Why is it so difficult to lose weight? Why do you seem to regain weight after losing it? Why do you feel hungry when you do? These are all questions that will be addressed here and in the upcoming series of articles reviewing weight gain and metabolism of fats and carbohydrates in the human body.
Your body gets energy from carbohydrates and fats. You consume digestible carbohydrates such as starches (long chains of glucose molecules) and table sugar (glucose and fructose molecules bound together). You also consume non digestible carbohydrates or fiber which is digested by bacteria in your gut.
Your body also uses fat as energy. The fat that you eat is broken down into fatty acids which are distributed to your cells inside of a specific type of lipoprotein cholesterol package called a chylomicron. This special packaging allows fat particles to be transported in the blood- otherwise the fat would not dissolve in solution– think of an oil slick on water.
Different organs use different types of energy. Your brain can only use glucose. Your muscle can use glucose or fat for energy depending on the situation.
Where is energy stored in the body? Your liver stores glucose that you eat, and it also produces its own glucose (this is called gluconeogenesis). The fat that you consume is stored in your fat cells (adipocytes).
What happens during digestion? The starches that you consume are broken down into chains of glucose. Sugar is broken down into its components of glucose and fructose. Glucose and fructose enter the blood once they have reached the small intestine. As previously discussed, the fat that you consume is broken down into fatty acids which are packaged into special lipoprotein cholesterol packages called chylomicrons.
The glucose that enters your blood at the intestinal wall is monitored by your pancreas which releases a hormone called insulin when it detects glucose. Your pancreas ignores fats. For example, when glucose enters your body, insulin is released which causes your muscle to use glucose as fuel (instead of fat). At the same time, your fat cells will be directed to store fat and your liver will be directed to store excess glucose that is not used by your muscles for fuel.
In contrast, during prolonged exercise or during periods of fasting, your insulin levels drop. This low insulin state stimulates your fat cells to release fatty acids into the blood. Your muscle now switches to using fat as fuel. Your liver starts releasing glucose and it also makes its own glucose (gluconeogenesis).
Summary: high insulin levels tell your body to STORE fat and glucose. Low insulin levels tell your body to RELEASE fat and glucose into the blood to be used as fuel.
What is the problem with our modern diet? Well, we eat too much processed food. Processed food often has its fiber removed and additional sugar added. This is very problematic because it forces our body to absorb too much glucose way too quickly.
Think of an unprocessed food such as an apple- it has a lot of fiber mixed in with sugar, so it takes your body longer to digest. Your body has more time to break down the carbs and your pancreas will not release a large amount of insulin. But when you eat a doughnut (fiber removed, sugar added), the glucose in the doughnut is absorbed very quickly into your bloodstream.
To make matters worse, many processed foods contain large amounts of fructose. Fructose is a very damaging carbohydrate for your body and can only be processed by your liver. Your liver releases fat particles (fatty acids) as a waste product after processing fructose. These fatty acids are packaged with cholesterol and then released into the bloodstream as another type of lipoprotein cholesterol package (also known as very low-density lipoprotein or VLDL particles). If you eat too much fructose and your liver cannot keep up and process all of it, the fructose is stored in your liver instead.
In fact, alcohol and fructose are processed in the same way by your liver. Overconsumption of sugary soda can be damaging to your liver just as large amounts of beer can be.
Your liver can also fall behind in processing the fructose waste products including fat particles. When this occurs, fat is stored in the liver. This is abnormal. Your liver cannot function properly when it is infiltrated with fat cells. A fatty liver is also called nonalcoholic fatty liver disease (NAFLD) which can eventually lead to cirrhosis and liver failure. NAFLD occurs in approximately one out of four adults in the world and it is now a common reason for liver transplant. Most people are not aware that they have a fatty liver. Often a fatty liver is diagnosed incidentally on a CT scan or ultrasound which was done for another reason.
So how does type 2 diabetes develop? If you are eating a lot of processed foods with added sugar and very little fiber, your body is absorbing a lot of glucose very quickly. Remember that your pancreas senses the glucose entering your body and releases insulin in response. Your body tries to match the amount of insulin released to the amount of glucose consumed.
However, when too much glucose enters your blood at once, your pancreas releases too much insulin, causing an insulin overshoot. This leads to low blood sugar and low blood fat levels. (Recall that insulin is the fat and glucose storage hormone.) Your brain erroneously thinks that you are low in energy and it responds by making you feel hungry and tired. This causes you to eat more and release more insulin causing a vicious cycle. The problem is that you keep storing energy in a high insulin state and your cells are not able to use it as fuel.
The real danger in this story is inflammation. Your immune system constantly patrols your body looking for microbial invaders or accumulation of waste products, among other tasks. Your immune system will start to detect a buildup of half-digested glucose and accompanying waste products. This will cause your immune system to act by releasing attack cells and attack chemicals- this process is called inflammation. Chronic inflammation causes your insulin to become dysfunctional.
In response to all of this you will have persistently high levels of dysfunctional insulin in your blood. This is called insulin resistance and it is the cause of type 2 diabetes. Normally insulin release would tell your body to store glucose and fat.
However, now that your insulin is dysfunctional you will several problems. You will have persistently high blood glucose levels as the glucose you consume is not being taken up into the cells to be used as fuel. Your liver will keep making and releasing more glucose into the blood which is also not being taken up into the cells to be used as fuel as it should be.
To worsen the situation, your fat cells will be releasing fatty acids into the bloodstream. The fatty acids in your blood will be picked up by lipoprotein cholesterol packages released by the liver. These packages will then ferry the fatty acids back to your liver where the fat will be stored.
Keep in mind that these depleted lipoprotein cholesterol packages (also known as low density lipoprotein or “LDL cholesterol”) are also more likely to get stuck in the lining of your blood vessels. When these packages get stuck in your blood vessel lining, they attract immune cells and become inflamed. This inflamed fatty mess becomes a plaque inside your artery. If the plaque becomes dislodged and breaks off it can travel through the blood to your brain and block blood flow, causing a stroke. Alternatively, the fatty plaque can break off and travel through the bloodstream to your heart, where it can block the flow of oxygen-rich blood to your heart muscle, leading to a heart attack.
In summary, eating too much processed food with added sugar and very little fiber wreaks havoc on your body. You will absorb too much glucose too quickly and your pancreas will respond by releasing too much insulin. This will lead to low blood sugar and low blood fat levels which will make your brain think that you are low in energy, making you feel tired and hungry. You will eat more and release more glucose into the blood, feeding into a dangerous cycle. Your body will not be able to keep up and it will fall behind in processing all the glucose and the fatty waste products.
Your immune system will detect the accumulation of half-digested glucose and waste products. It will release attack cells causing chronic inflammation in your body. This inflammation will cause your insulin to become dysfunctional. Thus, you will have high levels of dysfunctional insulin most of the time. Because your insulin is not working, your liver will be releasing glucose and making its own glucose. Your fat cells will be releasing fat particles which will be picked up by cholesterol lipoprotein packages and taken back to your liver. Your liver will not have time to process all this extra glucose and fat and it will be stored in the liver, leading to non-alcoholic fatty liver disease.
High levels of dysfunctional insulin lead to insulin resistance which is the cause of type 2 diabetes. The high levels of fat particles in your blood also mean that you will have a lot of lipoprotein cholesterol packages carrying the fat back to your liver. Some of these lipoprotein cholesterol packages (specifically small LDL particles) are more likely to get stuck in the linings of your arteries causing fatty plaques. These plaques can break off and travel through your blood to your brain, causing a stroke, or to your heart, causing a heart attack.
In the next part, we’ll review how your brain controls your appetite and your ability to lose weight.
Recommended reading:
Nicholas A Lesica, A Conversation about Healthy Eating. London, UCL Press, 2017. https://doi.org/10.14324/111.9781911576754