Body Mass Index - What is BMI?

Body Mass Index (BMI)
or Quetelet Index is a statistical measure of the weight of a person scaled according to height. It was invented between 1830 and 1850 by the Belgian polymath Adolphe Quetelet during the course of developing "social physics".

BMI is defined as the individual's body weight divided by the square of their height. The formulas universally sed in medicine produce a unit of measure of kg/m2. Body mass index may be accurately calculated using any of the formulas below.BMI can also be determined using a BMI chart, 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 Body Mass Index categories.

BMI Usage
As a measure, Body Mass Index became popular during the early 1950s and 60s as obesity started to become a discernible issue in prosperous Western society. BMI provided a simple numeric measure of a person's "fatness" or "thinness", allowing health professionals to discuss over- and under-weight problems more objectively with their patients. However, Body Mass Index has become controversial because many people, including physicians, have come to rely on its apparent numerical authority for medical diagnosis, but that was never the BMI's purpose. It is meant to be used as a simple means of classifying sedentary (physically inactive) individuals with an average body composition.[1]

For these individuals, the current value settings are as follows: a Body Mass Index of 18.5 to 25 may indicate optimal weight; a BMI lower than 18.5 suggests the person is underweight while a number above 25 may indicate the person is overweight; a Body Mass Index below 17.5 may indicate the person has anorexia or a related disorder; a number above 30 suggests the person is obese (over 40, morbidly obese).

For a given height the Body Mass Index is proportional to weight; for example, if body weight increases by 50%, BMI increases by 50%. For a given body shape and given density, the BMI is proportional to height--if all body dimensions increase by 50%, the BMI increases by 50%. This tendency for taller people to have higher BMIs is partially offset by the fact that many taller people are not just "scaled up" short people, but rather tend to have narrower frames in proportion to their height. [2]

BMI Prime
Body Mass Index Prime, a simple modification of the BMI system, is the ratio of actual BMI to upper limit BMI (currently defined at BMI 25). As defined, BMI Prime is also the ratio of body weight to upper body weight limit, calculated at BMI 25. Since it is the ratio of two separate BMI values, BMI Prime is a pure, dimensionless number, without associated units. Individuals with BMI Prime < 0.74 are underweight; those between 0.74 and 0.99 have optimal weight; and those at 1.00 or greater are overweight. BMI Prime is useful clinically because individuals can quantify, at a glance, what percentage they deviate from their upper weight limits. For instance, a person with BMI 34 has a BMI Prime of 34/25 = 1.36, and is 36% over his or her upper mass limit. In Asian populations (see International Variation section below) BMI Prime should be calculated using an upper limit BMI of 23 in the denominator instead of 25. Nonetheless, Body Mass Index Prime allows easy comparison between populations whose upper limit BMI values differ.[3]

BMI Categories
A frequent use of the Body Mass Index is to assess how much an individual's body weight departs from what is normal or desirable for a person of his or her height. The weight excess or deficiency may, in part, be accounted for by body fat (adipose tissue) although other factors such as muscularity also affect BMI significantly (see discussion below and overweight). Human bodies rank along the index from around 15 (near starvation) to over 40 (morbidly obese).

This statistical spread is usually described in broad categories: underweight, normal weight, overweight, obese and morbidly obese. The particular Body Mass Index values used to demarcate these categories varies based on the authority, the CDC[4] and the WHO[5] regard a BMI of less than 18.5 as underweight and may indicate malnutrition, an eating disorder, or other health problems, while a BMI greater than 25 is considered overweight and above 30 is considered obese. These ranges of BMI values are valid only as statistical categories when applied to adults, and do not predict health.

Category BMI range - kg/m2 BMI Prime

Starvation less than 16.5 less than 0.60
Underweight from 16.5 to 18.5 from 0.6 to 0.74
Normal from 18.5 to 25 from 0.74 to 1.0
Overweight from 25 to 30 from 1.0 to 1.2
Obese from 30 to 35 from 1.2 to 1.4
Clinically Obese from 35 to 40 from 1.4 to 1.6
Morbidly Obese greater than 40 greater than 1.6

The U.S. National Health and Nutrition Examination Survey of 1994 indicates that 59% of American men and 49% of women have BMIs over 25. Extreme obesity — a BMI of 40 or more — was found in 2% of the men and 4% of the women. There are differing opinions on the threshold for being underweight in females, doctors quote anything from 18.5 to 20 as being the lowest weight, the most frequently stated being 19. A BMI nearing 15 is usually used as an indicator for starvation and the health risks involved, with a BMI <17.5 being an informal criterion for the diagnosis of anorexia nervosa.

BMI for age
Body Mass Index is used differently for children. It is calculated the same way as for adults, but then compared to typical values for other children of the same age. Instead of set thresholds for underweight and overweight, then, the BMI percentile allows comparison with children of the same sex and age.[6] A BMI that is less than the 5th percentile is considered underweight and above the 95th percentile is considered overweight. Children with a BMI between the 85th and 95th percentile are considered to be at risk of becoming overweight.

Recent studies in England have indicated that females between the ages 12 and 16 have a higher BMI than males by 1.0 kg/m² on average.[7]

International BMI variations
These recommended distinctions along the linear scale may vary from time to time and country to country, making global, longitudinal surveys problematic. In 1998, the U.S. National Institutes of Health brought U.S. definitions into line with World Health Organization guidelines, lowering the normal/overweight cut-off from BMI 27.8 to BMI 25. This had the effect of redefining approximately 30 million Americans, previously "technically healthy" to "technically overweight". It also recommends lowering the normal/overweight threshold for South East Asian body types to around BMI 23, and expects further revisions to emerge from clinical studies of different body types.

In Singapore, the BMI cut-off figures were revised in 2005 with an emphasis on health risks instead of weight. Adults whose BMI is between 18.5 and 22.9 have a low risk of developing heart disease and other health problems such as diabetes. Those with a BMI between 23 and 27.4 are at moderate risk while those with a BMI of 27.5 and above are at high risk of heart disease and other health problems.[1]

These ranges of Body Mass Index values are valid only for Southeast Asian body type.

Category BMI range - kg/m2
Starvation less than 14.9
Underweight from 15 to 18.4
Normal from 18.5 to 22.9
Overweight from 23 to 27.5
Obese from 27.6 to 40
Morbidly Obese greater than 40

Statistical BMI device
The Body Mass Index is generally used as a means of correlation between groups related by general mass and can serve as a vague means of estimating adiposity. The duality of the Body Mass Index is that, whilst easy-to-use as a general calculation, it is limited in how accurate and pertinent the data obtained from it can be. Generally, the Index is suitable for recognising trends within sedentary or overweight individuals because there is a smaller margin for errors.[8]

This general correlation is particularly useful for consensus data regarding obesity or various other conditions because it can be used to build a semi-accurate representation from which a solution can be stipulated, or the RDA for a group can be calculated. Similarly, this is becoming more and more pertinent to the growth of children, due to the majority of their exercise habits.[9]

The growth of children is usually documented against a Body Mass Index measured growth chart. Obesity trends can be calculated from the difference between the child's BMI and the BMI on the chart. However, this method again falls prey to the obstacle of body composition: many children who primarily grow as endomorphs would be classed as obese despite body composition. Clinical professionals should take into account the child's body composition and defer to an appropriate technique such as densiometry.

Clinical BMI practices
Body Mass Index has been used by the WHO as the standard for recording obesity statistics since the early 1980s. In the United States, BMI is also used as a measure of underweight, owing to advocacy on behalf of those suffering with eating disorders, such as anorexia nervosa and bulimia nervosa.[citation needed]

Body Mass Index can be calculated quickly and without expensive equipment. However, BMI categories do not take into account many factors such as frame size and muscularity.[10] The categories also fail to account for varying proportions of fat, bone, cartilage, water weight, and more. Body Mass Index is a statistical categorization and therefore is not appropriate for diagnosing individuals.

Despite this, Body Mass Index categories are regularly regarded as a satisfactory tool for measuring whether sedentary individuals are "underweight," "overweight" or "obese" with various qualifications, such as: Individuals who are not sedentary being exempt - athletes, children, the elderly, the infirm, and individuals who are naturally endomorphic or ectomorphic (i.e., people who don't have a medium frame).

One basic problem, especially in athletes, is that muscle is denser than fat. Some professional athletes are "overweight" or "obese" according to their Body Mass Index - unless the number at which they are considered "overweight" or "obese" is adjusted upward in some modified version of the calculation. In children and the elderly, differences in bone density and, thus, in the proportion of bone to total weight can mean the number at which these people are considered underweight should be adjusted downward.

Methods for actually measuring body fat percentage are preferable to Body Mass Index for measuring body fat. Body fat has been statistically linked to some health problems and trends, but again, this is often a spurious relationship and there are no simple proofs of health based on such measurement.

BMI Limitations and shortcomings
The medical establishment has generally acknowledged some shortcomings of BMI.[11] Because the Body Mass Index is dependent only upon net weight and height, it makes simplistic assumptions about distribution of muscle and bone mass, and thus may overestimate adiposity on those with more lean body mass (e.g. athletes) while underestimating adiposity on those with less lean body mass (e.g. the elderly).

In fact, some argue that the error in the Body Mass Index is significant and so pervasive that it is not generally useful in evaluation of health.[12] Due to these limitations, body composition for athletes is often better calculated using measures of body fat, as determined by such techniques as skinfold measurements or underwater weighing. However, recent studies of American football linemen, who undergo intensive weight training to make their muscles extremely large, show that they frequently suffer many of the same problems as people ordinarily considered obese, notably sleep apnea[13][14].

In an analysis of 40 studies involving 250,000 people, heart patients 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).[15] Patients who were underweight or severely overweight had an increased risk of death from cardiovascular disease. The implications of this finding can be confounded by the fact that many chronic diseases, such as diabetes, can cause weight loss before the eventual death. In light of this, higher death rates among thinner people would be the expected result.[citation needed]

Body Mass Index References
WHO Technical Report Series, #854, Physical Status: The Use and Interpretation of Anthropometry, Pg. 9 (2.1 MB PDF, here) Calculation of power law relationship between weight and height Gadzik J: "How Much Should I Weigh?" - Quetelet's Equation, Upper Weight Limits and BMI Prime Connecticut Medicine Feb 2006; 70: 81 - 88. About BMI for Adults
BMI Classification
BMI - Body Mass Index: BMI for Children and Teens
Health Survey for England: The Health of Children and Young People
Jeukendrup, A & Gleeson, M. (2005) Sports Nutrition Human Kinetics
Barasi, M. E (2004) Human Nutrition - a health perspective Jeukendrup, A & Gleeson, M. (2005) Sports Nutrition Human Kinetics
Aim for a Healthy Weight: Assess your Risk. National Institutes of Health (2007-07-08).
Is obesity such a big, fat threat?. Cox News Service (2004-08-30). Retrieved on 2007-07-08.
Brown, David; "Linemen More Likely To Have Sleep Condition", in The Washington Post, January 23, 2003
Ex.NFL Linemen unusually prone to Heart Disease Association of bodyweight with total mortality and with cardiovascular events in coronary artery disease: a systematic review of cohort studies.. Lancet (2006-08-19;368(9536):666-78). Retrieved on 2007-07-08.

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Obesity and My Health

What is it?
How is it measured?
What causes it?
What are the health risks?
What can you do about it?
What is obesity?

"Obesity" specifically refers to an excessive amount of body fat. "Overweight" refers to an excessive amount of body weight that includes muscle, bone, fat, and water. As a rule, women have more body fat than men. Most health care professionals agree that men with more than 25 percent body fat and women with more than 30 percent body fat are obese. These numbers should not be confused with the Body Mass Index (BMI), however, which is more commonly used by health care professionals to determine the effect of body weight on the risk for some diseases.

How is obesity measured?
Measuring the exact amount of a person's body fat is not easy. The most accurate measures are to weigh a person underwater or in a chamber that uses air displacement to measure body volume, or to use an X-ray test called Dual Energy X-ray Absorptiometry, also known as DEXA. These methods are not practical for the average person, and are done only in research centers with special equipment.

There are simpler methods to estimate body fat. One is to measure the thickness of the layer of fat just under the skin in several parts of the body. Another involves sending a harmless amount of electricity through a person's body. Results from these methods, however, can be inaccurate if done by an inexperienced person or on someone with extreme obesity.

Because measuring a person's body fat is difficult, health care professionals often rely on other means to diagnose obesity. Weight-for-height tables, used for decades, have a range of acceptable weights for a person of a given height.

One problem with these tables is that there are many versions, all with different weight ranges. Another problem is that they do not distinguish between excess fat and muscle. According to the tables, a very muscular person may be classified obese when he or she is not. The Body Mass Index is less likely to misidentify a person's appropriate weight-for-height range.

Body Mass Index - The BMI is a tool used to assess overweight and obesity and monitor changes in body weight. Like the weight-for-height tables, Body Mass Index has its limitations because it does not measure body fat or muscle directly. It is calculated by dividing a person's weight in pounds by height in inches squared and multiplied by 703.

Two people can have the same Body Mass Index but different body fat percentages. A bodybuilder with a large muscle mass and low percentage of body fat may have the same BMI as a person who has more body fat. However, a Body Mass Index of 30 or higher usually indicates excess body fat.

The Body Mass Index table below provides a useful guideline to check your BMI. First, find your weight on the bottom of the graph. Go straight up from that point until you come to the line that matches your height. A Body Mass Index of 25 to 29.9 indicates a person is overweight. A person with a Body Mass Index of 30 or higher is considered obese. Please review your findings with your health care provider.

If your Body Mass Index is outside of the normal range.

* Without Shoes**Without ClothesSourcesGeorge Bray, M.D., Pennington Biomedical Research Center. National Heart, Lung, and Blood Institute’s Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: The Evidence Report.

Body Fat Distribution
Health care providers are concerned not only with how much fat a person has, but also where the fat is located on the body. Women typically collect fat in their hips and buttocks, giving them a "pear" shape. Men usually build up fat around their bellies, giving them more of an "apple" shape. Of course some men are pear-shaped and some women become apple-shaped, especially after menopause. Excess abdominal fat is an important, independent risk factor for disease. Research has shown that waist circumference is directly associated with abdominal fat and can be used in the assessment of the risks associated with obesity or overweight. If you carry fat mainly around your waist, you are more likely to develop obesity-related health problems. Women with a waist measurement of more than 35 inches and men with a waist measurement of more than 40 inches may have more health risks than people with lower waist measurements because of their body fat distribution.

What causes obesity?
Obesity occurs when a person consumes more calories from food than he or she burns. Our bodies need calories to sustain life and be physically active, but to maintain weight we need to balance the energy we eat with the energy we use. When a person eats more calories than he or she burns, the energy balance is tipped toward weight gain and obesity. This imbalance between calories-in and calories-out may differ from one person to another. Genetic, environmental, and other factors may all play a part.

Genetic Factors
Obesity tends to run in families, suggesting a genetic cause. However, families also share diet and lifestyle habits that may contribute to obesity. Separating genetic from other influences on obesity is often difficult. Even so, science does show a link between obesity and heredity.

Environmental and Social Factors
Environment strongly influences obesity. Consider that most people in the United States alive today were also alive in 1980, when obesity rates were lower. Since this time, our genetic make-up has not changed, but our environment has.

Environment includes lifestyle behaviors such as what a person eats and his or her level of physical activity. Too often Americans eat out; consume large meals and high-fat foods, and put taste and convenience ahead of nutrition. Also, most people in the United States do not get enough physical activity. Environment also includes the world around us—our access to places to walk and healthy foods, for example. Today, more people drive long distances to work instead of walking, live in neighborhoods without sidewalks, tend to eat out or get “take out” instead of cooking, or have vending machines with high-calorie, high-fat snacks at their workplace. Our environment often does not support healthy habits.

In addition, social factors including poverty and a lower level of education have been linked to obesity. One reason for this may be that high-calorie processed foods cost less and are easier to find and prepare than healthier foods, such as fresh vegetables and fruits. Other reasons may include inadequate access to safe recreation places or the cost of gym memberships, limiting opportunities for physical activity. However, the link between low socio-economic status and obesity has not been conclusively established, and recent research shows that obesity is also increasing among high-income groups.

Although you cannot change your genetic makeup, you can work on changing your eating habits, levels of physical activity, and other environmental factors. Try these ideas:

Learn to choose sensible portions of nutritious meals that are lower in fat.
Learn to recognize and control environmental cues (like inviting smells or a package of cookies on the counter) that make you want to eat when you are not hungry.
Engage in at least 30 minutes of moderate-intensity physical activity (like brisk walking) on most, preferably all, days of the week.
Take a walk instead of watching television.
Eat meals and snacks at a table, not in front of the TV.
Keep records of your food intake and physical activity.

Other Causes of Obesity
There are some illnesses that may lead to or are associated with weight gain or obesity. These include:

Hypothyroidism, a condition in which the thyroid gland fails to produce enough thyroid hormone. It often results in lowered metabolic rate and loss of vigor.
Cushing's syndrome, a hormonal disorder caused by prolonged exposure of the body's tissues to high levels of the hormone cortisol. Symptoms vary, but most people have upper body obesity, rounded face, increased fat around the neck, and thinning arms and legs.
Polycystic ovary syndrome, a condition characterized by high levels of androgens (male hormone), irregular or missed menstrual cycles, and in some cases, multiple small cysts in the ovaries. Cysts are fluid-filled sacs.

A doctor can tell whether there are underlying medical conditions that are causing weight gain or making weight loss difficult. Lack of sleep may also contribute to obesity. Recent studies suggest that people with sleep problems may gain weight over time. On the other hand, obesity may contribute to sleep problems due to medical conditions such as sleep apnea, where a person briefly stops breathing at multiple times during the night.

Certain drugs such as steroids, some antidepressants, and some medications for psychiatric conditions or seizure disorders may cause weight gain. These drugs may slow the rate at which the body burns calories, stimulate appetite, or cause the body to hold on to extra water. Be sure your doctor knows all the medications you are taking (including over-the-counter medications and dietary supplements). He or she may recommend a different medication that has less effect on weight gain.

What are the consequences of obesity?
Obesity is more than a cosmetic problem. Many serious medical conditions have been linked to obesity, including type 2 diabetes, heart disease, high blood pressure, and stroke. Obesity is also linked to higher rates of certain types of cancer. Men who are obese are more likely than nonobese men to develop cancer of the colon, rectum, or prostate. Women who are obese are more likely than nonobese women to develop cancer of the gallbladder, uterus, cervix, or ovaries. Esophageal cancer has also been associated with obesity. Other diseases and health problems linked to obesity include:
Gallbladder disease and gallstones
Fatty liver disease (also called nonalcoholic steatohepatitis or NASH).
Gastro esophageal reflux or what is sometimes called GERD. This problem occurs when the lower esophageal sphincter does not close properly and stomach contents leak back—or reflux—into the esophagus.
Osteoarthritis, a disease in which the joints deteriorate. This is possibly the result of excess weight on the joints.
Gout, another disease affecting the joints.
Pulmonary (breathing) problems, including sleep apnea, which causes a person to stop breathing for a short time during sleep.Reproductive problems in women, including menstrual irregularities and infertility.

Health care providers generally agree that the more obese a person is, the more likely he or she is to develop health problems.

Psychological and Social Effects
Emotional suffering may be one of the most painful parts of obesity. American society emphasizes physical appearance and often equates attractiveness with slimness, especially for women. Such messages make overweight people feel unattractive. Many people think that individuals with obesity are gluttonous, lazy, or both. This is not true. As a result, people who are obese often face prejudice or discrimination in the job market, at school, and in social situations. Feelings of rejection, shame, or depression may occur.

Who should lose weight?
Health care providers generally agree that people who have a Body Mass Index of 30 or greater can improve their health through weight loss. This is especially true for people with a Body Mass Index of 40 or greater, who are considered extremely obese.

Preventing additional weight gain is recommended if youHave a Body Mass Index between 25 and 29.9Unless you have other risk factors for obesity-related diseases.Obesity experts recommend you try to lose weight if you have two or more of the following:
Family history of certain chronic diseases. If you have close relatives who have had heart disease or diabetes, you are more likely to develop these problems if you are obese.
Preexisting medical conditions. High blood pressure, high LDL cholesterol levels, low HDL cholesterol levels, high triglycerides, and high blood glucose are all warning signs of some obesity-associated diseases.
Large waist circumference. Men who have waist circumferences greater than 40 inches, and women who have waist circumferences greater than 35 inches, are at higher risk of diabetes, dyslipidemia (abnormal amounts of fat in the blood), high blood pressure, and heart disease.

Fortunately, a weight loss of 5 to 10 percent of your initial body weight can do much to improve health by lowering blood pressure and other risk factors for obesity-related diseases. In addition, research shows that a 5- to 7-percent weight loss brought about by moderate diet and exercise can delay or possibly prevent type 2 diabetes in people at high risk for the disease.

In a recent study, participants who were overweight and had pre-diabetes—a condition in which a person’s blood glucose level is higher than normal, but not high enough to be classified as diabetes—were able to delay or prevent the onset of type 2 diabetes by adopting a low-fat, low-calorie diet and exercising for 30 minutes a day, 5 days a week.

How is obesity treated?The method of treatment depends on your level of obesity, overall health condition, and readiness to lose weight. Treatment may include a combination of diet, exercise, behavior modification, and sometimes weight-loss drugs. In some cases of extreme obesity, bariatric surgery may be recommended.
(Visit www.win.niddk.nih.gov/publications/gastric.htm for more information on bariatric surgery.)

Remember, weight control is a life-long effort, and having realistic expectations about weight loss is an important consideration. Eating a healthful diet and getting at least 30 minutes of moderate-intensity physical activity on most, preferably all, days of the week have important health benefits. Sixty minutes of physical activity a day may be required to prevent gradual weight gain in adulthood. Previously overweight and obese individuals are encouraged to get 60 to 90 minutes of exercise a day to sustain weight loss.

Although most adults do not need to see their health care professional before starting a moderate-intensity physical activity program, men older than 40 years and women older than 50 years who plan a vigorous program or who have either chronic disease or risk factors for chronic illnesses should speak with their health care provider before starting a physical activity program.

Source: National Institutes of Health (NIH) Publication No. 04-4352. April 2004.