metabolism
The chains of glucose found in carbohydrates are digested or broken into smaller units of glucose for absorption by the body. The speed of digestion and absorption depends upon many factors. Refined carbohydrates, such as grains like rice that have been husked or polished, are absorbed almost instantly because processing removes most of the associated fiber, proteins, and fats that slow absorption. Unrefined carbohydrates, such as beans and legumes, are absorbed more slowly because none of the fiber or protein has been removed. Also, grinding grains like wheat into a very fine flour increases the speed of absorption.
The specific type of carbohydrate also makes a difference. Wheat contains mainly amylopectin A, which is quickly and easily absorbed by the body. In contrast, beans and legumes are high in amylopectin C, which resists digestion by the human body and is incompletely absorbed. The amylopectin C that remains in the intestines is eaten by the gut microbiome, which produces gas and is responsible for the flatulence associated with these pulses.
Blood glucose rises quickly when eating refined carbohydrates, stimulating secretion of the hormone insulin from the pancreas. The insulin sends a signal that moves glucose into cells to be burned for energy. With glucose stored in the body’s cells, blood glucose levels return to normal.
Figure 4.3. Carbohydrate metabolism
THE FED STATE: HOW THE BODY STORES FOOD ENERGY
THE BODY HAS two complementary methods of energy storage:
1.Glycogen (in the liver)
2.Body fat (in the fat cells)
When you eat more carbohydrates or proteins than your body needs, insulin rises. As we’ve seen, these macronutrients are converted into glucose and sent into the bloodstream, which causes your blood glucose levels to rise. This increase in blood glucose signals your pancreas to produce insulin, which indicates the availability of food and puts the body into the “fed” state. All the cells of the body (liver, kidney, brain, heart, muscles, etc.) can now help themselves to this all-you-can-eat glucose buffet.
If some glucose is left over, it must be stored away for future use. This is a relatively simple process, since the body just links all the glucose molecules into a long, branched chain called glycogen and stores it in the liver. Glycogen is made and stored directly in the liver. Our muscles also store their own supply of glycogen, but this source can only be used by the muscles. In other words, the glycogen within muscles cannot be used, for example, by the kidneys. In contrast, the glycogen in the liver can supply any organ by releasing glucose into the bloodstream.
In the fed state, insulin goes up, signaling the body to store excess food energy as glycogen. Liver-glycogen stores, if full, last approximately 24 hours. When the body’s glycogen stores are full, the body must use a second form of energy storage for unused glucose. The excess glucose from the liver is converted into triglycerides, or body fat, through a process called “de novo lipogenesis,” or creation of new fat. (The word “de novo” means “from new” and “lipogenesis” means “creation of new fat.”) Some of the glucose from which this body fat is created may have come from carbohydrates and some from dietary protein, which was changed from protein to glucose through gluconeogenesis.
Regardless of where the excess glucose comes from, the liver creates new fat (triglycerides) but cannot store it. Fat is designed to be stored in fat cells (adipocytes), not the liver. So the liver packages these triglycerides together with some transport proteins and exports them as very low-density lipoprotein (VLDL). In the bloodstream, insulin increases a hormone known as lipoprotein lipase (LPL), which helps the triglycerides move out of the VLDL particle and into the adipocyte. This effectively transforms excess glucose into triglycerides and moves them to the appropriate fat cells for long-term storage. If the rate of new fat creation from de novo lipogenesis exceeds the export capacity of the liver, these triglycerides back up in the liver and cause nonalcoholic fatty liver disease.
Remember, this process is not the same as ingesting dietary fat. The fat we eat is broken down into chylomicrons, absorbed by the small intestine, and sent directly into the adipocytes. There is no processing within the liver, no insulin signaling, and no possibility of using the glycogen storage system, which is exclusively for glucose.
This entire storage process for fat is much more laborious compared with the relatively simple glycogen storage. So why have the two different systems? The glycogen and body fat systems for storing food energy complement each other perfectly. Glycogen is easy to get to and convenient, but limited in storage space. Body fat is harder to get to and inconvenient, but unlimited in storage space.
Think of glycogen like a wallet. You can move your cash into and out of your wallet without much difficulty, but you would not hold six months’ worth of cash in your wallet. Think of body fat like your bank account. It is more difficult to move money back and forth: you have to go to a bank machine or teller, put money in, and perhaps buy investments. Getting your money out as cash is also not so simple, because you need to go back to the bank to withdraw it. But you can store your life’s savings in a bank account without worry. This balance between short-term and long-term storage in the body also applies when you want to use that stored energy, as we’ll see next.
Figure 4.4. How the body stores food energy (calories)
THE FASTED STATE: HOW THE BODY USES STORED FOOD ENERGY
THE WORD “FASTING” may sound scary, but it simply refers to any time you are not eating. When you sleep, for example, you are fasting. In the fasted state, your body reverses its process for storing food. When insulin falls, as with fasting, the body breaks glycogen back down into individual glucose molecules to supply energy to the whole body. This is why we don’t die in our sleep every single night. “Breakfast” is literally the meal that breaks our fast, so you can see that fasting is a part of everyday life.
Put another way, at any given time our bodies exist in only one of two states: the fed state or the fasted state. Our body is either storing food energy or using it up. High insulin is the body’s signal to store incoming food energy. Low insulin is the body’s signal to use the stored food energy because no food is coming in. In the fasted state, we must rely on our stores of food energy to survive.
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