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Stay Safe Fit to Survive Assessing Body Composition
 

Assessing Body Composition

Darrell Mendenhall, M.S.
Director of Fitness and Health Promotion
Public Safety Medical Services
Indianapolis, IN 46204
IAFF Peer Fitness Trainer


Over the past few decades, the prevalence of overweight and obesity in the United States has steadily risen and now is at epidemic proportions.

According to the National Health and Nutrition Examination Survey, more than half (64.5 percent) of all adults are overweight or obese. While fire fighters may be healthier, more physically fit and less overweight than the general population, they are not immune to this national epidemic. In addition, the firehouse culture itself at times can contribute to the prevalence of overweight and obese fire fighters.

Several medical conditions are associated with being overweight or obese, including coronary heart disease, high blood pressure and type II diabetes. Additionally, the medical implications of being overweight or obese are exacerbated in fire fighters due to the extreme physical demands of the job. Therefore, achieving and maintaining a healthy body weight is a very important part of health risk management for fire fighters.

Knowing Your Body Type
There are many different methods of assessing body composition, but all can be broken down into three categories: 1) direct methods; 2) indirect methods; and 3) field methods. Direct methods are invasive measures conducted on body organs or whole cadavers. By directly studying body composition on cadavers and specific organs, reference bodies (reference man, reference woman, reference adolescent, etc.) have been developed that describe the anatomic makeup of these bodies. These theoretical models (reference bodies) developed through direct methods form the scientific basis of all indirect and field assessments.

Indirect methods of body composition assessment include dual energy x-ray absorptiometry (DXA), densitometry (underwater weighing) and air displacement plethysmography (ADP). Dual energy x-ray absorptiometry divides total body weight into fat, water and mineral and protein. Densitometry and air displacement plethysmography break the body into fat mass and fat-free mass. These are arguably among the most accurate methods available and are generally recognized as the gold standards, however, expense and practicality make them impractical for most fire departments.

Using the BIA Method
Field methods include skinfolds, bioelectric impedance analysis (BIA) and anthropometry. Skinfolds is widely used and estimates body fat with reasonable accuracy when skinfolds are properly measured. Variations in the skill level of the technician obtaining the measures can have a significant impact on the accuracy of this method.

BIA uses a low level electrical current which passes through the body to measure the impedance to flow to estimate total body water and, subsequently, distinguish between fat-free mass and fat mass. This method is based on the principle that fat-free mass is a good conductor of electrical current because of its high content of water (approximately 73 percent), whereas fat tissue, in contrast, has a low content of water. Under standardized conditions, BIA can provide a relatively accurate estimate of body fat and is simple to administer with no special skill required on the part of the technician. However, fluctuations in hydration status, electrolyte levels and skin temperature can decrease the accuracy of this method.

In addition, the accuracy of both of these methods can be adversely affected by using prediction equations that are inappropriate for the population being tested.

Using Anthropometry
Anthropometry refers to the measurement of the size and proportion of the human body, such as skeletal breaths, body segment lengths and circumference measures. It is also used to describe both regional and total body composition in terms of fat mass and fat-free mass. Three popular anthropometric methods used to identify individuals at risk of disease secondary to being overweight or obese are body mass index (BMI), waist circumference (WC), and waist-to-hip ratio (WHR).

Although equations have been derived to estimate body composition from these measures, they also are population specific and are subject to substantial error if they are not applied appropriately. These anthropometric measures are not widely used to actually estimate body composition per se. They are more commonly used in epidemiological studies as a more crude index of obesity than other field methods and to assess risk of cardiovascular disease due to overweight or obesity.

Understanding BMI
BMI is simply a product of weight in kilograms divided by height in meters squared. The relationship between BMI and cardiovascular disease was originally observed in the Framingham Heart Study. Associations were also established between high BMI and hypertension, type II diabetes, and total cholesterol/HDL cholesterol ratio.

Based on BMI, an individual is classified as normal weight (<25), overweight (25-29.9) or obese (>29.9). BMI is predicated on the assumption that a high value is the result of a preponderance of fat in relation to height. BMI does not factor a high index due to a preponderance of muscle mass, nor does it take into account that a low BMI could occur in an overweight individual due to excess loss of muscle mass -- such as is common in geriatric populations.

Some research has demonstrated an increased risk of cardiovascular disease associated with a BMI of less than 18.5. Therefore, BMI does not provide any real insight regarding the regional distribution of body fat.

Regional fat distribution is described in two ways. Excess body fat in the abdominal region is referred to as android obesity, and is associated with increased risk of cardiovascular disease. It is for this reason that the regional distribution of body fat is important.

Excess fat distributed throughout the buttocks, hips and thighs is referred to as gynoid obesity and does not have the same increased association with cardiovascular disease as android obesity. So, while BMI does provide useful information regarding the characteristics of a population, it does have its weaknesses.

In large epidemiological studies, these weaknesses might not be as significant. However, in special populations (such as fire fighters) where a significant portion of the population is athletic, it is likely that a good percentage of those individuals would have a high BMI when they may not be over fat at all. A study conducted on 109 male body builders found that the correlation between BMI and percent body fat was very weak – a vivid example of the limitations of the sole use BMI to assess relative risk due to overweight or obesity.

Measurement of waist circumference and waist-to-hip ratio better distinguishes between gynoid and android obesity. Most individuals who carry excess fat do not typically have small waists. An international study, the International Day for the Evaluation of Abdominal Obesity (IDEA), of more than 170,000 people from across 63 countries showed that waist circumference is a stronger independent predictor of cardiovascular outcomes than BMI.

This data was presented at the American College of Cardiology Annual Scientific Session in March 2006. Other studies have revealed similar findings. According to the American College of Sports Medicine, waist circumference should not exceed 102cm (40 inches) for men or 88cm (35 inches) for women. The Expert Panel on the Identification, Evaluation and Treatment of Overweight and Obesity in Adults has provided a classification of disease risk based on both BMI and waist circumference.

Classification of Disease Risk Based on Body Mass Index and Waist Circumference
   

Disease Risk Relative to Normal Weight and Circumference

   

Men=/<102cm
Women =/< 88cm

Men>102cm
Women>88cm

 

BMI (kg.m-2)

   

Underweight

<18.5

-- --

Normal

18.5-24.9

   

Overweight

25.9-29.9

Increased

High

Obesity , Class 1

30.0-34.9

High

Very High

Obesity Class 2

35.0-39.9

Very high

Very high

Obesity Class 3

=/>40

Extremely high

Extremely high

Source: ACSM’s Guidelines for Exercise Testing and Prescription, 7th edition

Waist-to-hip ratio is simply the product of the circumference of the waist divided by the circumference of the hips. Unlike BMI, waist-to-hip ratio provides a better differentiation regarding regional body fat distribution. Health risks have been shown to increase with waist-to-hip ratio. The table below provides risk classifications for waist-to-hip ratio.

Classification of Disease Risk Based on Waist to Hip Ratio

Risk Classification

Men

Women

High Risk

>1.0

>.85

Moderately High Risk

0.90-1.0

0.80-0.85

Lower Risk

<0.90

<0.80

Source: American Council on Exercise Personal Trainer Manual, third edition

It is important to understand the importance of assessing body composition for evaluating the associated risks with various diseases. Used in conjunction with each other, these three anthropometric measures can provide very valid and useful information regarding a fire fighter’s health risks associated with overweight or obesity.

 



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