BMI. Do you believe?

Personally I don't. Or at least not entirely. A guideline? Perhaps. But cut in stone? Surely not. It only takes two things into account. Height and weight. I am no mathematician, but that equation seems short a few variables.

The BMI does not take into account distribution of weight, body type, bone density or any other variables that can affect weight. I believe we all know that muscle weighs more than fat. It's actually possible to have a very low body fat count and an elevated BMI. Of course you'd look like the Hulk, but it's possible.


The body mass index (BMI), or Quetelet index, is a heuristic proxy for human body fat based on an individual's weight and height. BMI does not actually measure the percentage of body fat. It was invented between 1830 and 1850 by the Belgian polymath Adolphe Quetelet during the course of developing "social physics".^[1] Body mass index is defined as the individual's body weight divided by the square of his or her height. The formulae universally used in medicine produce a unit of measure of kg/m². BMI can also be determined using a BMI chart,^[2] which displays BMI as a function of weight (horizontal axis) and height (vertical axis) using contour lines for different values of BMI or colors for different BMI categories.


**So according to this I'd have to weigh between 165 and 210 pounds. Yah, OK sure, that's doable.****

Limitations and shortcomings

This graph shows the correlation between body mass index (BMI) and percent body fat (%BF) for 8550 men in NCHS' NHANES 1994 data. Data in the upper left and lower right quadrants show 

some limitations of BMI.^[17]

The medical establishment has generally acknowledged some shortcomings of BMI.^[18] Because the BMI formula depends only upon weight and height, its assumptions about the distribution between lean mass and adipose tissue are not always exact. BMI sometimes overestimates adiposity on those with more lean body mass (e.g., athletes) while greatly under-estimating excess adiposity on those with less lean body mass. A study in June, 2008 by Romero-Corral et al. examined 13,601 subjects from the United States' Third National Health and Nutrition Examination Survey (NHANES III) and found that BMI-defined obesity was present in 21% of men and 31% of women. Using body fat percentages (BF%), however, BF%-defined obesity was found in 50% of men and 62% of women. While BMI-defined obesity showed high specificity (95% of men and 99% of women presenting BMI-defined obesity also presented BF%-defined obesity), BMI showed poor sensitivity (BMI only identified 36% of the men and 49% of the women who presented BF%-defined obesity).^[17]
Mathematician Keith Devlin and a restaurant industry association The Center for Consumer Freedom argue that the error in the BMI is significant and so pervasive that it is not generally useful in evaluation of health.^[19][20] University of Chicago political science professor Eric Oliver says BMI is a convenient but inaccurate measure of weight, forced onto the populace, and should be revised.^[21]
A study published by JAMA in 2005 showed that "overweight" people had a similar relative risk of mortality to "normal" weight people as defined by BMI, while "underweight" and "obese" people had a higher death rate.^[22]
In an analysis of 40 studies involving 250,000 people, patients with coronary artery disease with "normal" BMIs were at higher risk of death from cardiovascular disease than people whose BMIs put them in the "overweight" range (BMI 25–29.9).^[23] In the "overweight", or intermediate, range of BMI (25–29.9), the study found that BMI failed to discriminate between bodyfat percentage and lean mass. The study concluded that "the accuracy of BMI in diagnosing obesity is limited, particularly for individuals in the intermediate BMI ranges, in men and in the elderly. ... These results may help to explain the unexpected better survival in overweight/mild obese patients."^[17]
A 2010 study that followed 11,000 subjects for up to eight years concluded that BMI is not a good measure for the risk of heart attack, stroke or death. A better measure was found to be the waist-to-height ratio.^[24]
BMI is particularly inaccurate for people who are fit or athletic, as the higher muscle mass tends to put them in the "overweight" category by BMI, even though their body fat percentages frequently fall in the 10-15% category, which is below that of a more sedentary person of average build who has a "healthy" BMI number. Body composition for athletes is often better calculated using measures of body fat, as determined by such techniques as skinfold measurements or underwater weighing and the limitations of manual measurement have also led to new, alternative methods to measure obesity, such as the body volume index. However, recent studies of American football linemen who undergo intensive weight training to increase their muscle mass show that they frequently suffer many of the same problems as people ordinarily considered obese, notably sleep apnea.^[25][26]
BMI also does not account for body frame size; A person may have a small frame and be carrying too much excess fat, but their BMI reflects that they are "healthy". Conversely, a large framed individual may be quite healthy with a fairly low body fat percentage, but be classified as "overweight" by BMI. Accurate frame size calculators use several measurements (wrist circumference, elbow width, neck circumference and others) to determine what category an individual falls into for a given height. The standard is to use frame size in conjunction with ideal height/weight charts and add roughly 10% for a large frame or subtract roughly 10% for a smaller frame. For example, a chart may say the ideal weight for a man 5'10" (178 cm) is 165 pounds (75 kg). But if that man has a slender build (small frame), he may be overweight at 165 pounds (78 kg) and should reduce by 10%, to roughly 150 pounds (68 kg). In the reverse, the man with a larger frame and more solid build can be quite healthy at 180 pounds (82 kg). If one teeters on the edge of small/medium or medium/large, a dose of common sense should be used in calculating their ideal weight. However, falling into your ideal weight range for height and build is still not as accurate in determining health risk factors as waist/height ratio and actual body fat percentage.
A further limitation of BMI relates to loss of height through aging. In this situation, BMI will increase without any corresponding increase in weight.
The exponent of 2 in the denominator of the formula for BMI is arbitrary. It is meant to reduce variability in the BMI associated only with a difference in size, rather than with differences in weight relative to one's ideal weight. If taller people were simply scaled-up versions of shorter people, the appropriate exponent would be 3, as weight would increase with the cube of height. However, on average, taller people have a slimmer build relative to their height than do shorter people, and the exponent which matches the variation best is between 2 and 3. An analysis based on data gathered in the USA suggested an exponent of 2.6 would yield the best fit for children aged 2 to 19 years old.^[27] The exponent 2 is used instead by convention and for simplicity.
As a possible alternative to BMI, the concepts fat-free mass index (FFMI) and fat mass index (FMI) were introduced in the early 1990s.<sup>[28]

*****what I said, just with more fancy words.</sup>
In 1998, the National Institutes of Health lowered the overweight threshold for BMI 27.8 to 25 to match international guidelines. The move added 30 million Americans who were previously in the "healthy weight" category to the "overweight" category. Today, the NIH advises doctors and their patients to include BMI in a complete assessment of a person's body size and overall health.
****Let's just change the rules for fun. What the heck.

Here's a great article from webMD

How Accurate Is Body Mass Index, or BMI?

Is BMI still the best way to measure fatness? Some experts aren't so sure.

By Kathleen M. Zelman, MPH, RD, LD
WebMD Weight Loss Clinic-Feature

Reviewed by Louise Chang, MD

What's your number -- under 25 or over 35? Body mass index (BMI) may not be a term that's on everyone's lips, but it's important for your health to understand what it is and to know your number.
Essentially, BMI is a simple mathematical formula, based on height and weight, that is used to measure fatness. You should be aware of your BMI because of the health risks of being overweight (that is, having a BMI of 25 or over). According to a report in the August 2006 New England Journal of Medicine, excess body weight during midlifeis associated with an increased risk of death.

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On the other hand, being too thin and having a BMI that's below the healthy range (18.5 to 24.9) can also be a health concern.
Many health care experts think BMI is a useful tool to measure weight and health risks, but others question its accuracy. Some believe a better way might be to take out the tape measure and check your waist circumference. Or is there a place for both methods?

What Is BMI?

In June 1998, in an effort to make sure doctors, researchers, dietitians, and government agencies were all on the same page, the National Institutes of Health announced its BMI guidelines. They replaced the old life insurance tables as a method to gauge healthy weight.
To calculate your BMI, divide your weight in pounds by your height in inches squared, then multiply the results by a conversion factor of 703. For someone who is 5 feet 5 inches tall (65 inches) and weighs 150 pounds, the calculation would look like this: [150 ÷ (65)²] x 703 = 24.96. An easier way is to use WebMD's BMI calculator.
(This BMI calculator is for use in the evaluation of adults, not for children.)
According to the NIH definitions, a healthy weight is a BMI of 18.5-24.9; overweight is 25-29.9; and obese is 30 or higher.

The Measurement of Choice

BMI is the measurement of choice for most health professionals.
''I think BMI is a very good and easy screening tool,'' says obesity expert, Cathy Nonas, MS, RD, a spokeswoman for the American Dietetic Association.
But while it is a simple, inexpensive method of screening for weight categories, it is not a diagnostic tool. Health professionals need to do further assessments to fully evaluate health risks. These assessments would include measurements of body fat percentage, diet history, exercise patterns, and family history.
Further, BMI does not take into account age, gender, or muscle mass. Nor does it distinguish between lean body mass and fat mass. As a result, some people, such as heavily muscled athletes, may have a high BMI even though they don't have a high percentage of body fat. In others, such as elderly people, BMI may appear normal even though muscle has been lost with aging.


Take for example, basketball player Michael Jordan: ''When he was in his prime, his BMI was 27-29, classifying him as overweight, yet his waist size was less than 30,'' says Michael Roizen, MD.
That's one reason some experts think waist circumference can be a better overall health measurement than BMI.
Another is that your health is not only affected by excess body fat, but also by where the fat is located.  Some people gain weight in their abdominal regions (the so-called ''apple'' body shape.) Others are ''pear-shaped,'' with excess weight around the hips and buttocks. People with apple shapes are at higher risk for health problems associated with being overweight.
''Fat around your waist is more biologically active and can do more damage to your body than weight around your hips," says Roizen, co-author of You: On a Diet. "The data show that waist circumference is more reliable and more closely correlated with diseases associated with obesity.''
According to the National Institutes of Health, a bigger waist circumference (greater than 40 inches for men and 35 inches for women) is linked to a higher risk of type 2 diabetes, high blood pressure, abnormal cholesterol levels, and heart disease when BMI is 25 to 34.9.
To properly measure your waist, no math is needed. Just use a soft tape measure around your bare midsection at your belly button. Find your upper hip bone, and measure around the abdomen above the bone. The tape should be snug, but not dig into your skin.
Nonas argues that waist circumference is not a better tool than the BMI ''because we do not have good criteria or cut points for levels of overweight, obesity, age or height.'' She also thinks that properly measuring the waistline is a little more difficult than measuring height and weight.
One thing that experts agree on is that weight is only one factor in our risk for disease. When it comes to evaluating weight and its impact on health, your percentage of body fat, waist circumference, BMI, and physical activity patterns are all important.
The National Heart, Lung, and Blood Institute recommends that health care providers assess BMI, waist circumference, and any other risk factors for obesity-related conditions. Combining all of the information provides the best assessment.

What Can You Do?

The first step toward shrinking your waistline and getting your BMI in line is to start eating a healthier diet and getting regular exercise. Preventing any further weight gain and slowly reducing weight into a healthier range is an excellent goal.
And while you might want to lose more, dropping as little as 5%-10% of your body weight can bring dramatic improvements in blood pressure, blood cholesterol, and blood sugar.
Nonas recommends four steps to a healthy lifestyle:

• Being physically active.
• Making healthy food choices.
• Avoiding overeating.
• Scheduling an annual physical examination.

''These are the vital parts to maintaining a long and healthy life,'' she says.

Sources

• "Aim for a healthy weight." National Heart, Lung and Blood Institute.
  http://www.nhlbi.nih.gov/health/public/heart/obesity/lose_wt/risk.htm
• "Bioelectrical Impedance Analysis." Impedimed Limited, March 10, 2005.
  http://florey.biosci.uq.edu.au/BIA/whatitis.html
• CDC: BMI
  http://www.cdc.gov/nccdphp/dnpa/bmi/bmi-means.htm
• CDC: BMI-for-age
  http://www.cdc.gov/nccdphp/dnpa/bmi/bmi-for-age.htm
• CDC: Growth Charts
  http://www.cdc.gov/nchs/about/major/nhanes/growthcharts/charts.htm
• KidsHealth.org: BMI Charts
  http://kidshealth.org/PageManager.jsp?dn=KidsHealth&lic=1&ps=107&cat_id=148&article_set=22610
• Kuczmarski, R. J. and K. M. Flegal. "Criteria for definition of overweight in transition: background and recommendations for the United States." American Journal of Clinical Nutrition, Vol. 72, No. 5, 1074-1081, November 2000
  http://www.ajcn.org/cgi/content/full/72/5/1074
• Mackey, Carole S. "Body Mass Index." Nutrition and Well-Being A to Z. Vol. 1. New York: Macmillan Reference USA, 2004. 71-74.
• Platkin, Charles Stuart. "BMI is a better way to measure." North Virginia Daily, 2005.
  http://www.nvdaily.com/Food/2005/salad_051805/BMI.html