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Speaker 1: Welcome to brain stuff from how stuff works. Hey, brain stuff,

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Speaker 1: Lauren Vogel Bomb here. For many of us humans, our

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Speaker 1: body size and shape our things in flux from one

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Speaker 1: month to the next, depending on a host of factors

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Speaker 1: both within and without our control. Our genes might be

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Speaker 1: a little tighter or a little looser. Our question of

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Speaker 1: the day is when we lose weight, where does that

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Speaker 1: lost weight go. The short answer is that our bodies

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Speaker 1: convert molecules in fat cells to usable forms of energy,

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Speaker 1: thus shrinking the cells. But getting this to happen isn't

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Speaker 1: just about sweating to the oldies or however you prefer

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Speaker 1: to work out. Understanding how our bodies perform this tummy

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Speaker 1: trimming trick requires a little more detail. We know that

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Speaker 1: weight loss hinges on burning calories. Calories are the measure

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Speaker 1: of the potential energy in the food you eat, in

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Speaker 1: the form of fats, proteins, and carbohydrates. If our bodies

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Speaker 1: were cars, energy would be the gas that keeps everything running.

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Speaker 1: Lounging in front of the television is like cruising the strip,

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Speaker 1: while sprinting around a track is more like drag racing

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Speaker 1: at maximum speeds. In short, more work means more energy

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Speaker 1: is needed, the body uses some of the calories we

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Speaker 1: ingest to digest that very food. Once the food is

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Speaker 1: broken down into its respective parts of carbohydrates, fats, and proteins,

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Speaker 1: it either uses the remaining energy or converts to fat

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Speaker 1: for storage in fat cells and as doctor who fans know,

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Speaker 1: fat cells live in adipose tissue, which basically acts like

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Speaker 1: an internal gas station, storing away fuel reserves. To lose weight,

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Speaker 1: you must burn more calories or energy than you consume

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Speaker 1: to start using up that fuel reserve. Essentially, if you're

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Speaker 1: not ingesting enough calories to fuel your additional work, your

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Speaker 1: body must pull from fat stores. According to the law

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Speaker 1: of conservation of mass, matter is neither created nor destroyed,

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Speaker 1: but it may alter its form through chemical reaction. Essentially,

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Speaker 1: that tells us that while we lose mass in our

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Speaker 1: bodies by burning up fat, it doesn't just disappear, It

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Speaker 1: simply changes form. When we eat, the glucose and other

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Speaker 1: sugars harnessed from carbo high drates are the first things

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Speaker 1: our bodies use as fuel stores. The liver stores the

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Speaker 1: glucose in the form of glycogen and releases it into

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Speaker 1: the blood stream as necessary to keep our bodies trucking along.

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Speaker 1: Think of your blood stream as an interconnected conveyor belt

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Speaker 1: that takes necessary nutrients to the body parts that need them.

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Speaker 1: Once that glucose runs out, fat takes over. Harnessing energy

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Speaker 1: by burning fat is referred to as catosis. It works

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Speaker 1: like this. Hormones regulating our blood sugar levels activate an

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Speaker 1: enzyme in the blood vessels of fat tissue called liepace.

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Speaker 1: Liepase ignites fat cells to release macro molecules called triglycerides,

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Speaker 1: which are what makes fat cells fat. Triglycerides are made

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Speaker 1: up of glycerol and three fatty acid chains. When they

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Speaker 1: receive the signal from liepace to exit the fat cells,

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Speaker 1: the triglycerides break up into their respective components and enter

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Speaker 1: the bloodstream for use. The liver snatches up the glycerol

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Speaker 1: to break it down for energy, and some of the

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Speaker 1: fatty acids move to the muscles that can form them

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Speaker 1: for energy as well. The action of breaking down triglycerides

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Speaker 1: into usable energy is old like polysis. Once the components

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Speaker 1: of the glycerol and fatty acids are inside are muscle

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Speaker 1: or liver cells, organelles called mitochondria shuffle and reshuffle those

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Speaker 1: compounds to harness their potential energy, sort of like a

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Speaker 1: furnace burns would the mitochondria breakdown and recombine those components

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Speaker 1: of our fat cells and produce heat, water, carbon dioxide,

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Speaker 1: and a denisine triphosphate or a t P. A t

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Speaker 1: P halls potential energy and its molecular bonds for use.

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Speaker 1: When we exercise, like intercellular carb loading, the water exits

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Speaker 1: our bodies as sweat and urine, and we exhale the

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Speaker 1: carbon dioxide. Now that the body has relieved fat cells

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Speaker 1: of some glycerol and fatty acids, the cells get smaller,

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Speaker 1: and so cell by cell, our body shape changes. Today's

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Speaker 1: episode was written by Christine Conger and produced by Tyler Claying.

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Speaker 1: To hear more from Christin check out her podcast Unladylike,

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Speaker 1: available wherever you listen to podcasts, and, of course, to

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Speaker 1: get the skinny on this and lots of other topics,

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Speaker 1: visit our home planet, how Stuff Works dot com. H