LESSON 3.2 WORKBOOK What is fast and slow metabolism? In the last lesson we saw data showing that the extent of obesity in the United States has risen dramatically, and we evaluated how obesity is measure In this lesson we will explore how exercise and body composition relate to metabolic rates. The concepts we will cover include the idea that fast and slow metabolism is largely a consequence of lean muscle mass. We will also explore other factors that may contribute to metabolic rate, such as efficiency of food absorption. Our metabolism determines our caloric needs In Unit 2 we learned a lot about metabolism. Metabolism is the process of breaking down macronutrients to release energy, which in turn is used by every cell in the body. How does this process of metabolism relate to how much we should eat and the idea of fast and slow metabolism? Are you stuck with the metabolism you were born with? Can you alter your metabolism? If you want to gain or lose weight, slowing or speeding up your metabolism seems like a good way to do it. Although there are some factors that you may be able to change about your metabolism, like muscle mass and your diet, others like your age, you are stuck with! 115
DEFINITIONS OF TERMS Thyroid A large gland in the neck that secretes thyroid hormone. Muscle mass can influence your metabolic rate A person that can eat a lot of food and never gain weight may be enviously labeled as having a 'fast metabolism', but are there biological reasons that explain this phenomenon? It is true that some people burn through calories quicker than others, but this effect is not so much based on luck, but rather based on the body composition of each individual. We have previously introduced the concept that muscle tissue is more metabolically active than fat tissue. Therefore, a person with more muscle mass will need more calories to maintain that tissue than a person that weighs the same but is made up mostly of fat tissue. In fact, just maintaining 5 pounds of lean muscle is estimated to costs about 100-250 calories per day! The thyroid controls metabolic rate The thyroid is a gland located in your neck that produces thyroid hormone from amino acids and iodine. One of the main roles of the thyroid is to regulate metabolism by making thyroid hormone and sending it throughout the body. Thyroid hormone passes into cells where they change the expression of genes that are involved in metabolism of carbohydrates, lipids and proteins. By changing the amount of thyroid hormone in a cell, the rate that carbohydrates, lipids and proteins are metabolized is also change Thyroid disorders can speed up or slow down metabolism, depending on whether the thyroid is making too much or too little thyroid hormone. People with hyperthyroidism have too much thyroid hormone, and may feel sensitive to heat, be hyperactive and generally have a faster metabolism. People with hypothyroidism do not make enough thyroid hormone and are often Figure 2: The thyroid is a butterfly shaped gland located in your neck. Thyroid hormone is an important regulator of metabolic rate. Figure 1: Muscles use more calories to maintain than fat. cold, have little appetite, are sluggish and have a slower metabolism. Having either an under or overactive thyroid can alter body composition because of these changes in metabolism. 1. Which of the following is NOT associated with a fast metabolism? Muscle mass. Younger age. Hyperthyroidism. Iodine deficiency. 116
Thyroid disorders are relatively common in the U.S. (about 5-8% of the population is affected), and are more likely to affect older adults and women. If someone is diagnosed with a thyroid disorder they can receive medication to correct thyroid hormone concentrations. Because iodine is necessary for the production of thyroid hormone, iodine deficiency results in hypothyroidism. To reduce cases of hypothyroidism due to iodine deficiency, most salt in the United States is fortified with iodine. Low calorie and low carbohydrate diets slow down metabolism It is true that if you consume fewer calories in a day than you burn, you will eventually lose weight. Unfortunately, the simple equation of energy input equaling energy output isn t always true. When a very low calorie diet is consumed, and not enough calories are supplied to fuel cellular processes and physical activity, metabolism actually slows down. A drop in caloric intake below 1000 calories a day can lead to a reduction in metabolic rate by 50%! The body will adapt by increasing metabolic efficiency, so each cell will use less calories to perform its normal functions. Non-essential cellular functions will cease, and a feeling of extreme fatigue will set in. Additionally, eating a calorie restricted diet may hinder the muscle s ability to grow, so if you are eating few calories but still exercising your muscles will not be able to repair themselves and grow, leading to a further reduction in muscle mass. As we know, your muscle mass affects your metabolic rate, so a decrease in muscle mass will also reduce the 'speed' of metabolism. Figure 4: Cutting carbohydrates completely from the diet may reduce your muscle s ability to grow. Figure 3: Iodine is added to salt to prevent thyroid problems. Similarly, a diet that is restrictive in carbohydrate intake will rely on digesting proteins from muscles for glucose. Recall that the liver performs gluconeogenesis using amino acids to make new glucose to export to the blood for the brain and red blood cells. Without carbohydrate consumption gluconeogenesis is the only source of glucose, and amino acids from the muscle and the diet will be processed into glucose. If a person is consuming a carbohydrate-free diet and still exercising regularly, amino acids may be shuttled away from the muscles and to the liver instead, reducing the person s ability to build new muscle tissue. 2. Which of the following can reduce metabolic rate? Increasing muscle mass. Hyperthroidism. Eating too few calories. Eating too many calories. 117
DEFINITIONS OF TERMS Basal metabolic rate The rate that the body uses energy while at rest to keep vital functions going, such as breathing and keeping warm. Energy expenditure The total amount of energy used by a body; can by calculated by adding up basal metabolic rate, energy used for physical activity and the thermic effect of foo Figure 5: Resistance training, like weight lifting, can help maintain muscle mass as we age. Some things you just can t change As we age our metabolic rate tends to decrease. Children will burn through calories much faster than older adults. This is in part because organs (like the heart, liver, brain and kidneys) make up a larger proportion of a child s body weight than an adult, and organ tissue accounts for large portion of the calories burne Moreover, children s bodies are growing rapidly, and calories must be used to grow their tissues and organs. Metabolism drops substantially after puberty is completed, and then slowly declines throughout the rest of life largely because of a loss of lean muscle mass. While we cannot stop ourselves from aging, we can reduce its impact by participating in resistance training, which builds muscle, and eating more protein rich foods, which helps us maintain muscle mass. There is some evidence that you may have inherited your metabolic from your parents. Studies in twins have demonstrated that people with identical genetics have similar metabolic rates, however the contribution of genetics to metabolism hasn t been completely sorted out. It seems that your genetics may play a role in determining your metabolism, but your metabolic rate is not set in stone, and can be altered by exercise and diet. How is energy from metabolism used? We have now learned that once energy is released from food it is used for physical activity and biological processes. We will now break down energy expenditure into three categories: basal metabolic rate, thermic effect of food, and physical activity. Basal metabolic rate uses the most calories Basal metabolic rate represent the minimal amount of energy used in a fasting state to keep a resting, awake body alive in a warm, quiet environment. For a sedentary person, basal metabolism accounts for about 60-70% Figure 6: Energy expenditure can be divided into three categories: basal metabolic rate (60-70% of energy expenditure, physical activity (25-40%) and the thermic effect of food (5-10%). 3. Physical activity can increase energy expenditure by: Burning more calories. Increasing muscle mass. Reducing muscle loss. All of the above. 118
DEFINITIONS OF TERMS Physical activity Movement that we consider exercise, including resistance training, sports and dancing. Thermic effect of food Energy used to digest, absorb and metabolize the food that we eat. of energy expenditure. Some of the processes involved include the beating of the heart, respiration of the lungs, and activity of the liver, brain and kidneys. The building up and breaking down of your muscles also uses energy that contributes to the basal metabolic rate. We can t change the size or metabolic rate of most organs in our body, but we can increase the size (and therefore the metabolic rate) of the muscle tissue. Amount of calories used for physical activity depends on you! Physical activity increases energy expenditure above and beyond basal energy needs by as much as 25-40%. By choosing to be active or inactive we can change how much energy we expend in a day. Unfortunately, energy expenditure from physical activity varies widely among people, so it s not possible to give a formula for how much activity is needed to increase energy expenditure by a certain number of calories. Physical activity may have the most effect on energy expenditure by changing our body composition (increasing muscle mass), in turn changing the basal metabolic rate. Figure 7: People that fidget burn calories through NEAT: non-exercise activity thermogenesis. Energy is used to digest food Another type of energy expenditure due to exercise is called NEAT, which stands for non-exercise activity thermogenesis. NEAT is the energy that is used for everything that is not sleeping, eating or what we consider exercise. This means that walking to school or the grocery store, cooking a meal, gardening, fidgeting, and basically any movement you do that is not intended to be exercise is considered NEAT. Because NEAT encompasses most of our movement a person that does a lot of exercise but has a low NEAT level may still be at risk for becoming overweight of obese. On the flip side, people that tend to fidget more, or get their exercise by walking to work or taking stairs instead of elevators may be able to stave of weight gain over time. In addition to BMR and physical activity, the body uses energy to digest, absorb and metabolize the food that we eat. This is called the thermic effect of food, and is like a sales tax. We re charged 5-10% of the total energy that we eat to cover the cost of processing the foo For every 100 calories consumed, 5-10 of those calories are used to simply process the foo In addition, food composition influences how many calories are lost to the thermic effect. For example, a protein rich meal has a higher thermal effect than a carbohydrate or fat rich meal, because it takes more energy to metabolize amino acids than glucose or fatty acids. In addition, large meals result in a higher thermal effect of food than the same amount of food eaten over many hours. 4. Going to soccer practice would be considered, while using energy to sit up straight all day is considered. Physical activity; basal metabolic rate. Exercise; physical activity. NEAT; basal metabolic rate. Physical activity; NEAT. 5. Energy used to break down nutrients free from a fibrous food is considered? NEAT. The thermic effect of foo Basal metabolic rate. All of the above. 119
STUDENT RESPONSES Mary is a 68-year-old woman who feels her metabolism has 'slowed'. What would you tell Mary about the possible causes of this change? How can she speed up her metabolism? Remember to identify your sources 120
TERMS TERM Basal Metabolic Rate Energy Expenditure Physical Activity Thermic Effect of Food Thyroid DEFINITION The rate that the body uses energy while at rest to keep vital functions going, such as breathing and keeping warm. The total amount of energy used by a body; can by calculated by adding up basal metabolic rate, energy used for physical activity and the thermic effect of foo Movement that we consider exercise, including resistance training, sports and dancing. Energy used to digest, absorb and metabolize the food that we eat. A large gland in the neck that secretes thyroid hormone. 121