I know that exercising will help me ‘build’ muscle. I also know that that’s a kind of weird way to describe a systematic tearing and repair of muscle tissue, eventually resulting in more muscle mass overall. I should also be able to ‘burn’ fat or ‘lose’ weight, although those terms are a little more opaque. It sounds like the amount of fat in my body will be reduced, but does that mean smaller fat cells? Fewer fat cells? What does it mean to get fat in the first place, and why do our bodies even bother in the first place?
Storing energy to ensure survival
Fat does a number of jobs in an animal’s body, from providing insulation from the elements to padding or protecting more sensitive anatomy deeper in the body. Most humans aren’t hitting the gym because they’re too comfortable swimming in cold water though. The issue we struggle with today is the way fat can store energy. It’s an ability that has helped organisms survive intermittently-low food supplies for millions of years, but thanks to modern farming and food storage, is basically working better than our bodies really need. As we eat more food than we can use in a day, our bodies try to store extra energy in fat cells a hungrier time that just never seems to arrive.
To store energy, your body packages excess sugars into molecules called fatty acids, stuffing them into fat cells for storage. As your personal energy reserves continue to grow, your body will increase both the size and number of fat cells you’re carrying. As normal deposits are stuffed full, fat cells will even get deposited on muscles and major organs, leaving you with more fatty acids than you’re likely to need in most modern circumstances.
Saving more than your body can spend
Carrying fat obviously doesn’t make you immune from hunger, and your body seems to only tap into these reserves of fatty acids in certain circumstances. Highly aerobic activities, such as fleeing from a predator on foot, is one way to gain access to the energy stored in fat. On a longer timescale, reducing your overall calorie intake can also convince your body to start cracking open those cells to make use of your stored energy. This is done by releasing fatty acids into the bloodstream so your muscles, heart and lungs can literally break them apart to make use of the energy stored in their molecular bonds. The remaining molecular debris, as well as the emptied fat cells, are eventually expelled from the body in both our breath and urine as a waste product. So you sort of ‘burn’ fat, but you’re also breaking and exhaling it.
As efficient a system as that may be, it only seems to work when we regularly dip into our fat reserves. Once we have an excessive number of fat cells, they weirdly become harder to use. These cells, known as adipocytes, are often oversized and produce inflammation-causing hormones. They also end up storing extra energy, and releasing it to our muscles and organs at an abnormally slow rate. In a way, they become too good at their jobs, hindering the original function of fat cells as a way to get through lean times.
Source: How does your body 'burn' fat? by David Prologo, The Conversation