Sometimes foods are described as having empty calories. This means that the item is made mostly of sugar, probably sucrose, and not much of anything else. When carbohydrates are ingested along with proteins, lipids, vitamins, and minerals, they form part of a balanced diet that fills our nutritional needs.
of proteins starts out simple, and then becomes more complex, depending on the protein.
The function of the protein depends on its structure. The chain of amino acids will bend and twist to a three-dimensional form, depending on the sequence of the amino acids. In general, the structure and appearance of proteins can be classified as fibrous or globular.
Fibrous proteins are strand-like in appearance. Fibrous proteins, which are the main building material of the body, are called structural proteins. They include collagen, keratin, and contractile proteins of muscles. Collagen provides strength to the tendons and ligaments that hold bones and muscle together. Keratin is found in skin and "seals" the skin surface, preventing evaporation of water from underlying tissues and keeping invading microorganisms out. Contractile proteins of muscles allow muscles to contract or shorten.
Globular proteins, which are compact, spherical proteins, have a wide variety of functions. Some proteins are found in hormones, such as human growth hormone, which helps regulate growth in the body. Other types of globular proteins are called enzymes and they increase the rate of chemical reactions in the body.
The most complete sources of proteins are found in animal tissues. Plants can also provide amino acids. There are eight amino acids, called essential amino acids, which human beings cannot make. These are tryptophan, methionine, valine, threonine, lysine, leucine, histadine, and isoleucine. Because humans cannot make them, they must be supplied in the diet. If they are not supplied, proteins cannot be made, which results in a protein deficiency. Protein deficiency during childhood can result in developmental problems that restrict both mental and physical development. Deficiencies occurring in adults cause a number of problems, such as premature aging, problems in fighting infections, and bleeding in joints and the digestive tract.
Evaluation of the amount of proteins in the body is used to determine an individual's nutritional status, called nitrogen balance. If the person is healthy, his production of proteins is equal to the breakdown of proteins, and he is in neutral nitrogen balance. If the person is growing or repairing tissue damage and has adequate amino acid resources for protein production, his production of protein exceeds protein breakdown, and he is in positive nitrogen balance. If a person's proteins are being broken down faster than the body can replace them, the person is in negative nitrogen balance, which is not good. Negative nitrogen balance means that the person needs supplementation of proteins and amino acids to achieve a neutral or positive nitrogen balance.
Lipids are insoluble in water, and thus they are difficult to carry in the blood. They are categorized into triglycerides, phospholipids, and steroids. The principal dietary lipids in the body are cholesterol and triglycerides. Phospholipids are mostly tied up in cell membranes and do not play a significant role in energy metabolism.
Triglycerides, which are made in the liver to store excess energy from carbohydrates, make up a major portion of adipose tissue. This tissue provides the body with insulation to keep warm and cushions joints and organs for protection. Triglycerides are composed of three-carbon glycerol molecules with three fatty acids attached, one to each of the three carbons.
Fatty acids are long chains of carbon atoms, 12 to 30 carbons long. Attached to the carbons are hydrogen atoms. If all the possible hydrogen atoms are attached to the chain, the fatty acid is called a saturated fat. If any of the hydrogen atoms are missing, the fatty acid is called an unsaturated fat. These forms of fatty acids behave slightly differently in the body. Saturated fats contribute more to the buildup of plaque in arteries and are considered less healthy than unsaturated fats.
Saturated fats are found in all animal tissues, and unsat-urated fats are found in nearly all plants. As with proteins, two fatty acids are essential for human beings: linoleic and linolenic, and are called essential fatty acids. About 90% of the body's dietary fat intake consists of the fatty acids
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