Understanding the Significance of Body Composition

     Body composition refers to your body weight relative to the amount of muscle, bone, fat, organ and other vital tissues that make up that weight.  Much emphasis is placed on the amount of fat versus muscle tissue.  This is because optimal health is achieved through appropriate amounts of both.  Although the exact body fat percentage identifying risk has yet to be agreed upon, values that range between 10-22% for men and 20-32% for women are generally considered appropriate for health.

     Excess body fat is associated with heart disease, stroke, high blood pressure, type 2 diabetes, abnormal blood lipid (cholesterol) levels and the metabolic syndrome.  In addition to the amount of fat that you have, where it is located on your body is of concern.  Fat that is primarily located around your abdomen ("apple" shape) puts you at greater risk than if the fat is more concentrated around your hips and thighs ("pear" shape).

     There are different methods to estimate your body composition that vary in cost and complexity.  These include: 

• Skin-fold measurements in which various sites on your body are "pinched" by a caliper.  The values are put into a formula to determine percent body fat.  However, results are affected by skill level of the health professional conducting the measurements.
  
• Circumference measurements, such as the waist-to-hip ratio (WHR), involve measuring the girth of various body parts.  Different equations are used based on age and gender to compute the values.
  
• Bioelectrical impedance analysis (BIA) is a method in which the speed of a signal sent between electrodes placed on your feet and hands determines the amount of body fat.  A slower signal indicates greater amounts of fat because fat impedes the signal.  However, results can be skewed based on your state of hydration.
  
• Hydrostatic weighing (underwater weighing) is known as the "gold standard".  The amount of water you displace when submerged in a tank of water after blowing out all of the air in your lungs is measured.  The more water that is displaced the less fat you have.
  
• BOD POD is a fiberglass chamber in which you sit and the amount of air that is displaced by your body is measured to determine body composition.
  
• Dual energy X-ray absorpiometry (DXA) distinguishes the tissue densities of your body by exposing you to low amounts of radiation.
  

     Underwater weighing, the BOD POD, and DXA are typically used in university, hospital, and laboratory settings due to the cost and complexity of equipment and procedures required.  A simple way to estimate body fat percentage that many doctors use is the waist-to-hip ratio.  The WHR is the circumference of your waist at its narrowest point divided by the circumference of your hips at the largest point of your buttocks.  It provides insight into your body's distribution of fat.  If your WHR is high, it may indicate that you have a large amount of visceral fat, that is, fat deep within your abdomen.  An excess amount of visceral fat is associated with the chronic health conditions listed above.  

     The WHR's associated with a greater risk for disease based on age and gender are listed below.

Men:

• very high risk if under 20 years with a WHR that is at least 0.95
  
• very high risk if 60-69 years with a WHR that is at least 1.03
  
• high risk if 20-70 years with a WHR that is at least 0.89-0.99

Women: 

• very high risk if under 20 years with a WHR that is at least 0.86
  
• very high risk if 60-69 years with a WHR that is at least 0.90
  
• high risk if 20-70 years with a WHR that is at least 0.78-0.84

     Body Mass Index (BMI) is another means by which your physician may determine your risk for developing chronic health conditions.  The National Heart, Lung, and Blood Institute (NHLBI) defines BMI as "a measure of your weight relative to your height".  It is calculated by dividing body weight (in kilograms) by height (in meters) squared.  To easily determine your BMI, the NHLBI has a chart on their website that you can use www.nhlbi.nih.gov/guidelines/obesity/bmi_tbl.htm.  

     The NHLBI states that a BMI of 30 or more indicates obesity.  A BMI of 25-29.9 qualifies as being overweight.  Normal weight is a BMI of 18.5-24.9.  And, a BMI below 18.5 is underweight.  Health problems can occur at both extremes.  BMI's greater than 25 are typically associated with an increased risk for obesity-related health problems.  BMI's greater than 30 are associated with a higher incidence of high blood pressure, abnormal blood lipids, heart disease and death.

     There are limitations with calculating BMI.  It does not distinguish between fat and muscle weight.  If you have a muscular build, such as with athletes, it could classify you as being overweight or obese because muscle weighs more than fat.  Health risk tends to be underestimated in individuals who are older or who have lost muscle mass.

     You should consult your physician to determine what the appropriate weight is for you.  Your physician will give you advice on how to lose and maintain body weight in order to achieve optimum health.

Resources:

The National Heart, Lung, and Blood Institute www.nhlbi.nih.gov

ACSM's Guidelines for Exercise Testing and Prescription eight edition