BATgirl to The Rescue (Part 1)

Brown Adipose Tissue is actually “the good fat” and it’s an active part of the energetic side of your metabolism. Think of all fat as fuel and know that BAT is simply fuel with a built in furnace.  The secret is that a cool environment causes these furnaces to run at full blast and that creates a high “caloric utility bill.”

Let’s spend a little more time understanding BAT and then we’ll talk about some of the ideas behind the science of BAT referred to in The 4-hour body.  Inside your cells are many mitochondria and these tiny organelles are the body’s power plants. They produce ATP, which is ultimately the source of most of your energy.  BAT is a special kind of fat that is just loaded with mitochondria.  That and the vascularity to supply it with nutrients is what makes it “brown.”  Many cold water aquatic mammals, and even small mammals like rodents, have significant BAT to keep them warm.  BAT has this special way of short circuiting the traditional energy cycle and consuming fat to produce heat.

That’s a great thing, but all you need to remember is that BAT is a fuel that comes with a built-in furnace.

geek alert – it’s going to get a little technical.

Humans aren’t loaded with BAT, at least as adults. Our infants, on the other hand are an fat-oddity of nature by any mammal standard. They are born with the most amount of total adipose tissue (15%) of any mammal: more than harp seals, sea lions, reindeer, or elephant seals and  have the largest percentage of heat-generating BAT (1-3%) (2). It’s interesting that we start out better equipped than these other mammals designed to live completely in the cold.

Many studies have show that the BAT is located primarily in the supraclavical are and it is typically activated with cold exposure to make visible.

The problem is that we lose much of our BAT by the age of 10, but what remains of our baby-BAT keeps metabolically active for the rest of our lives. Even as an adult, there may be plenty of BAT to have a significant impact on metabolic rate if regularly activated. The main location for BAT is on the the supraclavicular region around chest and back of shoulders/neck (see diagrams). There is some evidence that we may also have disseminated brown adipocytes throughout our sucutaneous WAT stores.

The heat-generating capability of BAT comes from a evolutionary metabolic inefficiency. Typically, the mitochondria in cells consume energy in the form of free fatty acids (FFA) and glucose to create and store ATP. In BAT these tiny mitochondria power plants are influenced by a special uncoupling protein 1 (UPC 1), which makes the membranes of the mitochondria “leaky” and they release heat instead of storing ATP.

BAT uses only about 10% glucose and so FFA is primarily what is consumed.    Most of what we know comes from studying rodents, but we know that Humans have BAT and that it remains active into adulthood.

So how do they detect BAT? How do you know if you are BATty? It turns out to be pretty complex. By using a PET-CT scan and a specially marked form of glucose (F-FDG), researchers can cause BAT to activate and it shows on the scan. Typically, they do a biopsy at the same time and compare the images to microscopy of tissue.  It’s not enough to simply put the marker in the body, but the BAT also must be activated. This activation is accomplished through cold exposure(3).

Many different methods have been used to induce metabolic activity in BAT. Some studies have used cold rooms, others immersions, and some just use feet/hands. It seems that there are many biological stimuli that tell the body to crank up the heat.

To think about the body and heat production in another way, let’s walk through the various steps of response to cold. Normally as you walk around in every day living, there is an excess of heat given off by the many different biochemical processes of life – walking, breathing…everything living in your body generates excess heat as a byproduct. As long as there is plenty of excess, your body simply keeps what it needs to stay 98.6F/37C and rejects the rest. If it can’t get lose it fast enough, you sweat to speed up the process. This is why sitting in a sauna doesn’t by comparison require any extra energy (accept elevated heart rate and heat stress).

As the difference between the temperature of your body and the environment around you increases, the heat leaves at a faster and faster rate (Newton’s Law of Cooling). Cold doesn’t come in, heat goes out. At some point, and there are many triggers, your body senses the beginning of a potential core temperature drop. It begins a process of now telling all parts of your body to generate heat and begins to shut down non-essential heat “wasters” (hands, toes, nose, ears, etc…).

By shutting down flow to the extremities and to the skin, the body slows down the rate of heat loss.  If this isn’t enough, BAT and other non-shivering thermogenesis (heat generating) activities kick in.  It could be as subtle as a leg twitch, an urge to rub your hands together, or just moving around that solve the heat deficit. If that isn’t enough non perceptible muscle “tone” sets in and all of your muscle fibers start to just tighten thereby using energy and generating the excess heat your body needs.

Comparative PET–CT scans reveal the patterns of 18F-FDG uptake in the same subject from the lean group after exposure to cold and under thermoneutral conditions. Lichtenbelt, WD et al. N. Engl. J. Med. 360, 1500–1508 (2009)

Finally, when this is insufficient, a progression between non-perceptible shivering and full on “Andrew’s in the cold shower” uncontrollable shaking (still laughing Andrew).  What you should see is there is a range of response to the degree of exposure. Your body can generate nearly 4-5 times it’s resting metabolism rate by shaking like a hula bobble-doll on on a wooden roller coaster.

The downside, however, is that many studies have shown that BAT goes down with increasing obesity levels. There doesn’t seem to be any clear indication that it is a cause and effect. Rather, it’s more of an observation that obese individuals tend to have less BAT. It may be  obesity is related to the biochemical make up and your body’s ability to generate excess heat. Many are looking at ways to convert WAT into BAT or to induce the the growth of new BAT. Women seem to have more BAT than men according to Cypress(1) and gram-for gram, BAT is excellent at contributing to your caloric deficit by consuming energy stored as fat.

The good news is that in Part 2, we’ll learn that while BAT is useful if you have it, but it may not be necessary to benefit from thermal loading.  We’ll look at a suprising twist of biophysics and evolution involving mitochondria (remember? – the cell’s furnace), which may allow anyone to exploit the power of cool.

(1) Cypess AM et al. Identification and Importance of Brown Adipose Tissue in Adult Humans, N Engl J Med 2009;360:1509-1517.

(2) Aherne W, and Hull D (1966) Brown adipose tissue and heat production in the newborn infant. J. Pathol. Bact. 91:223–234.

(3) Lichtenbelt, WD et al. Cold-activated brown adipose tissue in healthy men. N. Engl. J. Med. 360, 1500–1508 (2009)