When We Lose Weight, Where Does the Lost Weight Go?

According to the law of conservation of mass, matter is neither created nor destroyed, but it may alter its form through chemical reaction. Essentially, that tells us that while we lose mass in our bodies by burning up fat, it does not just disappear. It simply changes form, like water and steam.

When we eat, the glucose and other sugars harnessed from carbohydrates are the first fuel sources. The liver stores the glucose in the form of glycogen and releases it into the bloodstream as necessary to keep our body trucking along. Think of your bloodstream as an interconnected conveyor belt that takes necessary nutrients to the body parts that need them. Once that glucose runs out, fat takes over. Harnessing energy by burning fat is referred to as ketosis.

Hormones regulating our blood sugar levels activate an enzyme in the blood vessels of fat tissue called lipase. Lipase ignites fat cells to release macromolecules called triglycerides, which are what make fat cells fat. Triglycerides are made of glycerol and three fatty acid chains. When they receive the signal from lipase to exit the fat cells, the triglycerides break up into their respective components and enter the bloodstream for use. The liver snatches up the glycerol to break it down for energy, and some of the fatty acids move to the muscles that can farm them for energy as well.

This action of breaking down triglycerides into usable energy is called lipolysis. Once inside the mitochondria, or power source, or muscle or liver cells, the components of the glycerol and fatty acids are shuffled and reshuffled to harness their energy potential, producing heat, water, carbon dioxide and adenosine triphosphate (ATP). ATP hauls potential energy in its molecular bonds for use when we exercise like cellular carb loading. The water exits our bodies as sweat and urine, and we exhale the carbon dioxide.

Now that the body has relieved fat cells of some glycerol and fatty acids, the cells get smaller. But if the fat tissue shrinks, what happens to our skin?

If our jeans hang loose when we lose weight, it would make sense that our skin would as well. In cases of moderate weight loss, our skin shrinks back to fit the body's new size, thanks to its elasticity. A protein called collagen in the skin gives it this special property, which helps explain how we don't outgrow our skin as we get older. But as we age, these collagen fibers in the skin do weaken, leading to wrinkles.

Collagen does have its limits. Rapid growth or weight gain can outpace collagen production in the skin, causing areas to overstretch. This can lead to striations called stretch marks. These are particularly common with pregnancy and adolescents going through puberty. Although several lotions and creams on the market claim to get rid of stretch marks, most naturally go away on their own.

Sometimes in cases of massive weight loss, people have folds of extra skin left over from their heavier days. Their skin had become so outstretched that it hangs limply from the thinner body, like worn out elastic.

As the number of obese people undergoing gastric bypass surgery increases, so do corrective surgeries for the unwanted and often uncomfortable loose skin. Although it may seem purely cosmetic, extra skin can actually cause infections, rashes and back problems. Since it's a highly invasive procedure, plastic surgeons may space out skin tucks and removals over months or a year. A tummy tuck costs $5,798, on average, according to 2016 stats from the American Society of Plastic Surgeons. That figure does not include the cost of anesthesia and operating room facilities or other expenses.

Even after the work is done, patients wear lifelong scars from the surgeries. Also, this type of body recontouring comes with risks. Since it's a relatively new procedure, doctors only know anecdotal evidence of complications, including dead skin, infection and open wounds [source: Singer]. But as beltlines continue to bulge, the numbers of patients will likely increase as it has the past decade.

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