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What happens to adipose tissue when it has excessive energy?



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One of the good things (or bad things, depending on your perspective) about adipose tissue is that it can store all the energy (in the form of fat) you throw at it. In fact, it's the only energy storage form of the body capable of this feat. So to a fat cell, there is no such thing as excessive energy; give it more fat and it'll store more fat.

Admittedly, the above description of the situation is a little simplified; it assumes that fat cells operate in isolation, which isn't true. Fat cells are very sensitive to hormones circulating in the blood. For example, the above only occurs under the influence of insulin, a hormone that circulates at higher levels during meals. When insulin levels are relatively low (eg, between meals), fat cells take the "excess energy" that was stored during the meal and break it down into usable forms of energy.

There are many other ways to illustrate that adipose tissue does not operate independently of the rest of the body, but I'll only give one more here. In addition to being a major site for storing energy, fat cells are also a major endocrine organ. One major hormone they produce is called leptin. In rodents (and in humans, to a lesser extent), leptin levels in the blood are roughly proportional to the amount of adipose tissue in the body. Leptin has important actions in the brain to regulate hunger and activity level. High leptin levels (which indicate lots of fat is present) suppress hunger, increase activity, and stimulate the breakdown of adipose tissue into usable energy.

These two examples of how adipose tissue is regulated illustrates the important concept of negative feedback, a concept very common in physiology and medicine. When an imbalance occurs (eg, having too much adipose tissue), a signal is made (eg, high leptin levels) that triggers to return of balance (eg, burning fat and returning adipose tissue to its normal amount).