When choosing from a menu or selecting food to prepare, we are often more concerned with the food’s taste than with its nutritional value. However, knowing about nutrition is important because the food we eat provides us with the energy and the building blocks necessary to synthesize new molecules. What happens if we don’t obtain enough vitamins, or if we eat too much sugar and fat? Health claims about foods and food supplements bombard us every day. Which ones are ridiculous, and which ones have merit? A basic understanding of nutrition can help us answer these and other questions so that we can develop a healthful diet. It can also allow us to know which questions currently do not have good answers.
Nutrition (noo-trish′ŭn; to nourish) is the process by whichfood is taken into and used by the body; it includes digestion, absorption, transport, and metabolism. Nutrition is also the study of food and drink requirements for normal body function.
Nutrients are the chemicals taken into the body that provide energyand building blocks for new molecules. Some substances in food are not nutrients but provide bulk (fiber) in the diet. Nutrients can be divided into six major classes: carbohydrates, lipids, proteins, vitamins, minerals, and water. Carbohydrates, lipids, and proteins are the major organic nutrients and are broken down by enzymes into their individual subunits during digestion. Subsequently, many of these subunits are broken down further to supply energy, whereas other subunits are used as building blocks for making new carbohydrates, lipids, and proteins. Vitamins, minerals, and water are taken into the body without being broken down. They are essential participants in the chemical reactions necessary to maintain life. The body requires some nutrients in fairly substantial quantities and others, called trace elements, in only minute amounts.
Essential nutrients are nutrients that must be ingestedbecause the body cannot manufacture them—or it cannot manu-facture them in adequate amounts. The essential nutrients include certain amino acids, certain fatty acids, most vitamins, minerals, water, and some carbohydrates. The term essential does not mean that the body requires only the essential nutrients. Other nutrients are necessary, but if they are not ingested, they can be synthesized from the essential nutrients. Most of this synthesis takes place in the liver, which has a remarkable ability to transform and manu-facture molecules. A balanced diet consists of enough nutrients in the correct proportions to support normal body functions.
Every 5 years, the U.S. Department of Health and Human Services (HHS) and the U.S. Department of Agriculture (USDA) jointly make recommendations on what Americans should eat to be healthy. The latest recommendations, the Dietary Guidelines for Americans, 2010, were published in January 2011. In light of the increasing problem of obesity in the United States, the latest rec-ommendations focus on two concepts: (1) balancing calorie intake to obtain and maintain a healthy weight, and (2) increasing con-sumption of healthy, nutrient-rich foods. In June 2011, the USDA also introduced MyPlate, a new food icon to replace the former food guide icon, called MyPyramid. MyPlate (figure 17.1) is a sim-ple visual reminder of how to build a healthy meal. The MyPlate icon shows a plate and glass with portions representing foods from the fruits, vegetables, grains, proteins, and dairy food groups. To emphasize the importance of making healthy food choices, half the plate is fruits and vegetables. In addition to the MyPlate icon, the USDA also launched ChooseMyPlate.gov, a website that includes information on how to make healthy dietary choices.
The importance of a healthy diet has been proposed as a key to a healthy life. Two studies completed in 2000 compared the eating habits of 51,529 men and 67,272 women to the government’s Healthy
Eating Index, a measure of how well a diet conforms to recom-mended dietary guidelines. Results from these studies demonstrate that eating a healthy diet does provide certain health benefits. People who ate the best, according to the index, were compared to those who ate the worst. Men who ate best had a 28% reduction in heart disease and an 11% decrease in chronic diseases compared to men who ate worst. Women who ate best had a 14% reduction in heart disease but no significant decrease in chronic diseases compared to women who ate worst. There was no significant difference in cancer rates between the men and women who ate best compared to those who ate worst.
The energy the body uses is stored within the chemical bonds of certain nutrients. A calorie (kal′ ō-rē; heat) (cal) is the amount of energy (heat) necessary to raise the temperature of 1 gram (g) of water 1°C. Akilocalorie (kil′ ō-kal-ō-rē) (kcal) is 1000 cal and is used to express the larger amounts of energy supplied by foods and released through metabolism. For example, one slice of white bread contains about 75 kcal, 1 cup of whole milk contains 150 kcal, a banana contains 100 kcal, a hot dog contains 170 kcal (not counting the bun and dressings), a McDonald’s Big Mac has 540 kcal, and a soft drink adds another 145 kcal. For each gram of carbohydrate or protein metabolized by the body, about 4 kcal of energy are released. Fats contain more energy per unit of weight than carbohydrates and proteins, and they yield about 9 kcal/g. Table 17.1 lists the kilocalories supplied by some typi-cal foods. A typical diet in the United States consists of 50–60% carbohydrates, 35–45% fats, and 10–15% proteins. Table 17.1 also lists the carbohydrate, fat, and protein composition of some foods.
A kilocalorie is often called a Calorie (with a capital C). Unfortunately, this usage has resulted in confusion of the term calorie (with a lowercase c) with Calorie (with a capital C). Itis common practice on food labels to use the term calorie whenCalorie (kilocalorie) is the proper term.