Research Review: Brown fat, the good kind of body fat

By Helen Kollias

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You probably think your fat is something like a smushy blanket.

After all, it just seems to sort of… sit there.

It doesn’t beat like a heart, or move like a muscle, or make funny noises like… ummm… never mind.

Frankly, it’s hard to believe that fat really earns its rent.

Well, I have good and bad news. The bad news is that you have even more fat than you thought.

The good news is that it might be helpful.

For years scientists thought that a specific type of fat, brown adipose tissue (BAT), didn’t exist in adult humans, just in small animals — rodents (mice, rats, etc) and babies — but as we get older we lost our BAT. Turns out they made a boo boo. Adults have BAT.

You’re probably thinking “Woohoo, I have even more fat than I realized! Fantastic!”

Okay, you’re probably not thinking that, but what if I tell you that thanks to BAT you may be able to skip your cardio and just hang out in a cold room?

Well now. Sounds like fat might make that end-of-the-month payment after all.

I know it sounds a bit like an infomercial: FREEZE YOURSELF SKINNY!

Give them a few months and I’m sure I’ll see an ad selling the new cutting edge fat burning technology on TV.  The gadget will be a refrigerator you sit in.

Don’t laugh! It can’t be worse than the vibrating ab machines.  At least you can store your groceries in there if things don’t work out.

"Dudes, I am getting TOTALLY RIPPED here!"
“Dudes, I am getting TOTALLY RIPPED here!”

The other fat

Just like any good soap opera, your body has a good twin and an evil goatee-wearing twin (Note: JB doesn’t have a twin – he can’t be an evil twin).

Everybody knows and hates evil goatee-wearing fat — known as white fat. That’s the fat you squish with the calipers when you measure your body fat and that’s the fat that is in your belly (visceral fat). It’s virtually metabolically inactive (uses no calories), is mostly for storage and protection, but is involved in regulating some hormones too.

Meanwhile, there is the clean-shaven good fat twin: brown fat (or BAT). Unlike the white fat, the twin you just want to go away, brown fat could be your best friend.

Also unlike white fat, brown fat is very metabolically active. 50g can increase your basal metabolic rate (BMR) by 20%! BAT is filled with a specialized type of sub-cellular machines (mitochondria) that make energy – in the case of BAT all that energy becomes heat.

How does BAT burn calories?

Usually, tissues like your liver or muscles use calories for energy to make something like protein, pass a message like a nerve impulse, or physical energy for things like muscle contractions and generating heat. BAT is different. BAT only makes heat.

Energy balance is energy in minus energy out. Energy in is simply and crudely everything you put in your pie hole.

Energy out has three parts:

  • Basal Metabolic Rate: energy you use to be alive. If you were in a coma, in a perfectly temperature controlled room, that would be your basal metabolic rate (BMR).
  • Exercise (physical activity): any voluntary movement you do, whether on a treadmill or brushing your teeth, contributes to this category no matter how small an action it is.
  • Thermogenesis: literally the making (genesis) of heat (therm). For your body to work it needs to stay in a small range of temperature. When it’s cold outside your body keeps you warm. This takes calories. This is where BAT could boost your metabolism.

BMR calculations

Women

655 + (9.6 X Weight (in kg)) + (1.8 X Height (in cm) ) – (4.7 X Age (years))

Men

66 + (13.7 X Weight (in kg)) + (5 X Height (in cm) ) – (6.8 X Age (years))

Research question

This week’s review explores a brand new study that looked whether BAT was activated in the cold.

van Marken Lichtenbelt WD, et al. Cold-activated brown adipose tissue in healthy men. N Engl J Med. 2009 Apr 9;360(15):1500-8. Erratum in: N Engl J Med. 2009 Apr 30;360(18):1917.

Methods

The BAT-men

This study examined 24 young men — ten lean men (BMI<25) and fourteen obese men (BMI≥25) and tried to figure out how much BAT they have and what happens when they were cooled to 16°C (61°F).

How did they figure out how active BAT was?

This is the hard part.  They injected a radioisotope (18F-flurodeoxyglucose) that BAT likes to absorb (normally BAT takes up glucose) into the participants. Then they did a PET scan and a CT scan.

The more active BAT was, the more of the radioisotope it would absorb. The more BAT and the more active the BAT is, the more dark gray/black you see on the PET scan (Figure 1).

bat-pet-scan-figure-1
Figure 1: Comparison of normal and cold temperature

Results

Of the 24 men, 23 had a definite BAT activity around the neck, collarbone area and abdomen. All lean men had an increase in BAT activity when they were “chilled” and 13 of the 14 obese men also responded to attempts at making them popsicles.

That’s a bit surprising, since for years nobody thought there was much BAT in adults, let alone that it might be active.

What is really cool is that there was a difference in BAT activity between the lean and obese men. In fact, the higher the guy’s BMI was, the less BAT activity he had!

For those who have issues with BMI, the same thing was found using percent body fat (Figure 2). The higher the percent body fat, the lower the BAT activity.

Another interesting thing was that resting metabolic rate was lower in the obese guys (even after correcting for fat-free mass) than the lean guys.

bat-pet-scan-figure-2
Figure 2: BAT activity by level of body fat (%)

Conclusion

There are two main points in this study:

  1. Adults have active BAT.  This is revolutionary as far as human physiology is concerned. Discovery of basically another tissue in adult humans doesn’t happen very often anymore.
  2. There is a difference in BAT between lean and obese men. Is this the cause of obesity or the results?

What you see on CSI is not science

First, before I get into the differences in BAT between lean and obese people, I have to point out that people have the wrong idea about science and that’s why they get so confused over all the contradictions they hear in the media and scientific studies that seem to conflict.

Between the media, CSI and scientists themselves, science is portrayed as “the truth”, but it’s really “the pursuit of figuring out what the heck is going on.” Some stuff has been around for a long time and everybody is pretty confident that it’s true — like gravity, for example. Other things not so much — for example, what causes cancer.

Are people in cold climates leaner?

The next obvious question is: if BAT is more active in the cold and that makes people skinner are people skinner in colder climates? Or is it that some people are genetically predisposed to more or less BAT and that makes them skinner?

Bottom line

Before you decide to go outside your bathing suit during the winter I suggest you wait and see if more studies prove that hanging out in the cold actually works for fat loss.

Update June 12 2009:

From Brown Fat: Where It’s At on ScienceNews.org:

At the Endocrine Society meeting in Washington, D.C., this week, researcher Aaron Cypess’ group will report finding that some people develop substantial stores of the somewhat preferable brown fat cells — but not where we’d usually expect to find fat. At least in adults, Cypess reports, brown fat develops predominantly around the neck, across the shoulder blades — and, if there’s enough of it, in a band that can spill down into the chest.

So far, his group has turned up 106 people — 5.4 percent of those screened — hosting “significant amounts” of brown fat. Women were twice as likely as men to fall into this group. Other characteristics of adults with substantial brown fat: being relatively young, lean and not taking beta-blocker drugs to manage high blood pressure.

Hosting 50 grams of metabolically active brown fat can burn 300 to 500 calories a day, Cypess told reporters at a briefing this afternoon. That’s the equivalent of jumping rope for a half hour or swimming for 45 minutes. Obviously, then, increasing stores of brown fat — or the activity of this fat — can help counter obesity, Cypess says.

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