Fat storage is a way for mammals to manage energy allocation. Fat cells are not simple storage depots, but are metabolically very active. They respond to and produce hormones and other messenger molecules. A certain amount of body fat is essential for mammals, especially humans who have to maintain a large brain which uses a lot of energy. In adult humans 20% of basal metabolism is devoted to maintaining the brain. For infants, that number is closer to 80%. Fat storage gives humans the flexibility to manage environmental stresses such as variation in food supply.

In most of human history there was a balance between energy intake (eating) and energy output. Energy output includes energy to keep the body intact (basal metabolism), energy required to digest food (thermic effect of food) and exercise. Some of the energy from eating goes into fat storage, but the quantity of body fat is also kept in balance. This balance of energy intake, fat storage, and energy output is maintained by a complex feedback system. Most of the research into how energy balance happens has been done in animals, and we still don’t understand exactly how that translates to humans, but we are able to sketch the broad outlines of how the system works. This system involves messenger chemicals and the reward system in the brain. I won’t go into all the complexities here (It is so complicated it would make your eyes cross) but I will give you the simple version.

Leptin

Fat cells produce a hormone called leptin. The amount of leptin they produce is proportional to the amount of fat storage. Leptin decreases appetite and increases satiety (feeling that we have had enough to eat).

Ghrelin
When the energy output is greater than the energy input fat storage decreases and leptin decreases. The stomach makes a hormone called Ghrelin, which increases appetite. We then eat more food until the body fat gets back to the balance point.

The brain reward system

Humans evolved to find sweet high calorie density foods intensely pleasurable. That is probably because such foods were rare or at least seasonal for our hunter-gatherer ancestors. These kinds of foods cause dopamine release in the brain. This is the same system that makes drugs of abuse pleasurable. Eating in general stimulates this system to some extent. From an evolutionary standpoint eating being pleasurable has obvious survival benefit.

How fat storage gets out of balance

Our hunter gatherer ancestors took in lots of calories, but they also burned lots of calories in the hunting and gathering part. In our modern society we don’t have to exercise very much to get our food, so our daily calorie expenditure is a lot less on average than our hunter gatherer ancestors. Moreover, we have the big food industry which has learned very well about the kinds of foods that stimulate our brain reward system the most. The more calories we eat, the bigger their profits. The foods that maximize their profits are calorie dense processed foods with lots of sugar and starchy carbohydrates. We live in what the scientists call an “obeseogenic” environment. The easy (and cheap) availability of high sugar, high carbohydrate foods maximally stimulate the brain reward centers and overwhelm the rest of the energy and fat regulatory system. The result is that many of us increase fat storage up to the point that the extra calories it takes to move our large bodies around get most of us into balance again, but at a much higher level of body fat. Some unfortunate people continue to gain fat until they become disabled by the huge amount of body fat they carry. As I discussed in the previous post, that increased percentage of body fat increases the risks for heart disease, diabetes and cancer.

Addressing the epidemic of overfatness

While I intend to focus later on what individuals can do to lose fat permanently the thing that all of us need to do is to work at a political and social level to change the ”obeseogenic” environment. That will include things that increase the expense of high sugar high carbohydrate foods (like voting for taxes on sweetened beverages) and decrease the expense of healthy unprocessed foods like fresh fruits and fresh vegetables (like allowing food stamps to be used at farmers markets). It is unconscionable that ”food deserts” exist in some communities so that the only foods available are at convenience stores that sell only obeseogenic foods. We also need to work toward changing the exercise environment so that all communities (not just the wealthy ones) have safe walking trails and exercise facilities. All of us, fat or thin, need to do what we can to change the food and exercise environment.

The biology of fat loss

The only way to lose fat is for the calories we eat to be less than the calories we burn. That is a truism, but it is not very helpful in creating strategies for successful fat loss. So what does the body do when we change our energy balance to less energy in than energy out?

The first thing that happens when we have a negative energy balance is that leptin, the satiety hormone secreted by fat cells goes down. It actually goes down before we lose any fat. Ghrelin, the appetite enhancing hormone goes up.

As the body starts to lose fat several other things happen. The basal metabolic rate decreases. Muscles get more efficient, so the calories we burn per minute of exercise goes down as well. All these changes decrease energy output to try to match the decrease in energy input. This means that fat loss starts to slow down and eventually stop, even though we are eating the same number of calories that caused fat loss in the beginning. Not only does energy output decrease, the body sends powerful signals that increase appetite that make it more and more difficult to keep from increasing our calorie intake. The end result is that 90% of people who lose a lot of fat gain most or all of it back eventually, most within a year. Even more depressing, the low metabolic rate persists even after the fat is regained. Contestants in show The Biggest Loser still had low metabolic rates 6 years after the show, when almost all of them had gained back the fat they lost. In fact, their basal metabolic rates were even lower than when they lost the fat.

I’m sure at this point after hearing all this you are depressed and are saying to yourself, ”what is the point of even trying to lose fat.”

Bad News and Good News

The bad news is that all these mechanisms that operate to make you regain the fat you lost are proportional to the amount of fat you lose. Very fat people who lose enough fat to get close to ideal body fat have an almost impossible time staying there. The body mechanisms that work to bring the body back to the original amount of body fat are just too powerful to resist.

The good news is that permanent fat loss of around 10 percent of your body weight is possible by making permanent changes in your eating and exercise habits. The other good news is that modest fat loss makes a big difference in your risk of high blood pressure, diabetes, heart disease and cancer.

In the next post I will talk about the unconscious mechanisms that contribute to over eating. I will also talk about setting reasonable goals for weight loss and how to go about making permanent changes in your diet and exercise.