Artstract by Allegra Bentrim
Leptin and Insulin in Metabolic Homeostasis
Recent neuroscience has uncovered mechanisms of how unhealthy eating habits affect our brains. Eating a diet high in saturated and trans fats leads to long term potentiation of reward pathways that eventually cause morphological adaptations of the neurons involved. The melanocortin circuit is heavily influenced by what we put in our bodies. This pathway including the hypothalamus in the brain involves two important metabolic hormones called insulin and leptin. Insulin is a hormone synthesized in and secreted from the pancreas. It is commonly associated with metabolism, healthy energy use, and healthy energy storage. Its primary job is to facilitate the entry of glucose into cells to be broken down for energy use or stored as glycogen when blood sugar levels are elevated after we eat. When it does not do its job correctly, the body is unable to maintain blood glucose homeostasis (this is what goes wrong in diabetes). Leptin is largely produced in and released from fat cells in the body, and tells the hypothalamus in the brain that the body is no longer hungry. This hormone helps the hypothalamus to regulate hunger and satiety by monitoring the amount of available energy and adipose stores. When you have just eaten, the fat cells in your body will release leptin to tell your brain that it is not hungry anymore, and the hunger response will be inhibited. Both insulin and leptin are circulated at levels that match the body’s current nutritional state. They are both released when the body is at high levels of energy availability. Both insulin and leptin are affected by the foods we eat, and a diet consisting of a lot of high fat foods will eventually cause insulin and leptin resistance. The biggest consequence of insulin resistance is the development of type II diabetes, and occurs when the amount of energy consumed overcomes the effectiveness of the hormone (see my earlier post about Type II Diabetes for more information!). Leptin resistance, on the other hand, causes the melanocortin circuitry to shift such that your brain can keep telling you that you are hungry even after you have consumed enough energy. Individuals who experience leptin resistance lose the ability to discriminate between feelings of hunger and feelings of satiety.
High Fat Food as an Addiction
The hormone leptin does a good job of maintaining the state that your body is at: when it works, it prevents hunger signals from being sent when you are not hungry. However, the amount of leptin released is proportional to the amount of body fat on an individual because it is made in and released from adipose tissues. This makes weight loss hard because a loss of weight decreases the satiety signal that leptin produces which in turn means that an obese individual trying to lose weight would experience stronger feelings of hunger. Just like with conventional addictions, withdrawals from increased hunger signals and high fat foods is difficult to overcome. Obesity and food disorders are made more difficult to manage because every body needs food to live and our society markets the high fat foods cheaper and more accessible than healthier foods.
How full are you?
Your body will start sending signals to stop eating when it is full, regardless of the caloric intake of the food you have eaten. When you eat 400 calories of dense, high fat foods like burgers and fries, they take up less space in your stomach, and as a result you feel less full from that amount of food compared with the same caloric value of vegetables and leafy greens. 400 calories of kale looks a lot different than 400 calories of pizza! The kale is less dense and will be more spread out in your stomach. Because a lower number calories of veggies fill you up than high fat foods, a meal with high fat content will leave you feeling a lot less satisfied. If you eat until you are satisfied, then you will have consumed many more calories to feel full.
