Insulin is a vital hormone that your body makes to regulate blood sugar by transporting that sugar to where it can be stored.
When you eat, carbohydrates are broken down into sugar called glucose (C6H12O6). Glucose gets absorbed into your bloodstream via the small intestine. The rising blood glucose level signals beta cells in the pancreas to produce a hormone called insulin. Insulin helps to move the glucose from your blood into your cells. A hormone called GLP-1 (glucagon-like peptide 1) helps the pancreas to produce the right amount of insulin. It stimulates the pancreas to produce insulin based on the blood glucose level.
The body stores this glucose in muscle tissue and the liver in the form of glycogen. Glycogen is the stored form of glucose that’s made up of many connected glucose molecules. The muscles hold about 75% of the glycogen, and the liver holds the other 25%. The average adult male can store about 1,600 calories worth of glucose between the two locations.
The glycogen in your liver is primarily used to regulate your blood glucose levels. When your blood glucose level drops below a certain level, your pancreas makes a hormone called glucagon that triggers glycogen to convert back into glucose and enter your bloodstream so your body can use it for energy. Glucagon and insulin are the primary natural hormones that regulate your body’s blood glucose levels.
The glycogen stored in your muscles serves primarily as a source of metabolic fuel for your muscles. When you exercise (move), your muscles use this stored glycogen as fuel. The rate at which the glycogen is used depends on the ratio of fatty acids to glucose metabolized in the muscles, and also the ratio of aerobic to anaerobic respiration (metabolism). These two factors themselves are primarily governed by the intensity of exercise. Higher-intensity exercise uses glucose much faster.
So what happens to carbohydrates you eat if liver and muscle glycogen stores are already full? In this case, insulin converts those carbs you eat into fat and stores it in your body for later use. The body has an almost unlimited capacity to store excess calories as fat. In short, if your glycogen stores are full and you're not exercising, when you eat carbs, you get fat. In a typical sedentary person who is not depleting muscle glycogen rapidly, the excess energy from carbohydrates will largely end up in fat cells (specifically, as triglycerides contained within fat cells.)
One of the first places where this overflowing fat will cause problems is in your muscle, as it worms its way in between your muscle fibers like marbling on a steak. As this continues, microscopic fat droplets even appear inside your muscle cells. This is where insulin resistance likely begins. These fat droplets may be among the first destinations of excess energy/fat spillover, and as they accumulate they begin to disrupt the complex network of insulin-dependent transport mechanisms that normally bring glucose in to fuel the muscle cell. When these mechanisms lose their function, the cell becomes "deaf" to insulin's signals. Eventually, this insulin resistance will progress to other tissues such as the liver, but it is believed that it originates in the muscle. It's worth noting that one key ingredient in this process seems to be inactivity. If a person is not physically active, and they are not consuming energy via their muscles, then this fat-spillover-driven insulin resistance develops much more quickly.
Insulin resistance technically means that cells, initially muscle cells, have stopped listening to insulin's signals. Think of insulin as a key that unlocks the doors to allow glucose to enter the muscles. Insulin resistance is like those keys only working some of the time. When this happens the pancreas needs to produce more insulin in order to unlock the doors to get glucose into the muscles. If blood glucose levels continue to be elevated despite even more insulin being produced, then you get type 2 diabetes. This is very bad.
What is Diabetes?
Type 1 diabetes is an autoimmune disease where the immune system attacks and kills the beta cells in the pancreas that produce insulin. We don't know why this happens.
Type 2 diabetes is basically severe insulin resistance, and is usually caused by consuming excessive calories, mostly carbohydrates, and not exercising. The muscles have stopped responding to insulin's signal to store glucose. Today over 11% of the US adult population, has clinical type two diabetes. Another 38% of US adults meet at least one of the criteria for pre-diabetes.
What's the problem with chronically elevated blood sugar?
High blood sugar levels can cause blood vessel damage, heart disease, neurodegenerative disease, blindness, and kidney failure. Having chronically high blood sugar or type 2 diabetes dramatically increases your risk of heart disease, Alzheimer's disease, and cancer.
How to you determine if you're insulin resistant?
There are a few things you can do to check your insulin sensitivity. One of the most basic is to check your hemoglobin A1C (HbA1c) with a blood test. While not perfect, this represents your average blood glucose level over the past 3 months. You want this to be less than 5.7%. Above 6.5% is consistent with having diabetes.
While you can also check your fasting blood glucose level (you want this to be below about 100 mg/dL) this can vary a lot on a minute-to-minute basis depending on a variety of factors. You can check this with a blood test or an at-home glucometer (finger prick device).
You can also use a continuous glucose monitor (CGM). These are now becoming available to those without a diagnosis of diabetes. I currently use one to optimize my health. It measures your blood glucose all the time, so you can see how diet, exercise, sleep, and stress affect your blood sugar level. The one I currently use is called Nutrisense and you can get it here if you're interested.
How to avoid insulin resistance
You can increase your insulin sensitivity (decrease insulin resistance) by exercising regularly, eating less carbohydrates, intermittent fasting, and getting good quality sleep. Sleep has been shown to increase insulin sensitivity.
When you exercise, you're using some of your muscle glycogen stores. That means the next time you eat carbohydrates, insulin has a place to put that glucose rather than storing it as fat. Shortly after exercise, your cells are also more sensitive to insulin.
A common question is, "if I don't eat carbs regularly, won't that be a problem? Will I run out of glycogen and my blood sugar plummet to dangerously low levels?" No, you'll be fine. This is because the liver has the ability to make glucose from other molecules in the body, such as proteins, in a process called gluconeogenesis. In fact, a ketogenic diet (extremely low carb, high fat) is often prescribed as a way to treat insulin resistance.
** I am not a doctor and nothing I say should be taken as medical advice.