There are literally thousands and thousands of explanations of how the body converts food to useful 'building blocks' online. I could write an explanation of my own, but with so many great resources out there, I am going to simply post one from Merck and encourage you to do more research yourself if you feel so inclined.
From the Merck Manual Home Edition:
Carbohydrates, proteins, and fats are digested in the intestine, where they are broken down into their basic units: carbohydrates into sugars, proteins into amino acids, and fats into fatty acids and glycerol. The body uses these basic units to build substances it needs for growth, maintenance, and activity (including other carbohydrates, proteins, and fats).
Carbohydrates
Depending on the size of the molecule, carbohydrates may be simple or complex.
Simple carbohydrates: Various forms of sugar, such as glucose and sucrose (table sugar), are simple carbohydrates. They are small molecules, so they can be broken down and absorbed by the body quickly and are the quickest source of energy. They quickly increase the level of blood glucose (blood sugar). Fruits, dairy products, honey, and maple syrup contain large amounts of simple carbohydrates, which provide the sweet taste in most candies and cakes.
Complex carbohydrates: These carbohydrates are composed of long strings of simple carbohydrates. Because complex carbohydrates are larger molecules than simple carbohydrates, they must be broken down into simple carbohydrates before they can be absorbed. Thus, they tend to provide energy to the body more slowly than simple carbohydrates but still more quickly than protein or fat. Because they are digested more slowly than simple carbohydrates, they are less likely to be converted to fat. They also increase blood sugar levels more slowly and to lower levels than simple carbohydrates but for a longer time. Complex carbohydrates include starches and fibers, which occur in wheat products (such as breads and pastas), other grains (such as rye and corn), beans, and root vegetables (such as potatoes).
Carbohydrates may be refined or unrefined. Refined means that the food is highly processed. The fiber and bran, as well as many of the vitamins and minerals they contain, have been stripped away. Thus, the body processes these carbohydrates quickly, and they provide little nutrition although they contain about the same number of calories. Refined products are often enriched, meaning vitamins and minerals have been added back to increase their nutritional value. A diet high in simple or refined carbohydrates tends to increase the risk of obesity and diabetes.
If people consume more carbohydrates than they need at the time, the body stores some of these carbohydrates within cells (as glycogen) and converts the rest to fat. Glycogen is a complex carbohydrate that the body can easily and rapidly convert to energy. Glycogen is stored in the liver and the muscles. Muscles use glycogen for energy during periods of intense exercise. The amount of carbohydrates stored as glycogen can provide almost a day's worth of calories. A few other body tissues store carbohydrates as complex carbohydrates that cannot be used to provide energy.
Glycemic Index: The glycemic index of a carbohydrate represents how quickly its consumption increases blood sugar levels. Values range from 1 (the slowest) to 100 (the fastest, the index of pure glucose). However, how quickly the level actually increases also depends on what other foods are ingested at the same time and other factors.
The glycemic index tends to be lower for complex carbohydrates than for simple carbohydrates, but there are exceptions. For example, fructose (the sugar in fruits) has little effect on blood sugar.
The following also influence a food's glycemic index:
- Processing: Processed, refined, or finely ground foods tend to have a higher glycemic index.
- Type of starch: Different types of starch are absorbed differently. For example, potato starch is digested and absorbed into the bloodstream relatively quickly. Barley is digested and absorbed much more slowly.
- Fiber content: The more fiber a food has, the harder it is to digest. As a result, sugar is absorbed more slowly into the bloodstream.
- Ripeness of fruit: The riper the fruit, the more sugar it contains, and the higher its glycemic index.
- Fat or acid content: The more fat or acid a food contains, the more slowly it is digested and the more slowly its sugars are absorbed into the bloodstream.
- Preparation: How a food is prepared can influence how quickly it is absorbed into the bloodstream. Generally, cooking or grinding a food increases its glycemic index because these processes make food easier to digest and absorb.
- Other factors: The way the body processes food varies from person to person, affecting how quickly carbohydrates are converted to sugar and absorbed. How well a food is chewed and how quickly it is swallowed also have an effect.
The glycemic index is thought to be important because carbohydrates that increase blood sugar levels quickly (those with a high glycemic index) also quickly increase insulin levels. The increase in insulin may result in low blood sugar levels (hypoglycemia) and hunger, which tends to lead to consuming excess calories and gaining weight. Carbohydrates with a low glycemic index do not increase insulin levels so much. As a result, people feel satiated longer after eating. Consuming carbohydrates with a low glycemic index also tends to result in more healthful cholesterol levels and reduce the risk of obesity and diabetes mellitus and, in people with diabetes, the risk of complications due to diabetes.From: http://www.merck.com/mmhe/sec12/ch152/ch152b.html
In spite of the association between foods with a low glycemic index and improved health, using the index to choose foods does not automatically lead to a healthy diet. For example, the glycemic index of potato chips and some candy bars—not healthful choices—is lower than that of some healthful foods, such as brown rice. Some foods with a high glycemic index contain valuable vitamins and minerals. Thus, this index should be used only as general guide to food choices.
Glycemic Load: The glycemic index indicates only how quickly carbohydrates in a food are absorbed into the bloodstream. It does not include how much carbohydrate a food contains, which is also important. Glycemic load, a relatively new term, includes the glycemic index and the amount of carbohydrate in a food. A food, such as carrots, bananas, watermelon, or whole-wheat bread, may have a high glycemic index but contain relatively little carbohydrate and thus have a low glycemic load. Such foods have little effect on the blood sugar level.
Proteins
Proteins consist of units called amino acids, strung together in complex formations. Because proteins are complex molecules, the body takes longer to break them down. As a result, they are a much slower and longer-lasting source of energy than carbohydrates.
There are 20 amino acids. The body synthesizes some of them from components within the body, but it cannot synthesize 9 of the amino acids—called essential amino acids. They must be consumed in the diet. Everyone needs 8 of these amino acids: isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Infants also need a 9th one, histidine. The percentage of protein the body can use to synthesize essential amino acids varies from protein to protein. The body can use 100% of the protein in egg and a high percentage of the proteins in milk and meats.
The body needs proteins to maintain and replace tissues and to function and grow. If the body is getting enough calories, it does not use protein for energy. If more protein is consumed than is needed, the body breaks the protein down and stores its components as fat.
The body contains large amounts of protein. Protein, the main building block in the body, is the primary component of most cells. For example, muscle, connective tissues, and skin are all built of protein.
Fats
Fats are complex molecules composed of fatty acids and glycerol. The body needs fats for growth and energy. It also uses them to synthesize hormones and other substances needed for the body's activities (such as prostaglandins). Fats are the slowest source of energy but the most energy-efficient form of food. Each gram of fat supplies the body with about 9 calories, more than twice that supplied by proteins or carbohydrates. Because fats are such an efficient form of energy, the body stores any excess energy as fat. The body deposits excess fat in the abdomen (omental fat) and under the skin (subcutaneous fat) to use when it needs more energy. The body may also deposit excess fat in blood vessels and within organs, where it can block blood flow and damage organs, often causing serious disorders.
Fatty Acids: When the body needs fatty acids, it can make (synthesize) certain ones. Others, called essential fatty acids, cannot be synthesized and must be consumed in the diet. The essential fatty acids make up about 7% of the fat consumed in a normal diet and about 3% of total calories (about 8 grams). They include linoleic acid and linolenic acid, which are present in certain vegetable oils. Eicosapentaenoic acid and docosahexaenoic acid, which are fatty acids essential for brain development, can be synthesized from linolenic acid. However, they also are present in certain marine fish oils, which are a more efficient source.
Linoleic acid and arachidonic acid are omega-6 fatty acids. Linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid are omega-3 fatty acids. A diet rich in omega-3 fatty acids may reduce the risk of coronary artery disease. Lake trout and certain deep-sea fish contain large amounts of omega-3 fatty acids. In the United States, people tend to consume enough omega-6 fatty acids, which occur in the oils used in many processed foods, but not enough omega-3 fatty acids.
Kinds of Fat: There are different kinds of fat: monounsaturated, polyunsaturated, and saturated (see Coronary Artery Disease: Types of FatSidebar). In general, saturated fats are more likely to increase cholesterol levels and increase the risk of atherosclerosis. Foods derived from animals commonly contain saturated fats, which tend to be solid at room temperature. Fats derived from plants commonly contain monounsaturated or polyunsaturated fatty acids, which tend to be liquid at room temperature. Palm and coconut oil are exceptions. They contain more saturated fats than other plant oils.
Trans fats (trans fatty acids) are a different category of fat. They are man-made, formed by adding hydrogen atoms (hydrogenation) to monounsaturated or polyunsaturated fatty acids. Fats may be partially or fully hydrogenated (or saturated with hydrogen atoms). In the United States, the main dietary source of trans fats is partially hydrogenated vegetable oils, present in many commercially prepared foods. Consuming trans fats may adversely affect cholesterol levels in the body and may contribute to the risk of atherosclerosis.
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