- insulin is a hormone secreted to lower blood sugar levels
- insulin is a body wide signal for growth
- high levels of insulin promote the storage of energy in the form of glycogen and triglycerides (fat)
- high levels of insulin BLOCK the breakdown of fat
- insulin is affected by type of food, timing of food, exercise, sleep, and many other lifestyle factors
Insulin is one of the most important molecules in our body. Remember that hormones are molecules secreted by one part of the body in order to communicate a message to another part. They are able to relay information through the bloodstream, allowing systemic responses to certain environmental conditions. Blood sugar is one of the most tightly regulated parameters in our body, as we run into serious problems with both high and low blood sugar levels. Insulin is a hormone secreted by the pancreas when elevated blood sugar has been sensed. Although insulin is one of our body’s primary tools to keep our blood glucose (sugar) in check, it is not a master tool. Insulin only acts to lower blood sugar levels. Typically in response to eating, our blood sugar levels rise. This is when insulin is excreted from the pancreas into the bloodstream. Once insulin is flying around our blood vessels, it starts screaming its message to all the cells it comes into contact with, and its primary message is: Energy is available! GROW, STORE ENERGY, and GROW MORE!
Throughout all levels of biology, a primary task of the organism is to sense energy availability. In the evolutionary world, energy was always hard to come by, so the ability to detect available energy was a crucial advantage that essentially all organisms developed. It would be a catastrophic failure for an organism to try to grow and divide while resources were scarce, and it would be an equally fatal mistake for the organism to fail to grow and store energy when the resources were available. As it turns out, the molecular switches that control this decision of anabolism (building) versus catabolism (breaking down) are often central to our health and longevity. There are a handful of these high level decision makers in our body, but today’s post will focus solely on insulin.
First we must keep in mind the big picture: when insulin is in the blood, it is a body wide signal for anabolism or growth. From here we can zoom in on some of the details of insulin’s action. As we mentioned above, a primary task of insulin is to lower blood glucose levels. When insulin comes into contact with muscle cells and fat cells, it induces a specific effect, essentially unlocking the cell for glucose entry. When a muscle or fat cell grabs (binds) a molecule of insulin from the bloodstream, a cascade of events is set off inside the cell. The end result of this process is the the insertion of the GLUT4 transporter into the cellular membrane of a muscle or fat cell. A quick digression on cellular membranes; these are structures that form the boundary of cells and organelles (smaller structures inside of cells). The membrane is the outer layer controlling what comes in and what goes out. If the bloodstream is a superhighway connecting the different parts of our body, the membranes completely control who is allowed to exit the highway and enter the city (cells). Back to insulin. So insulin binds to the fat or muscle cell, resulting in GLUT4 transporters being shoved into the cellular membrane. The GLUT4 transporter essentially acts like a very specific claw, searching the bloodstream for molecules of glucose, grabbing the glucose from the bloodstream, and transporting it inside the cell. Without GLUT4 transporters in the membrane, glucose cannot enter the cell, and it simply remains in the blood. This is a primary action of insulin. Recruit GLUT4 transporters to the surface of fat and muscle cells, allowing glucose to enter the cell and reduce the amount of glucose in the blood.
This is only the beginning of the effect of insulin. We have brought glucose, single molecules of sugar, into the cell. However, this is about creating stable, usable forms of energy, so getting energy into the cell is just the first step. The cell still needs to convert these singular sugar molecules into a form of energy that can be stored long term. As we already stated, there is a deep, hardwired desire for the organism to capitalize on available energy and prepare for a day when that energy is not accessible. We convert glucose into two energy forms that are better suited for storage: glycogen and triglycerides. Glycogen is essentially a bunch of individual glucose molecules strung together, creating a single, larger molecule. This certainly helps for storage, but it also retains functionality as glycogen can be broken down into usable forms of individual glucose molecules quickly. The primary issue with glycogen is that we run out of space. Each cell can only hold so much glycogen, and when the reserves are filled up, the remainder of the glucose is used to create triglycerides. Triglycerides are the body’s best and most efficient way to store large amounts of energy. These molecules are compact, energy dense, while also retaining the ability to be broken down into usable forms of energy. Triglycerides are colloquially referred to as fat, and most of us can see the abundant stores of energy we carry around our waist.
This system of energy acquisition and storage at the cellular level is quite impressive and sophisticated. It truly highlights the body’s ability to adapt and respond to dynamic environmental conditions. But the world we live in today is much different than the world in which these systems were developed. With our basic understanding of how insulin works to pull glucose into the cell and create stable forms of energy, we will now turn to how this might be problematic in our modern life. Just as we have systems to build and store energy, we of course have systems to break down those stored forms of energy. We have processes that break down glycogen and triglycerides into molecules that can fuel our energy demanding cellular processes. However, because we have these opposing processes (anabolism versus catabolism, or storing energy versus using energy) our body has to know which protocol to run. If we are manufacturing triglycerides to store energy, it would be counterproductive if the cell next door was breaking down its triglycerides to use for energy. Once again, this is a situation our body has developed protection against. Remember what insulin’s primary message is: energy is available, grow and store energy. So not only does insulin provide a pathway for energy into the cell (GLUT4 transporter), it blocks and amplifies certain other processes inside the cell. We have discussed how insulin stimulates the building of fatty acids (energy storage in the form of fats), but the presence of insulin also blocks the cell’s ability to break down fat stores, aka insulin blocks lipolysis. This of course is the outcome of a highly intelligent system, but it certainly promotes issues for our modern lifestyle. WHEN INSULIN CONCENTRATION IS HIGH, YOU CANNOT BREAK DOWN FAT STORES. A similar process is at play with glycogen. When insulin concentration is high, the breakdown of glycogen is blocked, and the formation of glycogen is amplified. This all fits under our big picture of insulin. Insulin is a body wide signal for growth, and in turn, a body wide signal to suppress utilization of previously stored forms of energy.
Even with this basic understanding of insulin, it should be obvious that insulin levels are vitally important for anyone concerned with losing weight. As the weight we should want to lose is in the the form of triglycerides, and those triglycerides cannot be burned in the presence of high levels of insulin. I realize there is not much practical information here, or tips on how to actually utilize this information in our daily lives, but understanding this background biochemistry is fundamental to a sophisticated approach to weight loss and health in general. On this landscape we can explore how certain foods effect insulin levels, the fact that calories are NOT created equal, how movement can be leveraged to help with blood sugar control, how the timing of a meal directly affects its metabolic outcomes, how sleep is intimately connected to insulin sensitivity and glucose tolerance, and many other processes. There are so many pathways that all hinge on the metabolic control switch of insulin. Stay tuned for ideas on how to structure our lives in accordance with the biochemistry that governs our cellular processes.