The word metabolism means all the chemical processes that take place in the body. One type of metabolism is the breaking down of complex substances into simpler ones, like the “burning” of glucose (sugar) in the cells to produce energy, heat, carbon dioxide, and water. The other type of metabolism builds complex substances from simpler ones, like turning amino acids into proteins.
The way the body uses nutrients depends on many factors. Among these are whether you are in an absorptive state (you are still digesting nutrients), in a postabsorptive state (following digestion), or in a fasting state (ready energy is depleted). The situation is also changed if you have a large deﬁciency or excess of any major nutrient.
Shortly after you begin an adequate, balanced meal, and for up to about four hours after you have ﬁnished, your digestion is supplying and replenishing your body’s reserves. Here is what happens to the major nutrients during this time:
Carbohydrates Enzymes in the small intestine break up starches into individual glucose molecules, and break disaccharides (sucrose, lactose, and maltose) into glucose, fructose, and galactose. These sugars are transported to the liver, where glucose enters the bloodstream, and fructose and galactose are converted to glucose and then enter the bloodstream.
If blood sugar is high enough, insulin is released so glucose is converted to glycogen (animal starch) to ﬁll the reserves in the liver and other body tissue. After glycogen reserves are ﬁlled, excess glucose can be stored as fat.
Proteins Dietary protein is broken down into its amino acids in the stomach and small intestine. The amino acids are delivered to the liver, which keeps some and releases the rest into the bloodstream, where cells throughout the body use them to rebuild cells, make hormones and neurotransmitters, and so on.
An excess of protein will trigger the release of insulin also. If blood sugar is low (there was not enough carbohydrate ingested), then the amino acid will be converted to glucose. If there is enough glucose, excess protein will be changed to fat and stored.
Fats Since fats are not soluble in water, they do not circulate freely in the bloodstream. Instead, the liver packs them into little packages called lipoproteins, which transport them to fat cells for storage. Some triglycerides (fats) are always being used for energy. Resting skeletal muscles and the liver itself actually prefer using fat as an energy source, if there is enough oxygen present.
If there is not enough glucose (blood sugar) present, a small part of each triglyceride (the glycerol part) can be converted to glycogen, but the fatty acids cannot.
When most of the food is gone from the digestive tract, three or four hours after a normal meal, the body must shift gears into using its reserve supplies. These needs include energy from glucose and fats, and amino acids.
The main sources of energy in the body are glucose (blood sugar) and fats. Under normal conditions both are used.
Glucose When the digestive tract stops supplying sugars, the blood sugar level will start to drop. This triggers the release of the hormone glucagon, which causes conversion of glycogen to glucose. If the glycogen reserves were ﬁlled, you will have up to about four more hours before they are depleted. This means you can go up to four hours in the absorptive state, and about four in the postabsorptive state, before things start to get critical.
Protein The body cannot exist without amino acids. They are the building blocks for many structures and processes of the body. There is no actual storage site for amino acids, so in less than a day the free amino acids will be used up, and the body has to begin breaking down muscle and other tissues to supply the protein it needs.
Fats For most people, fat supplies are adequate to provide energy for a long time. Fats are energy-dense: about nine calories per gram, compared with carbohydrates and protein, which each supply about four calories per gram. Fats need both oxygen and glucose to be “burned” properly. When the stored glycogen runs out, the body begins to enter an emergency mode in the postabsorptive state that amounts to fasting.
What happens when the glycogen is used up and the glucose has stopped coming in or is inadequate? Most of your body can switch to burning fat to stay alive, but brain cells require glucose and will get what is available ﬁrst. And the brain uses a lot of it. Though the brain is only about 2 percent of your weight, it uses about 20 percent of your glucose!
Glucose sparing With reduced glucose available, the body starts adapting to conserve all the glucose it can for the brain’s use. Almost every other organ can switch to using fats for its major energy needs. Protein can also be converted to glucose through a process called gluconeogenesis. This means loss of lean muscle mass, which in the long run conserves more glucose (and fat).
To meet this need, in the first week of fasting, up to a pound of muscle may be lost each day, in addition to a half-pound of fat. Obese people will lose about ten to thirty pounds of ﬂuid, while nonobese people will lose about four to six pounds of ﬂuid during this time.
Ketosis Fortunately there is an emergency process that can help keep the brain alive. Without glucose, the fats are burned improperly, and ketone bodies (usually acids) are produced. After four or ﬁve days of fasting, the brain will begin to use these along with what glucose is available, cannibalizing less tissue protein. After the ﬁrst week or so, weight loss falls to about one-quarter pound of muscle and about one-third to one-half pound of fat a day.
When you fast or go on a low=calorie diet, you are signaling your body that there is a famine outside. The body slips into a survival mode, in which it tries to conserve all the energy it can. The body can slow its energy consumption to about half its normal rate. If this is done repeatedly, the body “learns” how to store fat more efﬁciently, and to resist losing it.
Stoking the furnace We can “fire up the furnace” by increasing exercise (supplying more oxygen to help burn fats) and increasing carbohydrate consumption. Both actions tell the body that the famine is gone, it does not need to spare glucose or adapt its metabolism to conserve energy, and it is OK to reduce its fat stores.
Updated 7 Sep 2015
Addictionary 2 by Jan & Judy Wilson
is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.