other things, lent support to the idea that static electric charge gives the moon an ephemeral periodic near-surface pseudo-atmosphere of electrically levitating dust.

No other Apollo experiments on the moon remain functional. All the missions except the first included experiment packages powered by radiothermoelectric generators (RTGs), which operated until they were ordered to shut down on September 30, 1977. This was done to save money, but also because by then the RTGs could no longer power the transmitters or any instruments, and the control room used to maintain contact was needed for other purposes.

Because of fears that some problem might force Apollo 11 to abort back to orbit soon after landing, Apollo 11 deployed a simplified experiment package including a solar-powered seismometer which failed after 21 days.

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

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