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HOW USEFUL IS BMI?
Transcript of HOW USEFUL IS BMI?
BMI is not always a reliable measure of obesity or of being overweight for an individual. This index does not take account of other factors such as muscle: fat ratio, waist size, fat distribution or fitness. These factors may have a greater effect on the overall health of a person than just height and weight. By using a broader spectrum of easily obtainable body measurements, a better index can be developed which would include more of the factors that correlate weight to a person's state of health.
• Larger sample group: All statistical investigations benefit from having larger samples as this reduces the distorting effect of outliers.
• Investigate in athletes: Athletes in certain sports (eg rugby, sprint racing) require a large muscle:fat ratio.
•Divide data in terms of different ethnic groups: The effect of genetic factors that determine body morphology would be reduced by comparing people from the same genertic background with each other.
Although Body Mass Index (BMI) is accepted by the World Health Organisation as the standard expression of weight status, it is of limited use because it is restricted to only two measurements, height and body mass. Its reliability as an indicator of health and risk of weight related illness is therefore questionable. It can be convincingly argued that with our modern understanding of risk factors, a more sophisticated tool can be developed to provide a better indicator of the statistical risks individuals may be at due to the fat content of their bodies. For the purposes of this project, the effect of waist measurement (which is not accounted for by the BMI) as a risk factor for cardiovascular health will be examined.
What is BMI?
The body mass index (BMI), or Quetelet Index, is a heuristic proxy for estimating human body fat based on an individual’s weight and height. BMI does not actually measure the body fat as a percentage of an individual's weight.
The aim of the project is to show that BMI is only very broadly reliable, more useful in assessing large populations rather than individuals. When used in an undiscerning way, it can produce many false positive results depending on fitness, bone density, body type, gender, muscle:fat ratio and fat distribution of an individual. This research exercise also intends to explain the risks raised by the rigid application of the BMI by many commercial dieticians. There is also a risk that the diet industry could misuse this index to maximise profits. Changing the diet and exercise regime of patients based on this single measure would clearly not be risk free.
In deciding on my research subject, designing the project and putting the presentation together, I have received much appreciated assistance and guidance from the following people:
(1) My science teacher, Dr Slade , who discussed the project with me and assessed my initial ideas.
(2) Mr Salyani, who is a family friend, helped me find research studies, proof read and edited the content of my presentation. He also acted as my mentor anbd adviser.
(3) My brother, Sami Khan, who helped me with my choice of presentation format, the IT issues involved and assisted in the creation of the graphs.
I am grateful for the invaluable assistance I have received from these people and others who have provided constructive criticism of my presentation.
BMI was devised between 1830 and 1850 by the Belgian polymath Adolphe Quetelet during the course of developing ‘social physics’. Body Mass Index is calculated as an individual’s body mass in kilograms divided by the square of his or her height in metres.
BMI was explicitly cited in 1971 by Ancel Keys, a leading physiologist and nutritionist, as being appropriate for population studies and inappropriate for individual diagnosis. Nevertheless, due to its simplicity, it became widely used for individual diagnosis despite its inappropriateness for this purpose.
BMI provided a simple numeric measure of a person’s body shape, allowing health professionals to discuss overweight and underweight problems more objectively and quantitatively with their patients. However, BMI has become controversial because many people, including some physicians and dieticians, have come to rely on its apparent numerical authority for medical diagnosis, although that was never the original purpose of BMI. It was meant to be used as a simple means of classifying sedentary (physically inactive) populations.
These results were plotted on the standard dietician’s chart as shown below.
The respondents with BMI values above 25, placing them in either the overweight or the obese sections of the chart, were then re-examined. Of the 15 individuals who fell into this category, only 4 had large waist measurements (above 87.5 cm for women and 100 cm for men). Several studies have found that those with smaller waist sizes would be at lower risk of cardiovascular disease than their BMI values would suggest.
6 out of the 25 respondents had BMI values of 30 or over, placing them in the obese sections of the standard BMI chart. This finding of 24% is concordant with the published national average for the UK of 26% (http://www.hscic.gov.uk/pubs/opad12).
Respondents 2, 14 and 19 have BMI values defined as obese but waist sizes below that defined as high risk for cardiovascular disease. These three results would represent a 50% false positive rate when using only the BMI as an indicator of higher risk of cardiovascular disease or diabetes by way of their being defined as either clinically overweight or obese.
In order to test my hypothesis, a survey was conducted with a random but representative sample of respondents who consented to have their weights, heights, and waist measurements taken and anonymously recorded. A sample of 25 voluntary individuals were measured. Care was taken to ensure that the respondents represented a wide range of body shapes, ages and from both genders equally. The BMI for each respondent was calculated using the formula:
The Python BMI Calculator
The results of my research project have indicated that when BMI is used on its own as an indicator of health it tends to throw up too many false positives for it to be a reliable tool. I have attempted to demonstrate this by focusing on the effect of one other measure, namely waist size, that could be used in conjunction with BMI to create a better measure of risk. There are several other indicators that could be similarly used.
This is not to say that the BMI has no place in the assessment of general health. Rather, a principle finding of this project is that when applied to individual cases, the BMI should be used in conjunction with other indicators so that the false positives produces by people with a high muscle to fat ration can be minimised. The usefulness of BMI as an indicator of the prevalence of obesity in large populations is not undermined in any way by the findings of this study.
Methods used to assess Body Composition
Some other suggested indicators that can be used in conjunction with the BMI to assess body composition are:
Waist to hip ratio
Dual-energy X-Ray absorptiometry
Air displacement (Bod Pod)
Bio-electrical impedance analysis
Each of these indicators have their weaknesses as overall indicators of overweight or obesity. Used together, however, they should have the desired effect of reducing the rate of false positive results.
To run a BMI code to stimulate a BMI calculator by entering height, weight and waist measurments as the input and the BMI as the outputt.
To present graphically a representation of my collected data as well as statistical computations
The results were tabulated as follows:
On this line graph, the weights of the respondets are plotted against their heights.
This pie chart displays the categories that the respondents fall into according to their BMI values
1832 - Belgian Polymath
Measure the general build of humans
Young adults worked best
Height mattered too much
Body Mass Index
1972. Nutritionist and psychologist
Use the BMI scale with care!
why do Doctors and dieticians like it?
Cheaper and quicker
Easier to explain to patients
The BMI is unable to differentiate between muscle mass and fatty mass. Muscle muscle, being that its denser, would alter the results of an individual and may even diagnose them as overweight.
BMI doesn't take into account the location of where the additional fat is located. For example, fat around the midsection is is much more detrimental the health than fat around the hips and thighs.
Due to it's accessibility and easy nature to calculate, anyone is able to obtain the information. This being the case, it isn't difficult for many to self diagnose incorrectly.
It underestimates body fat in older persons and others who have lost muscle
Chest-to-hip and Waist-to -hip ratios.
Chest-to-hip ratio and Waist-to-hip ratios are sometimes measured to provide a broad indication of body shape with high scores in each of these ratios sometimes described as being either 'pear shaped' or 'apple shaped' respectively. Some research indicates that the apple shape correlates more strongly with weight-related health conditions such as heart disease and diabetes. These ratios do not, however, take into account lean body mass or fat mass and the descriptions of the body shapes are often used qualitatively rather than quantitatively.
Using callipers, a clinician can take a skinfold measurement to estimate the ersentage of lean to fat tissue in specific body areas.
Dual-Energy X-Ray Absorptiometry
The dual energy X-ray absorptiometry (DXA) uses low-radiation X rays to determine ratios of bone, bone-free lean tissue, and fat tissue using the difference in attenuation of these tissues.
Commonly called ‘underwater weighing’, hydrostatic weighing is usually performed in research labs. This works on
the principle that greater buoyancy indicates lower density and consequently a lower muscle to fat ratio as fat is much
less dense than muscle.
Air Displacement (Bod Pod)
The Bod Pod is an egg-shaped chamber in which a person sits while a measurement is taken of the amount of air that is
displaced. Accuracy is similar to that of hydrostatic weighing.
Bioelectrical impedance Analysis
A bioelectrical impedance analysis (BIA) machine sends a small electrical current through the body. It distinguishes lean
from fat mass by measuring resistance of various tissues to the current.
How useful Is BMI?
Provides a quantitative measure
Your body has two types of fat: visceral and subcutaneous. Subcutaneous fat is found just under your skin, and is the type that causes dimpling and cellulite. Visceral fat, on the other hand, shows up in your abdomen and surrounds your vital organs including your liver, heart and muscles.
Several studies have shown that high BMI values actually correlate to a
risk of heart disease. This counter-intuitive finding is 'corrected' when waist size is taken into account. A large waist size (defined as 87.5 cm for women and 100 cm for men) represents a higher risk than a BMI over 30. A combination of a large waist size
a BMI > 30 represents the highest risk of cardiovascular disease (CVD).
Reference: Journal of the American College of Cardiology
By Jia Khan
This graph shows the respondents' BMI values against their waist sizes.