19.1 living things as chemical factories: matter and energy manipulators

Organisms maintain themselves by constantly processing molecules to provide building blocks for new living material and energy to sustain themselves. Autotrophs can manufacture organic molecules from inorganic molecules, but heterotrophs must consume organic molecules to get what they need. All molecules required to support living things are called nutrients. Some nutrients are inorganic molecules such as calcium, iron, or potassium; others are organic molecules such as carbohydrates, proteins, fats, and vitamins. All heterotrophs obtain the nutrients they need from food and each kind of heterotroph has particular nutritional requirements. This chapter deals with the nutritional requirements of humans.

The word nutrition is used in two related contexts. First, nutrition is a branch of science that seek to understand food, its nutrients, how the nutrients are used by the body, and how inappropriate combinations or quantities of nutrients lead to ill health. The word nutrition is also used in a slightly different context to refer to all the processes by which we take in food and utilize it, including ingestion, digestion, obsorption, and assimilation. Ingestion involves the process of taking food into the body through eating. Digestion involves the breakdown of complex food molecules to simpler molecules. Absorption involves the movement of simple molecules from the digestive system to the circulatory system for dispersal throughout the body. Assimilation involves the modification and incorporation of absorbed molecules into the structure of the organism.

Many of the nutrients that enter living cells undergo chemical changes before they are incorporated into the body. These interconversion processes are ultimately under the control of the genetic material, DNA. It is DNA that codes the information necessary to manufacture the enzymes required to extract energy from chemical bonds and to convert raw materials into the structure of the organism.

The food and drink consumed from day to day constitute a person’s diet. It must contain the minimal nutrients necessary to manufacture and maintain the body’s structure (bones, skin, tendon, musle, etc.) and regulatory molecules (enzymes and hormones), and to supply the energy (ATP) needed to run the body’s machinery. If the diet is deficient in nutrients, or if a person’s body cannot process nutrients efficiently,a dietary deficiency and ill health may result. A good understanding of nutrition can promote good health and involves an understanding of the nergy and nutrient content in various foods.

19.2 kilocalories, basal bolism, and weight control

The unit used to measure the amount of energy in foods is the kilocalorie. The amount of energy needed to raise the temperature of 1 kilogram of water 1 C is 1 kilocalorie. Remember the the prefix kilo- means “1000 times” the listed. Therefore, a kilocalorie is 1,000 times more heat energy than a calorie, which is the amount of heat energy needed to raise the temperature of 1 gram of water 1 C. however, the amount of energy contained in food is usually called a Calorie with a capital C. this is unfortunate because it is easy to confuse a Calorie, which is really a kilocalorie, with a calorie. Most books on nutrition and dieting use the term Calorie to refer to food calories. The energy requirements in kilocalories for a variety of activities are listed in table 19.1

Significant energy expeniture is required for muscular activity. However, even at rest, energy is required to maintain breathing, heart rate, and other normal body s. The rate at which the body uses energy when at rest is known as the basal bolic rate. The basal bolism of most people requires more energy than their voluntary muscular activity. Much of this energy is used to keep the body temperature constant. A true measurement of basal bolic rate requires a measurement of oxygen used over a specific period under controlled conditions. There are several factors that affect an individual’s basal bolic rate. Children have higher basal bolic rates and the rate declines throughout life. Elderly people have the lowest basal bolic rate. In general, males have higher bolic rates than women. Height and weight are also important. The larger a person the higher their bolic rate. With all of these factors taken into account, most young adults would fall into the range of 1,200 to 2,200 kilocalories for a basal bolic rate. Some other factors are: climate, altitude, physical condition, hormones, and previous diet, percent of weight that is fat, and time of the year.

Because few of us rest 24 hours a day, we normally require more than the energy needed for basal bolism. One of these requirements is the amount of energy needed to process food we eat. This is called specific dynamic action and is equal to approximately 10% of your total daily kilocalorie intake.

In addition to basal bolism and specific dynamic action, the activity level of a person determines the number of kilocalories needed. A good general indicator of the number of kilocalories neede above basal bolism is the type of occupation a person has. Since most adults are relatively sedentary, they would receive adequate amounts of women consumed 2,200 kilocalories and men consumed 2,900 kilocalories per day. Since approximately 60% of American are overweight or obese, the U.S. Department of Agriculture has developed a program aimed at educating people about the health hazards of obesity. One of the problems associated with obesity is identification—developing a good definition that can be easily understood. Table 19.3 shows guidelines for determining whether you are overweight or not. It is based on a specific method for determining body mass index—appropriate body weight compared to height. Body mass index is calculated by  determining a person’s weight (without clothing) in kilograms and barefoot height in meters. The body mass index is their weight in kilograms divided by their height in meters squared.

(the inside back over of this book gives conversions to the metric system of measurements.) for example, a person with a height of 5 feet 6 inches who weights 165 pounds has a body mass index of 26.6 kg/m2

Table 19.3 provides an easier way to determine your body mass index. Determine your weight withouth clothing and your height without shoes. Then go to table 19.3 to determine your body mass index.

The idea body mass index for maintaining good health is between 18.5 and 25 kg/m2. therefore, the person described above would be slightly over the recommended weight. Those with a body mass index between 25 and 30 kg/m2 are considered overweight, but there are no clear indications that there are significant health affects associated with this degree of overweight. Those with a body mass index of 30 kg/m2 or more hava a significant increased risk of many different kinds of diseases. The higher the body mass index the more significant the risk.

Why is weight control a problem for such a large portion of the population? There are several bolic pathways that convert carbohydrates or  proteins to fat. Stored body fat was very important for our prehistoric ancestors because it allowed them to survive periods of food scarcity. In periods of food scarcity the stored body fat can be used to supply energy. The glycerol portion of the fat can be converted to a small amount of glucose which can supply energy for red blood cells and nervous tissue that must have glucose. The fatty acid portion of the molecules can be bolized by most other tissues directly to produce ATP. Today, however, for most of us food scarcity is not a problem, and even small amounts of excess food consumed daily tend to add to our fat stores.

Although energy doesn’t weigh anything, the nutrients that contain the energy do. Weight control isa matter of balancing the kilocalories ingested as a result of dietary intake with the kilocalories of energy expended by normal daily activities and exercise. There is a limit to the rate at which a moderately active human body can use fat as an energy source. At most, 1 or 2 pounds of fat tissue per week are lost by an average person when dieting. Because 1 pound of fatty tissue contains about 3,500 kilocalories, decreasing your kilocalorie intake by 500 to 1,000 kilocalories per day while maintaining a balanced diet ( including proteins, carbohydrates,and fats) will result in fat loss of 1 or 2 pounds per week. ( a pound of pure fat contains about 4,100 kilocalories, but fat tissue contains other materials besides fat, such as water.)

Many diets promise large and rapid weight loss but in fact result only temporary water loss. They may encourage eating and drinking foods that are diuretics, which increase the amount of urine produced and thus increase water loss. Or they many encourage exercise or other activities that cause people to lose water through sweating. Low carbohydrate diets deprive the body of glucose needed to sustain nervous tissue and red blood cells. If glucose is not available the body will begin to use protein from the liver and muscles to provide the glucose needed for these vital cells. This kind of weight loss is not healthy. Finally, just reducing the amount of food in the gut by fasting resulting in a temporary weight loss because the gut is empty.

For those who need to gain weight, increasing kilocalorie intake by 500 to 1,000 kilocalories per day will result in an increase of 1 or 2 pounds per week, provided the low weight is not the result of health problem.

If you have calculated your body mass index and wish to modify your body weight, what are the steps you should take? First, you should chech with your physician before making any drastic change in your eating habits. Second, you need to determine the number of kilocalories you are consuming. That means keeping an accurate diet record for at least a week. Record everything you eat and drink and determine the number of kilocalories in those nutrients. This can be done by estimating the amounts of protein, fat, and carbohydrate (including alcohol) in your foods. Roughly speaking, 1 gram of carbohydrate is the equivalent of 4 kilocalories, 1 gram of fat is the equivalent of 9 kilocalories, 1 gram of protein is the equivalent of 4 kilocalories, and 1 gram of alcohol is 7 kilocalories. Most nutrition books have food-composition tables that tell you how much protein, fat, and carbohydrate are in a particular food. Packaged foods also have serving sizes and nutritve content printed on the package. Do the arithmetic and determine your total kilocalorie intake for the week. If your intake in kilocalories equals your output, you should not gain any weight! You can double-check this by weighing yourself before and after your week of record keeping. If your weight is constant and you want to lose weight, reduce the amount of food in your diet. To lose 1 pound each week, reduce your kilocalorie intake by 500 kilocalories per day. Be careful not too eat less than 600 kilocalories of carbohydrates or reduce total daily intake below 1,200 kilocalories unless you are under the care of a physician. It is important to have some carbohydrate in your diet because a lack of carbohydrate leads to a breakdown of the protein that provides the cells with the energy they need. Also you may not be getting all the vitamins required for efficient bolism and you could cause yourself harm. To gain 1 pound, increase your intake by 500 kilocalories per day.

A second ingredient valuable in a weight loss plan is an increase in exercise while keeping food intake constant. This can involve organized exercising in sports or fitness programs. It can also include simple things like walking up the stairs rather than taking the elevator, parking at the back of the parking lot so that  you walk farther, riding a bike for short errands, or walking down the hall to someone’s officer rather than using the phone. Many people who initiate exercise plans as a way of reducing weight are frustrated because they may initially gain weight rather than lose it. This is because muscle weights more than fat. Typically they are “out of shape” and have low muscle mass. If they gain a pound of muscle at the same time they lose a pound of fat they will not lose weight. However, if the fitness program continues they will eventually reach a point where they are not increasing muscle mass and weight loss will occur. Even so, weight as muscle is more healthy than weight as fat.

If, like millions of others, you believe that you are overweight, you have probably tried numerous diet plans. Not all of these plans are the same, and not all are suitable to your particular situation. If a diet plan is to be valuable in promoting good health, it must satisfy your needs in several ways. It must provide you with needed kilocalories, proteins, fats, and carbohydrates. It should also contain readily avaible foods from all the basic food groups, and it should provide enough variety to prevent you from becoming bored with the plan and going off the diet too soon. A diet should not be something you follow only for a while, then abandon and regain the lost weight.