Is BMI the official yardstick for all body types?
Scientists began correlating body fat with surface area and height, and came up with a statistical statement that could be used with population group studies: the Body Mass Index (BMI).
For many years, physicians have looked for easy ways to measure fatness, thinness, and normality and to follow them as they change, under either experimental or disease states. Early on, the methodology was to weigh the subject in air and then completely submerged in water. After that, some arithmetic produced the amount of fat in a person’s body. The problems with that were the very expensive equipment involved, hours of time required to make measurements then calculations, inability to use the method on sick patients, and inconvenience.
Subsequently scientists began correlating body fat with surface area and height, and came up with a statistical statement that could be used with population group studies: the Body Mass Index (BMI).
The formula: BMI = mass (lbs.) x 703/Height (inches)2
Studies have shown this to be a good correlate for body fat. If the height is cubed (inches3) the resulting correlation is better.
But before you knew it, the first formula was being used for individuals and much of the literature would be useless if different formulas were used because of non-comparability. In any event, most measurements in medicine are best approximations, indirect approaches and approximately parallel to the parameters they seek to evaluate - so one lives with limitations in terms of interpretation.
A 1994 population survey in the USA showed:
Males - 59% were more than 25
Females - 49% were more 25
Males - 2% were more than 40
Females - 4% were more than 40
It is these small percentages (2% + 4%) for whom gastric surgery is primarily intended. This group is likely to develop cardiovascular and diabetic problems sooner and has a difficult time controlling sugar and blood fat levels.
In 2007 a survey showed that:
63% were BMI 25-30, i.e. 63% were a bit overweight, and
26% were more than 30, i.e. 26% were fatter yet
The study also suggested that BMI 15 means starvation, while a BMI of less than 17.5 suggests possible anorexia nervosa.
For children, an age-related percentile system is used. Anyone who is below the fifth percentile, or above the 95th percentile for the age is considered abnormal. Use of the BMI is suitable for recognizing trends within sedentary or overweight individuals.
The virtue of the BMI is ease of computation and interpretation. But tall people wind up with higher numbers. Short people get lower numbers (i.e., fat content is underestimated), and normal is slightly different for Asians. It does not take into account frame size or muscularity. Using this formula to study professional athletes would not give satisfactory result. It also doesn’t work well in the very elderly because of loss of height with aging without corresponding weight change. In general:
A BMI of 15.5 or less means underweight, and suggests illness
18.5 - 25 means normal fat content
25 - 30 means overweight (most of the USA)
30 - 40 means obese (30% of the USA)
40 and over means morbid obesity
There are other ways to estimate body fat, all with their own limitations:
Underwater weighing - impractical
Skin fold measurement - problems with reproducibility
Body volume index - impractical
Using radioisotopes - impractical
3-D body scanner - expensive
MRI/CT - available and precise, but, x-ray exposure has to be considered
So, what do we do with the BMI? How do we use it?
In general it is used to estimate fatness and can be used to follow loss of fatness, which is what most of the USA needs. Why? Because fatness results in hypertension, heart disease, strokes, arthritis, diabetes and premature death. One can follow how well treatment is progressing by using measurements that every doctor makes in calculating BMI, i.e., height and weight. If the BMI is not falling, there is no progress. It does not guarantee any particular result. For the patient, the BMI is a way to follow what it is that the physician is trying to do. A normal BMI means that the patient is in the normal range. A normal BMI does not guarantee health, but an elevated BMI suggests the high statistical probability of some type of cardiovascular problem, diabetes, and/or hypertension.
It has been found that most people with BMI more than 40 are potential candidates for bariatric surgery. In addition, BMI of more than 30 predisposes the patient to excess mortality risk of 130 percent. Elevated BMI is an independent risk factor for heart failure and increases the risk for renal cancer. In general, an elevated BMI (more than 30) places an individual in all kinds of adverse categories.
What options are available?
Risks associated with bariatric surgery
- Immediate -
- Surgical risk in high-risk patients
- Immediate post-op infections
- Deep vein thrombosis (clotting)
- Leaking anastomoses
- Delayed -
- Problems with delayed intestinal hookups
- Electrolyte disturbances
- Low blood sugar
- Chronic -
- Vitamin malabsorption
There are multiple answers and, as usual, each answer has its good and not-so-good aspects. Currently we are in the bariatric surgery phase. Why? Because it is perceived as providing quick results: the surgeon does the work and the weight comes off. The trade-off is, the patient lives with the complications.
Adjustable gastric band - puts a band near the top of the stomach.
Gastric/intestinal bypass (this is major surgery)
- Results: No major surgery
- No malabsorption
- Slower (and less) weight loss
- Major complications unlikely
The rapid weight loss and changes in intestinal architecture produce quick improvement in diabetes, hypertension, cholesterol levels, and cardiovascular risk. Current medical treatment under best circumstances would take two or more years to produce the good results that the surgery does in six months.
- Results: Malabsorption, diarrhea, abdominal cramps
- Weight loss is rapid
Currently most medications for weight loss have had recent restrictions placed on them because of the appearance of complications: rare liver injury, heart attacks, and stroke.
Researchers are looking at trying to rev up “brown fat” and make it burn “white fat;” also, trying to make anti-angiogenesis drugs block fat-making; also drugs to make the body act as though it were exercising (i.e., make lazy cells act as if exercising).
Most of these work in mice. Getting to humans without causing harm will be something else again.
printer friendly page
"Is BMI the official yardstick for all body types?"
C. Robert Meloni, M.D., FACP, FACE, is board certified in both internal medicine and endocrinology. He is a graduate of Harvard College (BS), Georgetown University (MS), and New York Medical College (MD). The former Chairman and President of the Nort...