v   Explain how Body Mass (BMI) Index is determined and used. 

Introduction

Body composition refers to the relative percentage of fat to lean body mass, muscle, fat, bone, water and other lean body tissues.  The body is made up of fat and lean mass and neither is more important than the other; they are equally important.  The body parts, such as bone, teeth, muscle, skin and water are considered lean, so all that is not lean is classified as fat. 

Fat may be expressed as a percentage of the body weight in proportion to total body weight, the weight of fat and lean mass.  Since the percentage of body fat is known to influence the performance in sports there is a concern for determining the amount of fat in the body.  A high percentage of fat is known to decrease performance in comparison to a low percentage.  However, contrary to common beliefs about fat, as a source of energy in the body, it is the primary fuel of the body.  It is fat that runs the body.  Thus, fat is not as bad as it is believed to be.  As a matter of fact, fat is essential.  It is an essential fuel because it supplies more energy than the other two energy sources, carbohydrates and proteins, and it never runs out.  The body never uses all of its fat.  Essential fat not only serves as a continuous source of energy, it also protects internal organs.  When fat performs in ways that benefit the body other than providing energy, it is called essential fat.  Three more ways that fat benefits the body are as follows: 1) It regulates essential nutrients (vitamins A, D, E, and K), 2) It protects women with Amenorrhea against bone loss (osteoporosis), and 3) It regulates body temperature.  Thus, because of its many benefits, standards have been established to determine how much fat and individual should possess.  Every person should posses the minimal amount of fat, essential fat, needed for good health.

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Fat Mass

To understand body fat or adipose tissue, it is necessary to view it as a fuel and to know how it is introduced and stored in the body.  When food that contains fat is consumed, it is digested or acted upon by saliva, stomach acid, and pancreatic enzymes to form free fatty acids, monoglycerides, triglycerides, high-density cholesterol, low-density cholesterol, and other by products.  The fatty acid molecules that are formed from the food people eat are extremely small and highly mobile.  Therefore, it is easy for them to pass through semipermeable membranes, the porous walls of capillaries and cells.  And they can move out of the blood stream into muscle cells to be burned for energy.  When fatty acids are not needed because there is not enough physical activity, they can move back into the blood and travel to fat cells for storage.

Triglycerides and cholesterol make up the bulk of the fats in the bloodstream.  They sometimes build up on the inside of coronary arteries and are associated with heart attack, stroke, and diabetes.  These are the fats that physicians measure to give people and indication of their risk of having a heart attack.

Fatty acids do not attach to anything; they move around looking for an exercising muscle.  They are not associated with heart attacks or other cardiovascular diseases; they have two important roles: 1) They are the basic fuel for energy and 2) They are stored to make people fat.  When fatty acids collect in fat cells, they bond together in threes to form triglycerides and remain stored in the body until needed.  A big belly may be thought of as a big chunk of triglycerides.  When triglycerides continue to collect in large amounts, they sometimes spill out into the blood stream again.  This is what doctors find when they say the triglycerides are too high.  So, triglycerides do two things: they collect in fat depots, and they disassemble to go back into the blood as fatty acids, when muscles need fuel. 

Body fat is stored in different regions of the body, and the pattern of distribution can actually alter the health risks of obesity, over-fatness.  That is, the location of excessive amounts of fat may lead to certain health problems.  For instance, the storage of large amounts of fat in the upper body results in  increased risk of coronary artery disease, whereas, weight gain in the thighs and hips, due to excessive fat, does not have the same magnitude of increased risk.  Measurements of the waist and hips can be used to determine the waist-to-hip ratio, which can also be used to assess the degree of risk associated with fat patterning.  When people who exercise regularly and participate in sports have excessive weight, due to fat storage in either of these regions of the body or throughout the body, they experience a negative affects on their ability to perform in sports.  Improvements in performance are usually recognized, when a person's body fat is reduced.

There are several methods that can be used to asses the amount of fat in the body for the purpose of  determining whether a person is over-fat, which is not the same as overweight (excessive weight for height without consideration of body composition).  The most common method used is based on a normalized  index of estimates of body composition; it is called the Body Mass Index (BMI).  This approach to assessing body composition is based on a table of heights and weights, which represents a height normalized index.  This method is not a good estimate for sport participants, but it may be used as a quick reference. Three popular methods for measuring body composition are underwater weighing, bioelectrical impedance analysis, and skinfold measurements.  The results of these assessments are compared to standards that have been established to determine whether a person has achieved the desired level of body fat.  A trained female sports enthusiast 18 to 24 years of age should strive to achieve and maintain a body fat composition that is equal to or less than 11% fat.  A trained male sports enthusiast 18 to 24 years of age should strive to achieve and maintain a body fat composition that is equal to or less than 9% fat.  Of course, these goals are not definitive, as observed in the range of body fat percentages of a given group.  Also, it is important to note that the desired level of fat varies according to the sport of interest.  In some sports, more fat may be inconsequential.  However, as a general rule, there is a desirable level of fat that is recommended for any sport.

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Lean Mass

When fat is removed from the body, all that remains is fat free mass or lean mass, which is the second dimension of a person's weight.  This aspect of the body composition includes bones, muscles, blood, water, nervous tissue and other tissues making up the body.  It is this lean mass in the body that must be considered in the study of the components of fitness: flexibility, cardio respiratory endurance, and strength.  Therefore, when a person participates in exercises that are designed to improve these three components of fitness, they are exercising to improve the quality of his or her lean body mass.

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Physical Activity and Body Mass Composition

If sufficient exercise is performed, the triglycerides stored throughout the body will diminish.  The most effective way to manage body mass composition is through a balanced diet and regular participation in physical exercise.  While the energy available from fat is used throughout the body, exercising muscles in specific ways can utilize a tremendous amount of fat to produce the energy they need for contraction.  Thus, achieving and maintaining a desirable body composition is an important result of regular fitness training that prevents excessive accumulation of fat.   When this occurs, the percentage of fat stored in the body in proportion to total body weight cannot be high.

The lean mass emphasized in the study of fitness constitutes the structural or organizational hierarchy of the body.  From the chemical level to the system level, there is a response to physical activity.  All levels of this structural complexity is lean mass.  And it is this part of the body that responds to physical activity in ways that can be observed and assessed to determine what activities must be performed to progress toward optimum fitness.  Included in this model is the body's use of fat as fuel. 

Thus, as lean mass carries out the many functions of the body, during physical activity, fat is the fuel used to produce the energy needed.  Some of the the functions of the body that are influenced by physical activity that require the production of energy, according to the level of physical activity experienced, are nerve impulses, cardio respiratory system functions, muscle contractions, and the excretion of waste. The end result of proper management of exercise and diet is a desirable body mass composition and system adaptations that bring about optimum fitness.  With proper exercise a desirable percentage of fat, by weight, in proportion to total body weight, will be achieved.  This includes the weight of lean mass present.

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Summary

This lesson provides an introduction to some of the basic principles of exercise physiology that are essential for an understanding of fitness training .  First you must understand the meaning of essential fat, over-weight, and obesity.  Then, with knowledge of how fat is introduced and used in the body, you can gain an appreciation of the need to determine its percentage of  fat.  Appreciation of this concept is heightened by an understanding of  the different methods that are used to measure body fat.

Knowledge of fat and lean mass and the roles that they play in the accomplishments of higher levels of fitness are important concepts about fitness training that must be acquired.  The ability of the body to utilize fat to produce energy for the many body functions that occur in response to physical activity is the basis of fitness training.  Thus, designing programs to improve the components of fitness: (flexibility, cardio respiratory endurance, and strength) can only be achieved through understanding this model, which includes an understanding of the use of  fat as fuel and the ability of the body to store it all over for future use in accordance with the demands of physical activity.

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References

Andersen, R., Wadden, T., S., and Weinstock, R. (1995).  The Effects of Weight Loss and Exercise on Changing Blood Lipid and Lipoprotein Profiles in Obese Women.  American Journal of Clinical Nutrition, 62: 350-357.

Andersen, R/. Brownell, K., and Haskell, W. (1992).  Health and Fitness Leaders Guide: Administering a Weight Management Program.  Dallas: American Health Publishing.

Durmin, J. and Womersley, J. (1974).  Body Fat Assessed from Total Body Density and its Estimation form Skinfold Thickness: Measurements on 481 Men and Women Aged 16 to 72 years.  British Journal of Nutrition.

Howley, Edward T. and Franks, Don B. Heath Fitness Instructor's Handbook  Human Kinetics, Champaign, IL. 1997. 

McArdle, W. D., Katch, F.  I.., and Katch, V. L (1991).  Exercise Physiology: Energy, Nutrition, and Human Performance. Lea & Febiger, Malvern, PA.

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