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THE ROLE OF INSULIN IN ENERGY STORAGE

HERE’S A STARTLING fact: I can make you fat. Actually, I can make anybody fat. How? It’s really quite simple. I prescribe insulin. Although insulin is a natural hormone, excessive insulin causes weight gain and obesity.

Hormones are essentially chemical messengers. They are produced by the endocrine system, a network of glands found throughout the body to maintain proper function. The pea-sized pituitary gland in the brain is often called the master gland because it produces many different hormones that control metabolic processes in other parts of the body. For example, it secretes growth hormone, which signals the rest of the body, including the bones and muscles, to grow bigger. The butterfly-shaped thyroid gland in the neck produces thyroid hormone to deliver its message to the rest of the body. When it receives this signal, the heart may beat faster, breathing may accelerate, and the basal metabolic rate may increase. Similarly, the pancreas produces insulin, a hormone that delivers several different messages mostly relating to the intake and storage of food energy.

INSULIN BASICS

WHEN WE EAT, foods are broken down in the stomach and small intestine for easier absorption. All foods are composed of three main constituents, called macronutrients. These are proteins, fats, and carbohydrates, and they are all handled differently by the digestive system. Proteins are broken down into amino acids. Fats are broken down into fatty acids. Carbohydrates, composed of chains of sugars, are broken down into smaller sugars, including glucose. Micronutrients, as the name implies, are nutrients that are necessary for good health in far smaller quantities, such as vitamins and minerals.

One of insulin’s roles is to facilitate the uptake of glucose into cells for energy, by opening a channel to allow it inside. Hormones find their target cell by binding to receptors on the cell surface, much like a key fitting into a lock. Only the correct hormone can open the receptor and deliver the message. Insulin works like the key, fitting snugly into the lock on the cell to open a gateway for glucose. Every cell in the body can use glucose for energy. Without insulin, glucose circulating in the blood cannot easily enter the cell.

In type 1 diabetes, autoimmune destruction of insulin-secreting cells leads to abnormally low levels of insulin. Without keys to open the gates, glucose cannot enter to provide energy for the cell and builds up in the bloodstream, even as the cell faces internal starvation. As a result, patients continually lose weight, no matter how much they eat, since they are unable to properly use the available food energy. Unused, this glucose is eventually excreted in the urine, even as the patient wastes away. Untreated, type 1 diabetes is usually fatal.

When people without type 1 diabetes eat, insulin rises, and glucose enters the cell to help us meet our immediate energy needs. The excess food energy is stored away for later use. Some carbohydrates, particularly sugars and refined grains, raise blood glucose effectively, which stimulates the release of insulin. Dietary protein also raises insulin levels, but not blood glucose, by simultaneously raising other hormones, such as glucagon and incretins. Dietary fats only minimally raise both blood glucose and insulin levels.

Another of insulin’s key roles is to signal to the liver that nutrients are on their way. The intestinal bloodstream, known as the portal circulation, delivers amino acids and sugars directly to the liver for processing. On the other hand, fatty acids are absorbed directly and do not pass through the liver before entering into the regular bloodstream. Since liver processing is not required, insulin signaling is not necessary and insulin levels remain relatively unchanged by pure dietary fats.

Once our immediate energy needs have been met, insulin gives the signal to store food energy for later use. Our body uses dietary carbohydrates to provide energy for working muscles and the central nervous system, but the excess also provides glucose to the liver. Amino acids are used to produce protein, such as muscle, skin, and connective tissue, but the liver converts the excess into glucose, since amino acids cannot be stored directly.

Food energy is stored in two forms: glycogen and body fat. Excess glucose, whether derived from protein or from carbohydrates, is strung together in long chains to form the molecule glycogen, which is stored in the liver. It can be converted to and from glucose easily and released into the bloodstream for use by any cell in the body. Skeletal muscles also store their own glycogen, but only the muscle cell storing the glycogen can use it for energy.

The liver can only stockpile a limited amount of glycogen. Once it is full, the excess glucose is turned into fat by a process called de novo lipogenesis (DNL). De novo means “from new” and lipogenesis means “making new fat,” so this term means literally “to make new fat.” Insulin triggers the liver to turn excess glucose into new fat in the form of triglyceride molecules. The newly created fat is exported out of the liver to be stored in fat cells to supply the body with energy when it is required. In essence, the body stores excess food energy in the form of sugar (glycogen) or body fat. Insulin is the signal to stop burning sugar and fat and to start storing it instead.

This normal process occurs when we stop eating (and begin fasting), which is when the body needs this source of energy. Although we often use the word fasting to describe periods in which we deliberately limit certain foods or abstain from eating altogether, such as before a medical procedure or in conjunction with a religious holiday, it simply applies to any period between snacks or meals when we are not eating. During periods of fasting, our body relies on its stored energy, meaning that it breaks down glycogen and fat.

Figure 5.1. Storage of food energy as sugar or fat

Several hours after a meal, blood glucose drops and insulin levels begin to fall. To provide energy, the liver starts to break down the stored glycogen into component glucose molecules and releases it into general circulation in the blood. This is merely the glycogen-storage process in reverse. This happens most nights, assuming you don’t eat at night.

Glycogen is easily available but in limited supply. During a short-term fast (twenty-four to thirty-six hours), glycogen will provide all the glucose necessary for normal body functioning. During a prolonged fast, the liver will manufacture new glucose from stored body fat. This process is called gluconeogenesis, meaning literally “the making of new sugar.” In essence, fat is burned to release energy. This is merely the fat-storage process in reverse.

Figure 5.2. Gluconeogenesis: The reverse of the glycogen storage process

This energy storage-and-release process happens every day. Normally this well-designed, balanced system keeps itself in check. We eat, insulin goes up, and we store energy as glycogen and fat. We fast, insulin goes down, and we use our stored glycogen and fat. As long as feeding (insulin high) is balanced with fasting (insulin low), no overall fat is gained.