Brown fat vs white fat | what is each and how do they differ?

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Aaron M. Cypess, MD, PhD, MMSc Assistant Professor, Harvard Medical School Assistant Investigator and Staff Physician Joslin Diabetes Center

Our recent posts on research being done at Joslin Diabetes Center on brown fat prompted a number of our readers to send in questions on this subject.  We picked three questions that have come up repeatedly and asked Dr. Aaron Cypess if he could help us out.  Here’s the first question.

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 the body 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, they store 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.  Other cells in the body would normallyconvert the fat calories into chemical energy.  Instread, brown fat mitochondria have a unique protein called uncoupling protein 1 (UCP1) that allows its mitochondria turn the fat into heat instead.  Brown fat cells may also release hormones, but they haven’t yet been identified.

A simple analogy is to 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.

Why is research into the two types of fat important?

Brown and white fat cells are part of the regulatory system 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. 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?

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