Carbohydrates, Proteins, and Fats

Carbohydrates, proteins, and fats supply 90% of the dry weight of the diet and 100% of its energy. As sources of energy, carbohydrates, proteins, and fats are interchangeable in proportion to their energy content. The energy content is 4 calories in a gram of carbohydrate or protein and 9 calories in a gram of fat (1 gram equals ¹/₂₈ ounce). However, these nutrients differ in how quickly they supply energy; carbohydrates are the quickest, and fats are the slowest.

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 are various forms of sugar, such as glucose and fructose. Because they are small molecules, they can be broken down by the body quickly and are the quickest source of energy. 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 are composed of long strings of simple carbohydrates. Because complex carbohydrates are larger molecules than simple carbohydrates, they are slower sources of energy. However, energy can be generated from them relatively quickly. Complex carbohydrates occur in wheat products (such as breads and pastas), other grains (such as rye and corn), beans, and root vegetables (such as potatoes).

The body stores very small amounts of excess energy as carbohydrates. The liver stores some as glycogen, a complex carbohydrate that the body can easily and rapidly convert to energy. Muscles also store glycogen, which they use during periods of intense exercise. The amount of carbohydrates stored as glycogen provides almost a day's worth of calories. A few other body tissues store carbohydrates as complex carbohydrates that cannot be used to provide energy.

Most authorities recommend that about 50 to 55% of the total daily calories should consist of carbohydrates.

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 nine of the amino acids--called essential amino acids. They must be consumed in the diet. They include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

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.

Adults need to eat about 60 grams of protein per day (0.8 grams per kilogram of weight or 10 to 15% of total calories). Adults who are trying to build muscle need slightly more. Children also need more. If more protein is consumed than is needed, the body does not build more protein (or muscle). Instead, the body breaks the protein down and stores its components as fat, which can be broken down and used for energy later as needed.

Fats: These complex molecules are composed of fatty acids and glycerol. 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 also stores excess fat in blood vessels and within organs, where it blocks blood flow and damages the organs, often with devastating results.

The body cannot synthesize some fatty acids--called essential fatty acids. They 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, linolenic acid, arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid. Linoleic acid and linolenic acid are found in vegetable oils. Eicosapentaenoic acid and docosahexaenoic acid, which are essential for brain development, are found in fish oils. In the body, arachidonic acid can be formed from linoleic acid, and eicosapentaenoic and docosahexaenoic acid can be formed from linolenic acid, although fish oil is a more efficient source.

There are different kinds of fat: monounsaturated, polyunsaturated, and saturated (see Section 3, Chapter 33). In general, saturated fats are more likely to increase cholesterol levels and increase the risk of atherosclerosis. Some evidence suggests that consuming trans fatty acids, a different category of fat, may increase cholesterol levels in the body and thus may contribute to the risk of atherosclerosis (see Section 3, Chapter 33).

Authorities generally recommend that fat be limited to less than 30% of daily total calories, or fewer than 90 grams per day, and that the amount of saturated fat, trans fatty acids, and cholesterol in the diet be reduced. However, people with high cholesterol levels may need to reduce their fat intake even more. When fat intake is reduced to 10% or less of daily total calories, cholesterol levels tend to decrease dramatically.