All Fat is not created Equal
A new Nature news story discusses the little known fact that there are two different types of adipose (fat) tissue: white and brown. White fat tissue stores excess calories that are not used for energy as lipids, and typically accumulates around the hips and thighs of the girls, and around the belly of the guys. Simply put, it’s the excess inches we try to get rid of through diet and exercise. Brown adipose tissue (BAT), on the other hand, typically accumulates around the collarbone, shoulder blade, and neck area. Originally thought to only be present in human newborns and animals, BAT is unique in that it burns excess fat calories, as opposed to storing them, to keep the body warm.
However, in recent studies published in The New England Journal of Medicine, researchers found metabolically active BAT in an unexpected place — on human adult volunteers. The studies used Positron Emission Tomography (PET), which measures where consumed radio-labeled glucose is metabolized in the body. Subjects were scanned either at room temperature, or in a cold room (17-19 deg Celsius), while their feet were repeatedly immersed in cold water (7-9 deg Celsius). It turns out that with the cold room and ice-cold foot bath, there was a significant increase in the metabolic activity of the fat tissue around the collarbone and shoulder blades, compared to scans taken at room temperature. Cold temperatures activate the sympathetic nervous system, and epinephrine (adrenaline) is released, which causes the body to warm itself. These results show that in colder temperatures, calories may not be stored on your waist or hips, but rather, metabolized by the brown adipose tissue to keep you warm.
Despite their findings, it’s not suggested you take your lunch and head for the nearest walk-in freezer. But the key finding is that BAT metabolism is triggered by adrenaline, the same hormone responsible for the “fight or flight” response. Therefore, these results open the possibility that new drugs that activate the sympathetic nervous system to release adrenaline may be a viable treatment for obesity.
Adrenaline as a treatment for obesity? I would worry about the long-term effects of excessive adrenaline, similar to the effects of long-term stress. Headaches, insomnia, tachycardia, palpitations, fatigue, elevated cortisol and cholesterol levels – these can hardly be beneficial side effects. Why not focus on massive lifestyle interventions first? Diet, exercise, reducing stress.
Hi Alexandra,
Great comment! You are right — in no way would this be an ideal first-line-of-defense strategy. Lifestyle modification can be very effective — just look at the
success of people on “The Biggest Loser” TV show! However, there are
cases where lifestyle intervention fails, and the authors suggest
pharmacological intervention as one possible alternative. I completely agree, stimulating the production of adrenaline could have very serious consequences and side effects. I think the authors were just trying to describe an early discovery of a new pathway for fat metabolism. An interesting point in the studies was that as BMI increased, the percentage of BAT decreased. So obese people may not even direcltly benefit from cold/sympathetic nervous system stimulation.
Given obese people tend to not have a lot of BAT, the authors talk about 2 ways to use this finding for a new treatment.:
1.) stimulate BAT activity when a person is at risk for obesity (e.g. maybe has screened positive for the obesity gene??). Animal models have shown that groups with high BAT are protected from obesity and diabetes (from Cypress et.al., NEJM 360, pp. 1509-1517)
strains with higher levels of intermuscular brown adipose tissue are
protected from diet-induced obesity and diabetes.9,37 Humans, like
mice, have a range of metabolic rates. Thus, methods to stimulate
generation and activation of brown adipose tissue might lead to new
approaches to promoting weight loss and increasing insulin sensitivity.“
2.) find a way to increase the existence of BAT in obese individuals. This discussion is a bit nebulous in the paper, but they mention the future possibility of coaxing stem cells to become BAT cells.
I chose to blog about this paper to try to generate some feedback on how researchers view epidemics such as obesity, so I am really glad you responded. Many time rmedical esearchers think drugs/surgery/devices first, lifestyle modifications second. This is quite different than the way we preventive medicine advocates reason. Thanks so much for your insights.