Five Things We Know About Fat

Jules Hirsch, a pioneer in obesity research, died in 2015. When he began his career, the dominant belief was that people were overweight because they ate too much—pure and simple. Thin people restrained themselves. People who were fat had no self-control. Fat was a passive repository of stored energy.

We know now that this is not true.

Hirsch recognized that fat is an active organ that has major effects on our metabolism by producing hormones that change how our body stores fat. He also discovered that while the number of fat cells in our body remains fairly constant, their size can change considerably. This is important in understanding why it is so easy to regain weight we have worked so hard to lose.

Hirsch focused on his patients and their needs, trying to understand their struggles to maintain a healthy weight—often fighting what seemed like a losing battle with their bodies. In honor of his life's work, five facts about fat:

• Fat storage varies a lot day-to-day—even hour to hour. When we eat and metabolize food, it raises our blood sugar. This is monitored by our pancreas, which tries to keep it within a fairly narrow range. When blood sugar gets low, it sends signals to brain telling us we're hungry. When it is high, it secretes insulin. Insulin does two things. First, it takes that glucose in our bloodstream and turns it into fat that gets stored in our fat cells (adipose tissue). Second, it stops us from burning fat for energy for around two hours. We can draw on that stored fat—potential energy—to use to do what our bodies need to do: work muscles, think, build new bones. So much of the fat we store is burned the same day it is deposited. It is only when we burn less than we store that we get a slow buildup of adipose tissue. If we're already a healthy weight—that can be a problem.
• Fat controls our metabolism through leptin. Fat is an organ and it fights to maintain itself. Fat secretes the hormone leptin. It opposes the hormone that makes you hungry—ghrelin—and tells you that you are full. It also speeds up metabolism so that you burn calories faster. When you are heavy, your body becomes less and less efficient, adding relatively less fat per calorie consumed. When you get thinner, your fatty tissue decreases. This has three effects: it reduces leptin, makes you hungrier, and slows down your metabolism so that you add more fat per calorie. Leptin is the reason that when you're dieting those first few pounds peel right off and it gets tougher and tougher when you get closer to your goal.
• There are different types of fat — brown fat, white fat, and visceral fat. Brown fat keeps your warm. Children and people who are lean have relatively more brown fat and relatively less of other fat types. When brown fat starts working to produce energy for your body - which is the point of fat - it burns white fat. Brown fat makes up a very small proportion of our body fat, but it is active, burns white fat and uses a LOT of calories.

So what is white fat? White fat stores energy for later use. Fat cells that are small (have little stored energy) produce a hormone called adiponectin that communicates with the liver and insulin-sensitive tissues, reducing the risk of diabetes. When fat cells become large, with lots of stored energy, they reduce adiponectin production, making tissues resistant to insulin and increasing diabetes risk.

Subcutaneous and visceral fat are the types of fat under the skin (subcutaneous - primarily in the thighs and buttocks) and in the abdomen. They increase the risk of heart attack and stroke. Visceral fat also has a large effect on insulin-resistance. Visceral fat produces most leptin.

• Fat fights to maintain itself - which makes it really hard to lose weight. Fat begets fat. "Full" large fat cells produce leptin. Because it is leptin that quiets the hunger hormone, this means when you start getting thinner and your fat cells shrink, your leptin decreases and you are hungrier more of the time.
• People who maintain lower weight after substantial weight loss exercise a lot to do so. Fat fights dieting in another way too. When you lose fat, you lower your leptin. Leptin speeds up your metabolism so that you add more fat per calorie. Leptin is the reason that when you're dieting those first few pounds peel right off and it gets tougher and tougher when you get closer to your goal.

What Hirsch taught us about fat is both discouraging and optimistic. On the one hand, it shows just why it is so very hard to lose weight. Not only do we have to control our behaviors and get out of the habits that make us eat more and exercise less, our body is fighting us too. As we try to lose weight—and successfully do so—our bodies hold on more fiercely to every calorie and make us hungrier. If weight loss were easy, everyone would do it.

Optimistically, though, Hirsch demonstrated that some people really just do have an easier time staying slim than others: it's not just a matter of willpower, it's also a matter of genes and how our bodies work.

And, as in almost all complex systems, the truth is a complex combination of both.