Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. How on earth does this happen?
Smaller amounts of energy are stored in your liver and muscles as glycogen. When you diet , you take in fewer calories than your body needs. Because of this deficit, your body turns to fat reserves for energy. Your muscles first burn through stored glycogen for energy. They also recommend weightlifting and resistance training. One of the best-characterised physiological systems involves leptin, a protein secreted by our fat stores adipose tissue — that tells the brain that there is plenty of energy available stored in the form of fat.
When discovered, there was a great deal of excitement about whether leptin could be administered as a treatment for obesity — perhaps in an injection that would trick the body into thinking that there were large amounts of fat available so that we do not eat as much. Sadly, these potential treatments were not effective and we now understand why. As we store more and more fat the leptin level in our blood will increase in proportion to the increase in stored fat. Our brains get used to this higher level of leptin, so administering more leptin over and above this higher level of leptin does not seem to help.
Instead, it is when leptin levels fall that leptin becomes a very important signal. When we try to lose weight, there is a disproportionately large fall in circulating leptin in spite of only modest fat loss. A fall in leptin is an attempt to defend fat stores with leptin functioning as the signal to the brain in a negative feedback loop that maintains the stability of fat mass. Reduced leptin is also a trigger for depressive symptoms in animals. So, when we try to lose weight, our fat tissue sends signals to the brain to try to resist any further loss of fat; we feel hungry, we seek rewards, and we might feel a little down or depressed.
What can we do about this? Well, a recent study indicated that fat tissue may have some other properties that we might be able to manipulate to help. Some specialised types of fat cells adipocytes have the ability to burn energy to help keep us warm a process called thermogenesis. In these cells, under stimulation by insulin, fatty acids are made into fat molecules and stored as fat droplets.
It is also possible for fat cells to take up glucose and amino acids, which have been absorbed into the bloodstream after a meal, and convert those into fat molecules. The conversion of carbohydrates or protein into fat is 10 times less efficient than simply storing fat in a fat cell, but the body can do it. If you have extra calories in fat about 11 grams floating in your bloodstream, fat cells can store it using only 2.
On the other hand, if you have extra calories in glucose about 25 grams floating in your bloodstream, it takes 23 calories of energy to convert the glucose into fat and then store it.
Given a choice, a fat cell will grab the fat and store it rather than the carbohydrates because fat is so much easier to store. When you are first born, your body does not have much white fat to help insulate and retain body heat; although there are white fat cells, there is not much fat stored in them.
Brown fat cells are somewhat smaller than white, are composed of several smaller fat droplets and are loaded with mitochondria , which can generate heat. A newborn baby produces heat a process called thermogenesis primarily by breaking down fat molecules into fatty acids in brown fat cells. Instead of those fatty acids leaving the brown fat cell, as happens in white fat cells, they get further broken down in the mitochondria and their energy is released directly as heat.
This same process occurs in hibernating animals, which have more brown fat than humans. Once the newborn baby starts eating more, developing layers of white fat, the brown fat goes away. Adult humans have little or no brown fat.
0コメント