I have long been curious about what happens to the body when we gain and lose weight. People talk about ‘fat cells’ but I was not clear as to whether we have a relatively fixed number of such cells and weight fluctuations simply consist of the size of these cells changing, or whether the number of cells also changed. David Prologo, a professor of radiology and imaging sciences at Emory University, has a nice article explaining what is going on when we gain and lose weight.

The normal fat cell exists primarily to store energy. The body will expand the number of fat cells and the size of fat cells to accommodate excess energy from high-calorie foods. It will even go so far as to start depositing fat cells on our muscles, liver and other organs to create space to store all this extra energy from calorie-rich diets – especially when combined with a low activity lifestyle.
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The major problem with this excess fat is that the fat cells, called adipocytes, do not function normally. They store energy at an abnormally high rate and release energy at an abnormally slow rate.

So that explains how we add weight. But what happens when we lose weight? Do the fat cells once created hang around forever and just get smaller or do they get eliminated entirely? It turns out that both occur.

When a person begins and maintains a new exercise regimen and limits calories, the body does two things to “burn fat.” First, it uses the energy stored in the fat cells to fuel new activity. Second, it stops putting away so much for storage.

The brain signals fat cells to release the energy packages, or fatty acid molecules, to the bloodstream. The muscles, lungs and heart pick up these fatty acids, break them apart, and use the energy stored in the bonds to execute their activities. The scraps that remain are discarded as part of respiration, in the outgoing carbon dioxide, or in urine. This leaves the fat cell empty and renders it useless. The cells actually have a short lifespan so when they die the body absorbs the empty cast and doesn’t replace them. Over time, the body directly extracts the energy (i.e., calories) from food to the organs that need them instead of storing it first.

As a result, the body readjusts by decreasing the number and size of fat cells, which subsequently improves baseline metabolism, decreases inflammation, treats disease, and prolongs lives. If we maintain this situation over time, the body reabsorbs the extra empty fat cells and discards them as waste, leaving us leaner and healthier on multiple levels.

Fascinating.