Dear Dr. K;

Dear Dr. K;

As you know I’m overweight and I’m always reading about the health liabilities of fat.  But recently, I saw something about brown fat being good for health.  Can you elaborate?

Yes, I can, or at least I’ll give it my best shot.  Adipose tissue (fat) is an underappreciated and misunderstood vital organ in the human body.  It consists of two types of fat:  white adipose tissue (WAT) and brown adipose tissue (BAT).  In lean adults WAT accounts for 30 to 40% of total body mass in women and 15 to 25% of total body mass in men.  While BAT accounts for roughly 1% of total body mass. 

You are correct about the negative implications of too much WAT but it’s the “too much” which is the operant concept.  The Goldilocks scenario of “just right” pertains to WAT.  Believe it or not children born with congenital absences of fat have multiple severe health consequences and shorter lives.  We all need the right amount of WAT.  It subserves four main functions: thermal insulation, mechanical protection, storage of readily available fuel energy and hormonal function.  Successful pregnancy requires the last two functions.

The problem with obesity is that the individual WAT cells expand from 30 to 100 µm in size. As they do so the swelling compromises blood delivery of oxygen to these cells which causes the cells to set off “fire alarm alerts” through a variety of mechanisms:  altering the expression (function) of more than 1,000 genes, triggering insulin resistance (type II diabetes), releasing a spectrum of inflammation causing chemicals, impairing normal immune function (hence increased risk with COVID), and leading to deposition of triglycerides inside blood vessels and in the liver (leading to vascular disease and fatty liver). 

BAT on the other hand, reduces inflammation in the body.  It also improves insulin sensitivity, thus preventing diabetes.  BAT generates heat (thermogenesis) when we are chilled and in doing so burns calories.  BAT helps improve bone density.  BAT releases a healthy hormone called adiponectin.  Most centenarians (people aged 100 plus) have high levels of adiponectin.  Regular exercise increases our stores of BAT even if the exercise does not lead to weight loss. 

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