By Ronald Hoffman, MD
It's a fat that bats have, but in med school we were taught adult humans lacked it. It's Brown Adipose Tissue (BAT). Found in many mammals, especially rodents, BAT is what keeps them warm because of its accelerated fat-burning capabilities. Unlike the more prevalent yellow blobby ("white") fat that predominates in obesity, it has a rapid metabolic rate.
Babies are known to have brown fat as a means of cold-adaptation because they can't shiver (most warm-blooded animals don't either). But in breakthrough research, the New England Journal of Medicine announced this week that adult humans have it, too.
How does brown fat rev the body's metabolic machinery? The answer is complex, but offers some insights into thyroid physiology. The main source of thyroid hormone, or T4, must be converted to T3 within the mitochondria inside cells to set the rate of metabolism and fat-burning. In brown fat, the T4 to T3 conversion is in overdrive, resulting in thermogenesis, or fat-burning.
This is why complementary docs often favor thyroid replacement with T4/T3 combinations instead of straight T4, as is found in Synthroid. Dosing with pre-formed T3 simulates the conversion that occurs in brown fat. But too much T3 can result in jitteriness and heart arrhythmias.
Overweight rodents are in luck these days. Scientists have developed a slew of prospective weight loss drugs that are dramatically effective in mice and rats. But because rodents have brown fat and humans were said not to have it, the experimental drugs disappoint in humans.
Now research shows that brown fat does not entirely disappear in adulthood. Small amounts of it are present in healthy, trim individuals. In overweight folks with hallmarks of metabolic slow-down like high cholesterol and Syndrome X, it has virtually disappeared.
So what are the practical implications? Are there ways we can enhance brown fat?
Remarkably, the New England Journal researchers have found a partial answer: Just CHILL!
In a series of experiments, they exposed healthy adults to cold temperatures. We're not even talking the Polar Bear Club here. Sitting around in light clothing for a couple of hours at temperatures around 60, or immersing feet in cold (not ice) water produced a detectable increase in brown fat, and it happened within just days!
Now, I'm thinking to myself, am I aspiring to rodent-hood? I make a point of turning off the radiators in winter, opening windows, and dialing down the thermostat to 60 or 62. I challenge myself to frigid runs in twenty-degree temperatures throughout winter. While I don't go so far as to take cold showers, I regularly swim in Shinnecock Bay after water temperatures top 60 around Memorial Day. Maybe I'm stoking my metabolism and building brown fat stores while saving the planet and reducing my carbon footprint!
It may be that one way that modern civilization conspires to make us fat is by shielding us from the evolutionary anvil of cold which honed the metabolisms of our ancestors for millennia. We sit in toasty homes, commute to work in cozy cars with seat warmers, and if we work out, we do so in heated gyms or tepid swimming pools.
Another take-home from the New England Journal research: Certain drugs rev brown fat, especially amphetamines. But their addictive properties and side effects make their use undesirable. According to the articles, natural ephedra works well, too, but it's been banned, despite a track record of mostly safe use among responsible users. EGCG from green tea may also hit brown fat receptors, and it's still available.
Unfortunately, there are drugs that shrink brown fat, thus slowing metabolism. The sad fact is that they are routinely prescribed to tens of millions of Americans. They are the beta-blockers, dished liberally for heart disease and hypertension, and the benzodiazepines, medicine's main remedies for anxiety. Common examples include Inderal, Lopressor, Toprol, Zebeta, Tenormin, and Valium, Ativan, Xanax, Halcion, and Klonipin.
So, for now, barring any major breakthroughs on the fat-drug front, embrace the cold, and try to stay healthy enough to eschew medications that are metabolic downers.