Carbohydrates are sugar compounds that plants make when exposed to light, the process called photosynthesis. A carbohydrate consists of carbon (C), hydrogen (H), and oxygen (O) atoms.
There are three types of carbohydrates:
- Simple carbohydrates. These have only one or two units or sugar.
- A carbohydrates with one unit of sugar is called a simple sugar, a monosaccharide. Examples are fructose (fruit sugar), glucose (blood sugar), the sugar produced when carbohydrates are digested and galactose, derived from digesting lactose (milk sugar).
- A carbohydrate with two units of sugar is called double sugar, disaccharide. Sucrose (table sugar) is an example.
- Complex carbohydrates. These have more than two units of sugar linked together. Examples are raffinose, stachyose and starch. As complex carbohydrates have more units of sugar it takes a longer time to digest them than simple carbohydrates. As a result, glucose (details below) is released more slowly into the bloodstream.
- Dietary fiber. These contain multiple sugar units but the links that hold the units together cannot be broken by human enzymes. Dietary fiber is not considered a source of energy (see more below).
Sources of Carbohydrates
The most important sources are plant foods, i.e. fruits, vegetables and grains. Also, the carbohydrate lactose (milk sugar) is found in milk and milk products.
Carbohydrates and Energy
Carbohydrates are a source of energy, but no carbohydrate is an essential nutrient in humans. We are able to obtain most of our energy requirement from protein and fats.
Inside our cell, glucose is burned to produce heat and adenosine triphosphate (ATP), a molecule that stores and releases energy whenever required by a cell.
The transformation of glucose into energy occurs in one of two ways – with or without oxygen. Glucose is converted to energy with oxygen in the mitochondria — tiny bodies inside every cell. This conversion yields energy (ATP, heat) plus water and carbon dioxide, which is a waste product. Red blood cells do not have mitochondria, so they change glucose into energy without oxygen. This yields energy (ATP, heat) and lactic acid. Glucose is also converted to energy in muscle cells. These are versatile when it comes to producing energy from glucose – they have mitochondria, so they can process glucose with oxygen. However, if the level of oxygen in the muscle cell falls very low, the cell can go ahead and change glucose into energy without it.
Carbohydrates and Fat
Our cells do not store more energy than they need right now. Any glucose the a cell does not need for its daily work is converted to glycogen (animal starch) and tucked away as stored energy in our liver and muscles. The body can pack about 400 grams of glycogen into liver and muscle cells. A gram of carbohydrates — including glucose — has four calories. If all the glucose stored in glycogen is added to the small amount of glucose in our cells and blood, it equals around 1,800 calories of energy. If the diet provides more carbohydrates than is needed to produce this amount of stored calories in the form of glucose and glycogen in our cells, blood, muscles, and liver, the excess will be converted to fat.
Other Uses of Carbohydrates
Besides providing energy, carbohydrates also protect our muscles. When energy is needed, our body looks for glucose from carbohydrates first. If none is available, e.g. because of being on a carbohydrate-restricted diet or having a medical condition that prevents us from using the carbohydrate foods consumed, our body starts to extract energy out of fatty tissue and then moves on to burning its own protein tissue (muscles). If this use of proteins for energy continues long enough, we run out of fuel and die. A diet that provides sufficient amounts of carbohydrates keeps our body from eating its own muscles. That is the reason why a carbohydrate-rich diet is described as protein sparing.
- Assist in our body’s absorption of calcium.
- Regulate the amount of sugar circulating in your blood so that all our cells get the energy needed.
- May help lower cholesterol levels and regulate blood pressure.
- Provide nutrients for the friendly bacteria in our intestinal tract that help digest food.
Carbohydrates, Insulin and Diabetes
When carbohydrates are eaten, the pancreas secretes insulin, the hormone that enables our body to digest starches and sugars. The release of insulin is sometimes referred to as an “insulin spike”. Eating simple carbohydrates such as table sugar provokes higher insulin spikes than eating complex carbohydrates such as starch. With a metabolic disorder like diabetes, that keeps the body from producing enough insulin, care must be given not to take in more carbohydrates than can be digested. For persons with diabetes, the glucose continues to circulate in the blood until it is excreted through the kidneys. That is why one way to tell whether someone has diabetes is to test the level of sugar in that person’s urine.
As noted above, dietary fiber is not a source for energy. There are two types of dietary fiber:
- Insoluble dietary fiber. These include e.g. cellulose and lignin, which is found in whole grains and plants. This fiber is a natural laxative. It absorbs water and stimulates our intestinal walls to contract and relax, to move food through our digestive tract.
- Soluble dietary fiber. These include pectins, found in fruit, and beta-glucans, found in oats and barley.
Dietary Fiber From Food
We get dietary fiber from plant food – fruits, vegetables and grains. There is no fiber in foods from animals.