How Carbohydrates are Used
Plants are rich in carbohydrate, as that is their storage form of energy. When we eat plant-based foods, we ingest this stored energy and put it to use within the body. Although we can use protein and fat to produce energy, carbohydrate is the source of fuel that is easiest for the body to use, and so it is preferred. This is due primarily to the basic chemical structure of carbohydrate -- those units of carbon, hydrogen, and oxygen. Monosaccharides, the simple carbohydrates, are absorbed directly into the bloodstream. But complex carbohydrates are also relatively easy for the body to disassemble, creating glucose that can be shuttled off to cells to provide energy. In fact, the body actually begins to break down carbohydrates in the mouth with the help of an enzyme that is found in saliva.
We need fat in our diets, but also in and on our body -- within limits of course. Dietary fat provides a concentrated source of energy (9 calories per gram). It makes us feel fuller longer, as it tends to leave the stomach slowly, and it transports and supports absorption of the fat-soluble vitamins A, D, E, and K. In the body, fat is the primary form in which humans store excess long-term energy. Fat is not a readily available energy source like carbohydrate; rather, it's meant to sustain us in times of starvation, self-imposed or otherwise. That's one reason why it's so difficult to lose body fat, especially when you cut calories too low. Our body views fat as a source of protection, and it's reluctant to give it up easily. Throughout the body, fat is a component of cell membranes that helps regulate the flow of materials into and out of cells, serves as a precursor to a variety of hormonelike substances that regulate many physiologic processes, acts as an insulator against heat loss, and protects vital organs such as the kidneys and heart.
Protein's power is often underestimated. Sure, protein is made up of amino acids, which the body uses to build new and repair old body tissue and muscle. But protein is a workhorse -- it's part of every body cell and tissue (including organs, skin, bone, and muscle), supports the body's immune functions, aids in the transport of nutrients, serves as a buffer to maintain a stable blood pH, and as enzymes and hormones, works to regulate body processes.
When there isn't enough readily available glucose from the breakdown of carbohydrate, the body first turns to stored carbohydrate reserves (glycogen). If there's still no new intake of carbohydrate and the reserves are depleted, the body is forced to use alternative sources (fat and protein) for energy. When this happens, protein is diverted from its intended job and there isn't much left over to support muscle repair, so muscles can become smaller and weaker.
In extreme cases, such as when a person is on a zero-carb diet, the body can break down muscle to convert into glucose. However, your heart -- the body's main muscle -- is usually protected. The energy needs of the brain and heart are the body's first priority, and it works hard to prevent damage to these and other major organs.
Stoking Our Engines
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Complex carbohydrates like fruit cause
your blood sugar to rise slowly and
stay at a constant level.
Our bodies have an amazing ability both to use glucose and to conserve it for future needs. There is a delicate regulatory system that maintains tight control over the level of glucose in the blood.
The regulatory system works like this: When you eat a carbohydrate-containing food or meal, the carbohydrate is absorbed from your digestive tract, causing blood sugar (glucose) levels to rise. Insulin, a hormone released from your pancreas, helps glucose enter your cells, where it is used to produce energy. Your body doesn't turn all of its blood sugar into energy at the same time. Insulin works to both clear glucose from the blood for use by cells and to store any excess in your liver and muscles. This storage form of glucose is called glycogen. Some glucose may also be converted to body fat if you eat more calories than your body needs.
If blood sugar levels drop too low, another hormone called glucagon triggers the conversion of glycogen from the liver back to glucose through a process called glycogenolysis (the breakdown of glycogen). This interplay between insulin and glucagon, in concert with several other physiologic control systems, works to keep blood sugar levels within a normal range at all times. Your body relies on a steady supply of glucose around the clock because your organs never stop working. In fact, a steady supply of glucose is so important that we can produce glucose by an alternate method during periods when we have low glycogen stores or an inadequate intake of carbohydrate. Through a process called gluconeogenesis, protein, and to a lesser extent fat, can be called on to supply glucose to the body, although this is a much more complicated, "fuel expensive," and inefficient process and not the preferred method of obtaining glucose.