Our posts on research being done at Joslin Diabetes Center on brown fat prompted anumber of our readers to send in questions on this subject. We picked some questions that have come up repeatedly and asked Aaron Cypess, M.D., Ph.D., if he could help us out.
Brown fat vs white fat—what is each and how do they differ?
The job of a white fat cell (“white adipocyte”) is to store fat calories for future use. The fat is stored in a single fat droplet that grows and shrinks depending on how much fat is being stored.
Insulin is a hormone that tells the white fat cell to take up glucose and fat from the blood and store them as fat. Hormones like epinephrine (a.k.a. adrenaline) norepinephrine (a.k.a. noradrenaline), glucagon, cortisol, and growth hormone all tell the white adipocyte to break down its fat stores and release them into the blood.
White fat cells also release hormones into the blood, such as leptin, which tell the rest of the body how many calories it has available in its storage depot.
When there is too much fat stored in the cell, it starts to release other hormones that can cause insulin resistance and diabetes.
The job of a brown fat cell (“brown adipocyte”) is to generate heat. To do that, it stores fat temporarily so that it can be used as a fuel source. When a body senses cold (and possibly other messages), signals from the brain are sent through the sympathetic nervous system (the “fight or flight” system) to activate a brown fat cell.
Specifically, the nerve from the sympathetic nervous system releases norepinephrine, which binds to a protein on the surface of the brown fat cell, and triggers a series of reactions inside the cell.
The fat inside is broken down from larger building blocks and fed into the mitochondria (the part of the cell that produces energy). Other cells in the body would normally convert the fat calories into chemical energy. Instead, brown fat mitochondria have a unique protein called uncoupling protein 1 (UCP1) that allows its mitochondria to turn the fat into heat instead. Brown fat cells may also release hormones, but they haven’t yet been identified.
A simple analogy is a tanker truck vs a sports car. Like white fat cells, tanker trucks store fuel, to be used by other cars to keep them going. Like brown fat cells, sports cars also have some fuel, but it is kept only temporarily for the purpose of being burned up. Unlike cars, brown and white fat cells are both “green” when it comes to energy use.
Where do the two types of fat come from, and where do they go?
White fat grows when we eat more calories. Our bodies are designed to store an enormous amount of calories, which allowed us to go days or even weeks without regular meal without dying from starvation. After we eat, the calories from carbohydrates and fats are stored mostly in the liver and muscle, but when those stores are filled up, white fat takes over and stores the rest.
Insulin, which causes white fat cells to take up calories from the blood, in the long-run, causes pre-fat cells lying around the white fat cells to become mature white fat cells. So we get “fat” two ways – our fat cells get bigger, and we also make more of them.
Brown fat grows in a similar manner. Norepinephrine, which turns on brown fat in response to cold, also sets in motion signals that cause brown fat cells to grow. These signals also turn pre-fat cells into brown fat cells.
Recent studies, including ours, showed that brown fat is frequently active in children even when they are not cold, suggesting that brown fat activity is important for normal living.
In an average person, the body needs 2000 Calories a day to function. If that person eats more that is required, then the calories get stored in the fat and weight gain is seen. On the other hand, if one is more active, eats less (or both), then the body consumes the calories in the fat.
Most of the energy in the fat is used to power the other cells of the body – muscle contraction, kidney filtering, heart pumping, nerve conduction, etc. But some of the fat is lost as heat, even in normal cells. Brown fat cells are special because they can turn a much higher percentage of the fat they use into heat.
Why is research into the two types of fat important?
Brown and white fat cells are part of the regulatory system that the body uses to make sure there are enough calories to function. Twenty years ago we first learned that white fat does more than store calories – it also releases hormones and can cause disease when there’s too much of it.
It was only two years ago that we learned that brown fat even had a function in adult humans. The big questions right now are 1.) does brown fat have the ability to burn enough fat calories to contribute to weight loss and lower blood sugar levels, and 2.) does brown fat releases hormones too?
Can we harness the power of brown fat for medical purposes?
Studies at Joslin have already demonstrated that increasing the amount of brown fat is enough to help prevent weight gain in mice that are offered a diet high in fat and carbohydrates – a typical “Western” diet. Other studies have shown that activated brown fat in mice can burn up more than half of the fat and glucose that they consumed.
The challenges facing us are identifying safe and effective therapies for people that increase the amount of brown fat in the body and then activate it. We believe there is a real possibility that there will be real treatments arriving in the next five to 10 years, so stay tuned!