Last week’s Dear Mark discussing cold thermogenesis  got some of you asking about brown adipose tissue. It’s a topic that deserves a full-fledged Primal Primer, especially since the idea of “good” body fat, a term many use to describe brown adipose tissue (BAT), is a foreign one. I mean, we’re talking about body fat here. Who wants it? Everyone I know is trying to get rid of their adipose tissue, not obtain more. It’s what brings many to this blog and what initiates this grand journey toward health and wellness. Even the people who say they “don’t care” about how they look  would rather not have excess body fat, if only because it’s a marker of poor health or hormonal disregulation. We might acknowledge that we technically “need” some body fat to survive, but most of us will pass on any more than is absolutely necessary, thank you very much.
So whenever brown adipose tissue is invoked as the “good” kind of body fat , a body fat that cannibalizes other body fat, flabbers audibly gast. Is such a thing even possible?
Yes. Brown adipose tissue is very different than white adipose tissue. While white body fat can be regarded as an endocrine organ involved in the release of hormones, it doesn’t “do” all that much. It leads a pretty sedentary existence. Brown adipose tissue is metabolically active, however, consuming fat and glucose , increasing metabolism, and generating warmth for the organism as needed. Animals without the ability to shiver or tie scarves around their necks – like rodents and newborns – have lots of brown fat, because that’s how they stay warm – through “non-shivering thermogenesis.” Brown fat is dense with mitochondria , the power plants of cells which normally use fat and glucose to produce ATP . BAT mitochondria use fat and glucose to produce heat, rather than ATP. Thermogenin, or UCP1, is the uncoupling protein within the mitochondria that enables BAT to oxidize fat without producing much ATP.
Until quite recently, researchers assumed brown fat was mostly absent in adult humans. And if adults did have any, it was probably just a vestige from childhood with little actual functionality. In actuality, recent studies show that men  and women  can and do have significant amounts of brown fat, usually located near the neck, the chest, and the upper back, with women tending to have more than men. Rather than being inert, this adult brown adipose tissue is metabolically active with some interesting potential effects:
- If “cold exposure” is indeed a proxy for “brown adipose tissue activity,” as I suspect, it clears triglycerides  from the blood once the fat in the BAT has been depleted.
- It is inversely correlated with obesity. The more body fat you have, the less BAT you have and the less activity you show in the BAT you do have .
- It is correlated with bone mineral density. Low, or nonexistent levels of brown fat are strongly linked to low bone mineral density .
- It is inversely correlated with fatty liver .
That all sounds pretty good, but how do we act on this knowledge? Is there anything we can do to start utilizing brown adipose tissue in our pursuit of health , leanness , and general Primal awesomeness ? Maybe.
If you want to activate BAT, you have to get cold. Seeing as how brown adipose tissue’s primary function is to maintain body temperature, cold exposure activates existing brown fat – it presents the necessary environmental stressor to tell brown fat to start burning triglycerides for energy. A recent study (PDF ) found that while exposing both lean and overweight men to “mild cold exposure” (61 degrees F, or 16 degrees C) activated brown adipose tissue in 23 out of 24 of them, thermoneutral temperatures resulted in zero BAT activity. Your brown adipose tissue doesn’t have much to do on a nice, warm day – nor, for that matter, on a miserably cold day so long as you’ve got the heater on inside.
Get cold, but not so cold that you can’t stand it without breaking down into a shivering mess. Brown fat keeps us warm up until the point of shivering, after which the physical act of trembling warms us and brown fat is deactivated (or down-regulated; it’s not clear whether it gets flipped off or gradually fades away). If you want to activate your BAT and only your BAT, don’t get so cold that you begin to shiver. Eventually, of course, your “shiver set point” will improve, you’ll get used to the cooler temperatures, and you’ll be able to tap into your BAT at lower and lower temperatures. Shivering also burns calories in its own way, but, well, shivering is kind of unpleasant and awful and it requires far lower temperatures. Go for goosebumps.
Although cold exposure is definitely the best way to activate brown fat, there’s also evidence that a person’s brown fat stores mediate the amount of energy they store after eating. Whenever you eat something, heat is generated, both from the physical and enzymatic breakdown of the food and from “diet-induced thermogenesis.” In patients with lower UCP1 expression (remember, UCP1 is the protein that enables combustion in the brown adipose tissue), the thermogenic response to a meal is lessened ; and patients with confirmed brown adipose tissue generate more heat  in response to a meal than patients without brown adipose tissue. Since that heat comes from energy that is not being stored, a greater thermogenic response to food means less (bad) body fat  accumulation.
All this revolves around the activation of existing brown adipose tissue. While that’s important, what about creating new BAT? There are two candidates – chronic cold exposure and exercise .
In rodents, temperature to which the animal is chronically exposed determines the total amount of BAT on the body. Rats in a heated lab will have less brown fat than rats living outdoors. Humans, even those living in cold climates, are rarely exposed to the cold weather. They sleep in heated homes, drive in heated cars, shop in heated department stores, and bundle up with multiple layers for those fleeting moments spent outdoors. It’s even been proposed that the advent of central heating is related to obesity . I suspect that the total amount of human BAT also depends on chronic exposure to cold, especially since one study (PDF ) showed that outdoor workers have more BAT than indoor workers. Acute exposure activates, chronic  exposure creates.
Irisin, the “exercise hormone,” appears to convert white adipose tissue to brown adipose tissue . As irisin increases in a rodent’s blood, energy expenditure increases without an increase in movement or food intake, suggesting an increase in thermogenesis mediated by the converted WAT. Humans also make irisin  in response to exercise, so this could work for us, too.
I don’t think we can ignore brown adipose tissue as a partial player in the metabolic mess we’re in. It’s not the one key to solving the obesity epidemic, but neither is anything else. It’s a piece of the puzzle, a contribution to the whole mess, and it’s completely plausible to think that people are fatter than they have to be because they’re too dang warm all the time. Sure, people have always avoided the cold, whether through central heating or animal pelt, but the way we avoid it today is way different – and far more effective. At any rate, it can’t hurt to give it a shot.
Hopefully, one of these Saturdays I’ll be able to include a recipe for stir-fried veggies in the rendered brown fat of pasture-raised hamster (sorry, hamster lovers; I had to pick a rodent). Until then, let’s hash things out in the comments. Tell me about your experiences with cold exposure, brown fat, and weight loss, or weight gain, or your plans to experiment. Take care!