![]() The cells look for all intents and purposes like brown fat cells, and like brown fat cells they express UCP1. These cells, resembling brown fat embedded in white fat reserves, were first observed in the 1980s, but it was not until 2012 that researchers came to understand their unique nature. Even more intriguing, the number of such cells can change in response to an individual’s metabolic needs. In fact, this third type appears different from the well-known forms of both brown and white adipose tissue. Another, newer part of the story, however, has emerged in just the past couple of years, with the discovery that the tissue hailed as brown fat in healthy adult humans does not have the same composition as the classical brown fat cells of newborn babies. What we have described is only one part of the story-the part explaining the function of brown fat in general, along with the relatively recent realization that brown adipose tissue, or something resembling it, exists in human adults. In this way UCP1 accelerates fuel consumption, while at the same time releasing energy as heat instead of storing the energy as ATP. Aptly named uncoupling protein 1 (UCP1), this channel conducts protons from the outside to the inside of the brown fat cell mitochondrion, thus bypassing proton movement through the ATP synthase. When the mitochondria of brown fat cells engage in thermogenesis, the trick they play is to allow the protons that are pumped out during respiration to move down their inwardly directed gradient through a proton channel distinct from the channel in the ATP synthase. These mitochondria are said to be coupled, because the combustion of carbon compounds is tightly linked to the production of ATP. When protons flow back through the ATP synthase, the electrochemical energy (the same type of energy stored in an electrical battery) in the proton gradient is converted to chemical bond energy by the combination of ADP with Pi to give ATP. This difference then powers the synthesis of ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi) when protons come back across the inner membrane through a channel in the enzyme (ATP synthase) that is responsible for catalyzing this reaction. The energy released by the oxidation of foodstuffs is used to pump protons across the innermost membrane of the mitochondrion from the inside to the outside to set up a proton gradient. In muscle and most other healthy cells, the energy released by the burning of fats, carbohydrates, and proteins is used, in part, for the synthesis of adenosine triphosphate (ATP), the universal energy currency of living things. Inside the cells of animals and plants, mitochondria-double-membraned, rod-shaped structures-serve as the power plants for turning foodstuffs into respiratory energy. ![]() These reserves then respond like a furnace switched on by a signal from the thermostat: The hormone noradrenalin, released from the terminals of sympathetic nerves, binds to receptors on the surface of brown fat cells, prompting them to fire up thermogenesis and distribute the heat generated throughout the body via the bloodstream. If it senses a sustained drop below the physiological set point of about 37 degrees Celsius (98.6 degrees Fahrenheit), it sends excitatory signals through the sympathetic nervous system to brown fat reserves. #SKINNY FAQT SKIN#Like the thermostat of a home heating system, thermoreceptors under the skin and in the body core transmit an electrical signal-in our case, through thermosensory neurons-to a part of the brain known as the hypothalamus. To take a very loose analogy, if we think of the mammalian body as a dwelling, white fat is something like a combination of the home’s fuel reserve and insulation.īy extension of this analogy, brown fat is the physiological equivalent of the furnace in a home heating system that is switched on when the temperature of the home falls below a set point. Without the calorie reserves stored in white fat, our ability to minimize heat loss and to subsist between meals would be severely limited hibernation, for those species that eke out their stored fat to survive the winter months, would simply not be an option. ![]() Although the calorie-dense diet popular in many Western societies, in combination with a sedentary lifestyle, is now leading to an unprecedented epidemic of obesity, we should not lose sight of the fact that throughout most of our evolutionary history the ability to hedge against starvation by laying down white adipose tissue has been crucial to human survival. ![]()
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