The food we cat provides us with energy, water, and molecules for growth, maintenance, and repair. The role of food as a source of energy was explored in Exercise A. In this exercise, we will examine the molecules needed for proper health. These can be classified as macronurrients, if needed in large amounts, or micronutricnts, if needed in small yet essential quantities. The macronutricnts are organic chemicals that includc carbohydrates, lipids, and proteins. These compounds, especially the carbohydrates and lipids, supply most of our energy. The macronutricnts are our source of building materials to make more living material molecules, cells, and tissues in our bodies.
Carbohydrates can be grouped as simple sugars (monosaccharides and disaccharidcs) and complex carbohydrates (polysaccharides). Simple sugars, such as glucose, fructose, or sucrose, are cheap sources ofcncigy. In our bodies, these can easily be oxidized by respiration to produce ATP. They arc also building blocks for more complex carbohydrates such as glycogen. When consumed in excess, our bodies usually convert sugars to fat Simple sugars are naturally found in many plant foods, such as fruits. Often, however. simple sugars are added to foods to sweeten and enhance the flavor. Most Americans consume much more sugar than they need. The simple sugars only provide energy and are not required for other nutritional needs.
Complex carbohydrates arc larger chemicals composed of long chains of simple sugars linked together. When we ear complex carbohydrates instead of simple sugars, the energy is released over a longer period of time, so we avoid sudden peaks or valleys in blood sugar levels. Complex carbohydrates include starch and glycogen, which arc both polymers of glucose units.
When plants photosyitthesize, thev store the carbohydrates they produce as starch. Because of this, many foods of plant origin arc high in starch. In laboratory Topics 11 and 13 we will explore starch more in specific classes of foods. Glycogen ditYers from starch only in the degree of branching in die glucosc chains. Although it is not found in foods of plant origin, our bodies temporarily store high levels of glucose as glvcogcn. Another complex carbohydrate is cellulose. It is also a polymer of glucose, but the bonds between the glucosc units arc dif ferent from those in starch or glycogen. This difference prevents our bodies from digesting cellulose. However, even though we do not digest it, cellulose and other indigestible polymers, such as lignin and pectins, play an important nutritional role as dietary fiber. The fiber absorbs water in the intestines and helps move waste material through the colon. High fiber diets are recommended to help rcducc the risk of colon cancer.
Most of the lipids in our diet are fats. Like carbohydrates, lipids primarily provide energy to our bodies. Fats provide even more energy per gram than carbohydrates. l:at consumption in the United States increased from 34% of the diet in the 1930s to 42 in the late 1950s and 1960s. The current levels have dropped off somewhat, but thev arc still higher than recommended levels. Animal products are often higher in lipid content than many plant products. However, as explored in Laboratory Topics 12 and 13, some foods of plant origin are also relatively high in fat, such as avocados (so watch the guacamole ;.
Diets high in fat, especially fats composed of saturated fattv acids, are associated with manv health . .
problems, including certain forms of cancer and cardiovascular diseases. Saturated fatty acids occur more commonly in tars of animal origin, but some plant lipids, such as coconut oil and palm oil, are also high in satu rated fatty acids.
L'suallv it is recommended that a person's total daily fat intake be less than 65 g for a 2,000 kcal diet (with less than 20 g as saturated fat). In most diets, especially if animal products arc consumed, the level is often higher. However, rwo fatty acids, linolinic acid and linolcic acid, arc essential in vour diet because vou cannot svnthesize them. • . .
Both are found in many seeds, nuts, and animal products.
Proteins are composed of various combinations of 20 amino acids. We use proteins in our bodies as enzymes, contractilc molecules in muscles, and structural components of bone, cartilage, skin, and blood. Although we consume proteins in our food, it is actually the amino acids we are after. If we have the amino acids, we can form new proteins. A typical adult needs 50 to 60 g of protein per day. In addition, we need to consume adequate amounts of eight essential amino acids fisoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine since our bmlics cannot synthesize them. A balance of proteins from grains and legumes can provide all the eight amino acids.
Micronutricnts are required in smaller quantities, yet are often needed for key metabolic reactions. Micronu-trients are either vitamins or minerals. Vitamins are organic compounds that often act as coenzymes in metabolic reactions. When left out of a diet, the deficiencies often lead to debilitating diseases and even death. Vitamins are either water-soluble or fat-soluble. The watcr-sol-uble vitamins are not stored in the body and tend to be
Fats and sweets (Use sparingly)
Meat group (2-3 servings)
Milk group (2-3 servings)
Fruit group (2-4 servings)
Vegetable group (3-5 servings)
Bread and cereal group' (6-11 seryings)\ J
FIGURE I 0.3 THE FOOD PYRAMID ILLUSTRATES THE RECOMMENDED DIETARY GUIDELINES SET FORTH BY THE U.S. DEPARTMENT OF AGRICULTURE.
removed by the kidneys. They must be added to die diet on a regular basis. The fat-soluble vitamins can be stored by the body and used as needed. High levels of water-soluble vitamins arc often washed away in urine, whereas liigh levels of fat-soluble vitamins can actually cause toxicity.
Minerals are elements also needed in the diet to help ensure proper health and to aid metabolism. People in different stages of their lives, such as infants or pregnant women, often have different mineral requirements, so general recommendations need to be adjusted according to life phase.
The USDA has advanced the food guide pyramid (lig. 10.3) as a simple guide to eating. In addition, nutritional labels that provide caloric and nutritional data to the consumer are required on all processed foods in the United States.
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