Fat: Who Says? (Measuring Obesity) What’s Your Resistance?
The Relationship Between Resistance and Water (D)
Student Data Page 3C Part 3
Problem Statement:What effect does water have on BIA (Bioelectrical Impedance Analysis) measurements?
Activity Background:BIA stands for a Bioelectrical Impedance Analysis. It is a quick, non-invasive, and painless testthat takes less than 10 minutes to perform. This test is a way to measure body composition.Understanding what makes up your total body weight and how it changes over time is importantso you can monitor your overall fitness levels. When a BIA is done, a person’s height and weightare taken when the test is administered.
The diagrams below show you how a BIA is actually be taken. Figure 1 shows a relatively inex-pensive machine that takes weight measurements as well as calculates your BIA. The machine inthe center is more commonly found at a medical facility. The last diagram shows how a person ispositioned to take the BIA in a medical laboratory setting.
Figure 1
IMPORTANT EQUATIONS
Body Fat Percentage = Body Fat Mass (lbs)/Total Body Weight (lbs) X 100
Total Body (100%) = Percent Body Fat + Percent Lean Body Mass
Water is of major importance to all living things on Earth. Some organisms contain as much as 90% water. Humans on average contain overall around 60% to 70% water.Approximately 70% of the human brain is water, 90% of the lungs, and 83% of the blood. Humans need to replace close to 2.4 liters of water each day and do so through drinking fluids and eating food.
There are many factors that affect the amount of water in our bodies. Age is one of these factors. Newborns have about 78% just after they are born. At one year of age, it drops to about 65%. In adult men, about 60% of their bodies are water. Women seem to have less water content than men and even race plays a very small part. On a tissue level, fat tissue has less water than lean tissue therefore fat men have less water mass than thin men.
When a BIA (Bioelectric Impedance Analysis) is taken, it is assumed that a person is within a normal hydration level. If a person is not within normal hydration levels, the resultsare not accurate. Dehydration causes the fat mass to be overestimated. Over hydration causesthe fat mass to be underestimated. It is easier to explain dehydration than the term hydration.Dehydration can occur when we loose as little as a 2% of our water.
Dehydration can occur when:• You over exercise i.e. competing in a marathon race.• You are sick such as having a high fever when you have the flu.• You perspire when it is extremely hot outside.• You don’t drink enough fluids in a day.• You consume too many caffeinated drinks or alcohol.• You take some prescriptions such as diuretics.• Women are menstruating.
The following list provides examples of what dehydration does to your body:t Your brain won’t work properly – you may be groggy, slow, and feel out of it. t You’ll lose muscle tone and may experience cramping. t Your kidneys won’t be able to function; toxins and wastes will build up in your body,
making you feel bad. t You may feel overheated, or chronically cold because you cannot regulate your
body temperature. t You may become constipated.t Fats stored in your body won’t get used up or metabolized.t You’ll think you’re hungry all the time, and so you’ll be likely to eat more. t Your skin will get dry, itchy, and saggy. t You may feel dizzy, light-headed, and/or nauseated.
Bioelectric Impedance is measured when a very small electrical signal carried by water and fluids is passed through the body. Impedance is greatest in fat tissue, which containsonly 10-20% water. Fat-free mass contains 70-75% water, allowing the signal to pass muchmore easily. BIA assumes that the body is within a normal hydration range. If a person is dehydrated, the amount of fat tissue is overestimated. If a person is over hydrated, the amount of body fat is underestimated.
Activity Materials:Scenario CardsCalculator 1 Copy Student Data Page per student
Activity Instructions:1. Read the scenario card selected by your group.
2. Use the information in your scenario card to calculate % body fat and the % lean body mass. Then, using the % Body Fat you calculated and Figure 2 Body Fat Tables, determineif the character in your scenario card is Underfat, Healthy, Overfat, or Obese.
Remember:
IMPORTANT EQUATIONS
% Body Fat = Body Fat (lbs)/Total Body Weight (lbs) X 100
Total Body (100%) = Percent Body Fat + Percent Lean Body Mass
Remember:
If a person is dehydrated, the amount of fat tissue is overestimated. If a person is over hydrated, the amount of body fat is underestimated.
3. Apply what you have learned to assess how accurate this BIA might be considering the medical information provided on your scenario card. Specifically, determine if the calculation % of body fat is correct – higher, lower, or normal. Explain your answer.
5. In spite of the inaccuracies in Bioelectrical Impedance Analysis (BIA), it remains a useful tool to help people maintain a healthy body composition. Explain why the inaccuracies would not be important to a person using a BIA scale at home every week.
Figure 3 Example of Fluctuation of Body Fat Percentage Using Bioelectrical Impedance
Source: Omron HBF-306 Body Logic Manual www.omronhealthcare.com
6. Look at Figure 3 – you can see that measurements of body fat percentage seem to fluctuate throughout the day, likely due to hydration changes. The information in this graph might help a person using a BIA scale at home obtain more accurate readings. What should be done to increase the accuracy?
