P eople eat a variety of foods, which are broken down by the body and used to build muscle, keep the body warm, and provide energy throughout the day. Metabolism is a term used to describe this breakdown of food into energy in the body. Each person has a different metabolic set point based on a variety of factors, including hormones, which are the main communication tool within the body. Examples of hormones include: insulin, glucagon, thyroid hormone, cortisol, estrogen, and testosterone. Each of these hormones plays a role in the regulation of metabolic rate, which ultimately controls hunger and weight gain or loss. The breakdown of food is the primary manner by which the body produces energy; the energy gained from the food is determined by the number of calories contained in each meal.
The thyroid gland is one of the key organs that regulates metabolic rate through the release of thyroid hormone. People who produce too much thyroid hormone will often be hot, hungry, and unable to gain weight. When someone produces too much thyroid hormone, the body is unable to store any of the energy from the food and, therefore, converts it all to heat. This is what makes the person hot all the time. The opposite of producing too much thyroid hormone is not producing enough. The thyroid gland was one of the first inborn errors of metabolism noted in individuals who were unable to lose weight even after dieting. An individual who does not produce enough thyroid hormone will be cold all the time and will gain weight. This condition is treatable by placing the person on oral thyroid hormones. People attempting to diet have, at times, even resorted to taking oral thyroid hormone to increase metabolic rate and, thereby, lose weight.
When a person limits the amount of food eaten by dieting, the metabolic rate can be affected in a number of ways. Basal or resting metabolic rate (RMR) is a term that describes the lowest number of calories a particular person needs to eat in order to fuel basic life functions. If a person consumes fewer calories than are needed for survival (fewer calories than the RMR), the body will go into starvation mode. In starvation mode, the body begins to shut down processes which are not essential for life. For example, the body will tear down muscle tissue in order to get protein and energy to maintain heart and kidney function, which provides an additional challenge for weight management because muscle uses more energy than fat. A person who has lost muscle after dieting, therefore, has the potential to gain fat after going back to eating regularly. This happens because the reduction in muscle leads to a reduction in RMR, which ultimately means the person needs less food to maintain body weight now than was needed before the diet began.
Severe reduction in the amount of food that a person consumes, as happens when a person is dieting, can also lead to feelings of anxiety, sluggishness, and irritability. These and other side effects have led to the development of dieting drugs that combat the slowing metabolism that sometimes occurs in dieting. Drugs like ephedrine and caffeine stimulate the metabolism, which helps to maintain higher energy levels. These drugs do not prevent the loss of muscle tissue that occurs if a person consumes fewer calories than are required by that person's RMR. For this reason, dieticians and doctors have recommended that people never consume less than 1,200 calories per day. Although 1,200 calories per day is a good estimate for the average person, many people have a higher RMR. In order to better determine RMR, many gyms and health clubs have purchased machines that measure personal RMR. Knowing this information helps dieticians and trainers to develop a dieting strategy that will produce more effective results for a person trying to lose weight.
See also Calorie; Homeostasis; Hormones Used in Dieting; Very-Low-Calorie Diets
References and Further Reading Black, A.E., Coward, W.A., Cole, T.J., and Prentice, A.M. (1996) "Human Energy Expenditure in Affluent Societies: An Analysis of 574 Doubly-Labeled Water Measurement," European Journal of Clinical Nutrition 50 (2): 72-92. Flatt, J.P. (1993) "Dietary Fat, Carbohydrate Balance and Weight Measurement," Annals of the New York
Academy of Sciences 683 (1): 122-40.
Jebb, Susan A. (2002) "Energy Intake and Body Weight," in Christopher G. Fairburn and Kelly D. Brownell (eds), Eating Disorders and Obesity: A Comprehensive Handbook, 2nd edn, New York: Guilford Press, pp. 37-42.
Ravussin, Eric (2002) "Energy Expenditure and Body Weight," in Christopher G. Fairburn and Kelly D. Brownell (eds), Eating Disorders and Obesity: A Comprehensive Handbook, 2nd edn, New York: Guilford Press, pp. 55-61.
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