The PCOS Plan. Jason Fung
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can be either used as building blocks for cell components or burned for energy. The sugars that make up carbohydrates are burned for energy, but they are not used to build other cell parts.
The chemical reactions involved in creating this energy and building cell parts are collectively called metabolism. And each macronutrient is metabolized differently. Why is this important? Because these differences affect how energy is stored and used.
Protein metabolism
Protein, like lean meat, is broken down into its component amino acids during digestion and transported to the liver. Amino acids are mainly used to rebuild proteins in blood cells, bone, muscle, connective tissue, skin, etc. Think of this process as being similar to taking the letters from a Scrabble board and reshuffling them to create new words. We eat animal and plant proteins, break them into amino acids, and then recombine them to form our own proteins.
The primary function of ingested protein is to rebuild cell components, and burning it for energy is only a secondary function. If you eat more protein than is needed for rebuilding, there is no way to store these extra amino acids. Instead, the liver changes them into glucose by a process called gluconeogenesis, or “the formation of new glucose.” (This word is derived from “gluco” meaning “glucose,” “neo” meaning “new,” and “genesis” meaning “the formation of.”) For an average American adult, an estimated 50 to 70 percent of ingested protein is turned into glucose for energy.4 However, this percentage varies greatly depending upon your body weight and how much protein you are eating.
Dietary protein takes significant processing by gluconeogenesis before it is converted to glucose. By this time, the body has activated multiple hormonal systems to deal with the expected increase in glucose availability. Thus, blood glucose remains stable even if you eat lots of protein.
Insulin is released when eating protein, especially in patients with type 2 diabetes, and signals the cell to start synthesizing new proteins. Certain animal proteins, such as the whey in dairy, generate almost as much of an insulin response as carbohydrates.
Fat metabolism
Digestion of dietary fat requires bile to mix and emulsify it. Bile is secreted by the liver, stored in the gallbladder, and released by the small intestine. Once the fat is absorbed by the small intestine, it is in droplets known as chylomicrons that are absorbed into the lymphatic system, which empties directly into the bloodstream. These chylomicrons are carried to fat cells called adipocytes, where they deliver a form of fat called triglycerides that are taken up for storage.
Dietary fat is absorbed more or less directly into our stores of body fat. While it may appear that dietary fat is far more conducive to increasing overall body fat, we will see later that this is not the case.
Carbohydrate 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