11.6: Energy and Body Weight
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The supply of energy from nutrients must be continuous because homeostasis and survival require that vital functions never cease. Since people eat intermittently rather than continuously, only some of the energy from food is used immediately after its nutrients have been absorbed. The remaining energy is stored in reserves until needed.
Most reserve energy is stored as fat in fat cells and as glycogen in the liver and in muscle cells. The body draws on these reserves when energy supplies from the nutrients being absorbed are lower than energy demands. The body can obtain energy from body proteins, but it usually does this only after most of the glycogen and fat reserves have been used because proteins make up essential body components.
Dietary Sources of Energy
Most energy from foods is obtained from carbohydrates, lipids, and proteins (see Chapter 2). The carbohydrates in foods are either single sugar molecules such as glucose or larger molecules made of combinations of sugar molecules. Common examples include the disaccharides sucrose (table sugar) and lactose (milk sugar) and the polysaccharides starch and glycogen. Dietary lipids occur in a variety of forms, though usually more than 90 percent of them are in the form of triglycerides, which are also called fat on food labels. Other dietary lipids include cholesterol, monoglycerides, diglycerides, and phospholipids.
Disaccharides and polysaccharides are broken down into single sugar molecules before being absorbed into the body. Most sugars are converted into glucose by the liver before being transported by the blood to other cells. Fat is also broken down before absorption but is reconstituted before entering the circulatory system. Proteins are broken down into amino acids before they are absorbed.
Obtaining Energy from Molecules
Energy in sugar, fat, and amino acid molecules is contained in the chemical bonds that hold the atoms composing each molecule together. Cells convert this energy into forms they can use by breaking the chemical bonds and transferring the released energy into motion, other chemical bonds, or heat.
Glucose molecules can yield useful energy almost immediately as they pass through a series of chemical reactions called glycolysis, which takes place in the cell cytoplasm. Glycolysis converts only approximately 5 percent of the energy in a glucose molecule into an immediately useful form. Some of the energy is released as heat, and the rest is in the remaining fragments of the glucose molecule. These fragments enter the mitochondria, where they are broken down further by complicated processes called the Krebs cycle, electron transport, and oxidative phosphorylation. More than half the energy released from glucose and other sugar molecules by these processes appears as heat. Most of the remaining energy is placed into adenosine triphosphate (ATP). The energy in ATP is used to power vital processes such as moving, manufacturing substances and body components, and transporting materials.
Fat and amino acids must also be broken down partially before they can release useful energy. Most of the fat and amino acid fragments enter the mitochondria, where chemical reactions convert most of their energy into heat and ATP molecules. Note that the initial partial breakdown of amino acids produces a toxic waste material (ammonia), which is immediately converted into a harmless substance called urea. The kidneys pass urea into urine for elimination.
The breakdown of molecular fragments from sugar, fat, and amino acids by mitochondrial processes requires the addition of oxygen and results in the formation of CO2 and H2O. If the oxygen supply is inadequate, glucose fragments are converted to lactic acid while most fat and amino acid fragments accumulate as ketones (ketoacids). Therefore, having an inadequate supply of oxygen limits energy extraction and leads to an excessive buildup of lactic acid or ketoacids, which disturb acid/base balance. Energy extraction can also be affected by limitations in the number of mitochondria, as occurs in unexercised muscle cells. Therefore, when oxygen supply or mitochondrial numbers are low, weakness and fatigue may result.
Kilocalories
Comparing the energy contents (kilocalories) of carbohydrates, fat, and proteins reveals that for a given weight of a nutrient (e.g., 1 gram or 1 ounce), carbohydrates and proteins contain nearly the same amount of energy. A sample of fat of the same weight contains approximately twice as much energy.
The high energy content of fat may lead to difficulties in interpreting labels on packaged foods. For example, a label indicating that a food is 95 percent fat-free may mean that only 5 percent of the weight of the food is fat. However, more than half the kilocalories may come from the fat since much of the remaining weight may come from water or other low-calorie components.
Energy Balance
If the total energy intake over a period equals the total amount of energy used by the body during that period, the body is in energy balance. If the energy intake is greater than the energy used, the body has a positive energy imbalance. The extra energy is stored as fat and glycogen, causing a gain in weight. If the energy intake is less than the energy used, the body has a negative energy imbalance. Additional energy needs are met by breaking down stored fat and glycogen, causing a loss of weight.
Energy Use
The energy used by the body can be placed into several categories. One category includes the energy needed to sustain body functions when a person is awake and in a state of complete rest. These conditions are called basal conditions, and the rate of energy use is called the basal metabolic rate (BMR).
About 20 percent of BMR energy use is due to muscle cells. Some of this energy is used by the few muscle cells that contract for breathing and for maintaining muscle tone, though muscle cells that are not contracting also use energy to tear down and rebuild their internal components. Although all cells constantly undergo this process of turnover, muscle cells account for a large proportion of the BMR energy used because of the relatively large amount of muscle tissue in the body. Therefore, individuals with more muscle mass have higher BMRs. Additional energy is used by muscles for respiration and heartbeat.
Other portions of BMR energy are used by turnover activities in other cells and in the normal replacement of cells in the skin, blood, GI tract lining, and in the uterine lining after menstruation. About 40 percent of BMR energy is used by ongoing brain and liver activities. Finally, some of this energy is used to keep the body warm. In children, the energy needed for growth adds to these uses. The BMR for an average young adult is approximately 1 kcal per minute, or 1,440 kcal per day.
A second portion of the energy used by the body powers the processes involved in digestion. This energy may constitute approximately 5 percent to 10 percent of the body's daily energy use. A third portion goes into muscle contractions during physical activity. Since the amount of an individual's physical activity usually varies greatly from one day to the next, so also does the amount of energy used. Days involving light physical activity (e.g., writing letters, talking with friends) may add a few hundred to more than 1,000 kcal of energy use to basal energy needs. Days involving many hours of strenuous physical activity (e.g., carpentry, caring for a household with children, hiking) may double the body's energy use at basal conditions.
A fourth portion of body energy use serves defensive and healing functions. For example, much energy is used to maintain a high fever, combat a major infection or widespread cancer, or recover from surgery, a burn, or another extensive injury. Like the energy for physical activity, this type of energy use is quite variable and can exceed the amount used at basal conditions. Pregnant women use energy in a fifth way by supporting the development of a fetus.
Age-Related Changes in Energy Use
Aging is usually accompanied by a decrease in BMR energy use that is mostly caused by declining muscle mass. The BMR energy use from non-muscle mass remains basically unchanged or declines slightly with age. A decline in non-muscle BMR energy use may result from slower cellular turnover or replacement, slower synthesis of secretions by glands, and, in women, cessation of menstruation.
Though some of the decline in muscle mass is due to age changes in muscles, frequently a much larger proportion of the decline in muscle mass, and therefore in BMR energy use, results from decreased physical activity. Adults who remain physically active as age increases retain much of their muscle mass and have a small decline in BMR energy use. Furthermore, sedentary elderly people who increase their muscle mass through exercise have increases in BMR energy use.
There is probably no significant change in energy use from digestive processes in healthy aging adults. However, certain diseases of the digestive system may increase (e.g., GI tract spasms) or decrease (e.g., atrophic gastritis) this energy use.
On the average, energy used for physical activity decreases with age. The reasons for this decline include age changes; physical and mental disabilities; diseases; retirement; children reaching adulthood and needing less care; institutionalization changes in priorities; and following the preconceived or stereotyped sedentary lifestyles of the elderly. However, aging individuals can maintain or increase their level of physical activity and thus maintain or increase their energy use for physical activity.
Since many defense and healing processes occur more slowly with age, their rates of energy use may also decrease with age. However, since the number, frequency, and severity of problems requiring these processes seem to increase with age, the total amount of energy use for these processes may also increase.
Combining age-related changes in the five categories of energy use results in an average continuous decline in energy use with age. Estimates of energy use for men ages 23 to 50, 51 to 74, and 75 and above are 2,700 kcal a day, 2,400 kcal a day, and 2,050 kcal a day, respectively. The corresponding estimates for women are 2,000 kcal a day, 1,800 kcal a day, and 1,600 kcal a day. These values are estimated averages for healthy adults performing light physical activity. The actual values for individuals may vary from these estimates by more than 1,000 kcal a day depending on body size, amount of physical activity, and health status.
Age-Related Changes in Energy Balance
On the average, changes in body weight, muscle mass, and bone material suggest that there is a positive energy imbalance until about age 50. After that, there is at most a slightly negative energy imbalance. However, many older individuals vary significantly from the average and have substantial positive or negative energy imbalances.
For many people beyond age 50, energy balance is fairly well maintained while the total amount of energy intake and use declines substantially. At the same time, the amounts of almost all nutrients needed by the body seem to remain stable or increase. This means that to obtain all required nutrients in adequate amounts while consuming a diet with fewer kilocalories, the foods being consumed must have higher concentrations of nutrients relative to the kilocalories contained in those foods. The ratio of the amount of a specific nutrient to the number of kilocalories in a portion of food is called nutrient density. Foods in the fats, oils, and sweets group in My Plate have a very low nutrient density.
A better way to ensure an adequate intake of all required nutrients while maintaining energy balance is to increase energy use by increasing physical activity. This allows a person to eat more food while avoiding a positive energy imbalance, which can lead to excess body weight from a disproportionate amount of body fat. This strategy also increases the other benefits derived from regular exercise.
Overweight and Obesity
To understand what excess body weight and a disproportionate amount of body fat mean, one must establish values for desirable body weight and percent body fat.
Problems Increased by Obesity
For the U.S. National Library of Medicine and NIH comprehensive web site about obesity, go to http://www.nlm.nih.gov/medlineplus/obesity.html .
For “Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults” by the National Heart, Lung, and Blood Institute, go to http://www.ncbi.nlm.nih.gov/books/NBK2003/
For a PDF file of report on obesity, go to http://www.nhlbi.nih.gov/guidelines/obesity/ob_gdlns.pdf . Go to page 12 of the report for the section on health risks and other outcomes from obesity (e.g., social, psychological). For the NHLBI BMI calculator, go to http://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm
For the NHLBI web page with tips and information about overweight, go to http://www.nhlbi.nih.gov/health/public/heart/obesity/lose_wt/index.htm .
For the Centers for Disease Control and Prevention web site on “Overweight and Obesity”, go to http://www.cdc.gov/nccdphp/dnpa/obesity/index.htm . It has links to diverse topics about obesity and weight control (e.g., contributing factors, trends, consequences, recommendations, BMI calculators) .
For the Merck comprehensive web site about obesity, go to http://www.merckmanuals.com/home/disorders-of-nutrition/obesity-and-the-metabolic-syndrome/obesity .
For the Endocrine Society's web site about obesity and Obesity In America, go to http://obesityinamerica.org/ .
Also see https://www.biologyofhumanaging.com/websites.htm#Nutrition .
Using Tables
Several tables of desirable body weights for people without serious disease have been developed. A table from the Metropolitan Life Insurance Company is based on the assumption that the weights of individuals with the longest life spans represent desirable body weights. The National Health and Nutrition Examination Surveys (NHANES) table and the Andres table include values for older people. The Andres table also considers the average age-related increase in the proportion of body fat resulting from loss of muscle tissue. Though these tables provide guidelines for desirable body weights, factors such as body frame size and amount of muscle tissue should be considered in establishing the desirable body weight for an individual. (Suggestion 243.01.02)
Using Body Mass Index
Another way to determine whether a person has a desirable body weight is to find the body mass index. This value is calculated by dividing the person's weight in kilograms by the square of the person's height in meters. Values between 25 and 30 are considered desirable because people whose body mass indexes are within this range have the greatest longevity and the lowest risk of contracting diseases such as diabetes mellitus and high blood pressure. Those whose body mass index is above 40 have considerably reduced longevities and are at high risk for a variety of weight-related diseases.
Using Percent Body Fat
Percent body fat can be determined in several ways. These include measuring the thickness of skin folds at one or more places; measuring how much electrical resistance the body provides; and comparing a person's weight in air with body weight when that person is completely submerged in water.
Desirable percent body fat values for adults are considered to be 15 to 18 percent for men and 20 to 25 percent for women. Adults with values above 25 percent for men and 30 percent for women have shorter longevities and higher risks for developing weight-related diseases. Since the risks are somewhat higher for individuals with high concentrations of fat around the waist rather than the thighs, determining the ratio between the circumference of the waist and that of the hips gives a further indication of desirable percent body fat. Waist-to-hip ratios should be less than 0.9 for men and less than 0.8 for women.
Definitions
We can now define terms indicating deviations from accepted standards. People whose body weights are 10 percent to 20 percent greater than the desirable body weights can be considered overweight. People whose body weights are more than 20 percent above the desirable body weights can be considered obese if their percent body fat exceeds 25 percent (men) or 30 percent (women) or if their body mass index is more than 30. Very muscular individuals whose weight is more than 20 percent above desirable body weights but whose percent body fat is less than 25 percent (men) or 30 percent (women) are overweight but are not considered obese.
Consequences
Being overweight but not obese has little effect on a person's longevity or risk of developing weight-related disease. Some authorities suggest that in general elderly people may benefit from being slightly overweight because the extra energy stored in their body fat helps them maintain nutritional homeostasis and endure the adverse effects of periods of illness or other undesirable circumstances. (Suggestion 243.02.04)
Though being slightly overweight may be beneficial, obesity is accompanied by increases in the incidence of a variety of problems, a greater negative impact from these and other problems, and lower longevity. The incidence and seriousness of weight-related problems are directly related to the degree of obesity. Longevity is inversely related to the degree of obesity. Problems Increased by Obesity
For a list of problems increased by obesity, go to https://www.biologyofhumanaging.com/tblobese - true.htm .
Prevention and Correction
The best way to avoid the consequences of obesity is to avoid becoming obese. This is much easier than trying to lose weight once obesity has developed. Furthermore, recurring and significant fluctuations in weight decrease longevity and increase the risk of developing weight-related disease. (Suggestion 243.02.04)
One of the most important steps in avoiding obesity as age increases is to maintain energy balance by decreasing total energy intake when energy use decreases. A key sign that this action is appropriate is a significant increase in weight.
See Limiting Intake of Fat, Saturated Fat, and Cholesterol.
Another important step is staying physically active to maintain a high level of energy use. Physical activity keeps energy use high both directly and by helping to maintain a large muscle mass, which sustains a high BMR energy use. It can also help keep energy intake down by suppressing appetite, distracting attention from eating, promoting interest in diverse activities while preventing boredom, and supporting a healthful psychological state. Finally, since people who get plenty of exercise can eat more and not gain weight, they have a higher chance of obtaining adequate amounts of all necessary nutrients without becoming obese (Chapter 8).
Solving obesity by losing weight and decreasing percent body fat can be very difficult since many factors may contribute to obesity. Key features in a successful program to lose weight include getting a physical examination; developing a long-range plan for gradual weight loss and weight maintenance; decreasing energy intake while eating foods with high nutrient densities; exercising; and receiving monitoring regularly to prevent malnutrition and other problems.
Underweight
Having a weight below the range for desirable body weight is called being underweight. The negative energy imbalance that causes a person to become underweight may result from inadequate energy intake, a reduced ability of the digestive system to make energy-containing nutrients available to the body, or excessively high energy use.
Consequences
Being underweight may have several undesirable consequences. These including muscle weakness; fatigue; lethargy; increased risk of low body temperature; reduced resistance to infection; and decreased ability to tolerate periods of adversity such as a prolonged disease. Since being underweight is often but not always accompanied by deficiencies in specific nutrients, body malfunctions and other problems are also commonly present, including specific diseases resulting from nutrient deficiencies. This variability helps explain why some people with very low body mass index seem to have a greater mean longevity, while others have a reduced mean longevity.
Often, being underweight results in slightly lower longevity for underweight persons or those who have a body mass index below 20. Some scientists believe that the decline in longevity is directly related to the degree to which a person is underweight, while others believe that having a low body mass index increases mean longevity. Adverse effects from being underweight may be greater for the elderly than for young adults.
Prevention and Correction
Preventing being underweight involves avoiding a negative energy imbalance. This may involve increasing energy intake when activity levels increase. When a substantial drop in weight becomes apparent, dietary energy intake may be increased, correction of or compensation for digestive system difficulties may be implemented, or activity levels may be reduced. These strategies also can be used to correct a chronic underweight condition. As with solving obesity, correcting underweight conditions in elderly persons requires special awareness of and attention to each individual's particular circumstances.