Investigating the Problems Associated with BMI

A major problem with the BMI goes largely unmentioned. The basis for this measurement is a largely White American male population – which means it is difficult to extrapolate to men of other races, women, and children.

Some researchers have established that body fat could range from 10% to 32% in men with the same BMI of 27. Interestingly, the NIH suggests that men with 25% or more of their body mass composed of fat are obese, vs. 35% or more for women.

Yet, in men and women with a “normal” BMI of 25 or more, the percentage of body fat fell between 14% and 35%, vs. 26% and 43%, respectively. Thus, low-normal to obese fat values were observed in these groups, despite the normal BMI. Moreover, the BMI does not reflect the differences between men and women regarding normal fat mass.

Another consideration is the documented increase in height as well as weight in Western populations over the last century. In Europe, for instance, those in the Northern nations were ~5 feet 3 inches in the 17^th century, while at present, it is nearer 6 feet. This is associated with a rise in mean BMI.

Thus, obesity should be determined not simply on the basis of a BMI of 30 or more. In fact, the BMI reflects lean mass better than fat mass, at least in men.

The build-up of fat in the abdominal region rather than around the hips and thighs is observed to be a risk factor for cardiovascular disease, diabetes, gallbladder stones, and gout. Thus, the abdominal circumference, or the abdominal-to-hip circumference ratio, is more meaningful as a predictive factor for these conditions in both men and women.

Fatty liver and the accumulation of fat in the skeletal muscle are best linked to abdominal or visceral fat deposition and may be considered a reflection of the disease process that results from abnormally high fat mass and insulin resistance. Interestingly, the BMI would not be able to differentiate the presence of significant masses of fat in these organs, and total body fat mass is also a poor predictor of fat at such locations.

In diabetics, the BMI is higher than in non-diabetics at the same age, but the lean mass percentage remains very similar in both groups.

Finally, the BMI does not recognize the strong genetic and familial component of fat accumulation in different parts of the body, nor the total fat mass. Older people also have more body fat, in general, than younger people with similar BMI.

The correlation of BMI with mortality risks should always consider family histories of relevant metabolic disorders, including diabetes, metabolic syndrome, dyslipidemias, cancers, hypertension, and cardiovascular disease. With the latter, it is now known that fully half of the risk comes from gene variants rather than from environmental factors, including obesity.

Smoking, excessive drinking, mental illness, and chronic obesity are also important factors in tracing a correlation between BMI and illness or death. Occupational factors as well as the fat deposition incidental to the use of certain medications, also have a bearing on the BMI.

Studies that do not consider these factors may not draw valid inferences about the association between BMI and mortality risk, especially in relation to cardiovascular disease. First, a significant minority of people termed obese because of their BMI have no cardiovascular risk factors. They may be termed to have “metabolically healthy obesity (MHO).” Secondly, once known risk factors are adjusted for, BMI is no longer a risk factor by itself for cardiovascular events.

The ongoing EPIC observational study is one of many that has found that “overweight” people have the lowest risk of death, with a BMI above 30 or below 21 only showing any increase in mortality risk. The rest of the BMI spectrum appears to have a stable risk over the 10-year period of observation.

To assess the percentage of body fat mass and its accumulation in different spots, indirect methods are available, such as bioelectrical impedance, underwater weighing, density determination, and isotope-labeled water mass. Skin-fold thickness is a direct method of measuring fat mass.

Dual-energy x-ray absorptiometry (DEXA) scans build a 3-D image of the density of the body organs and the location of fat mass. Modified forms of computed tomography (CT) that involve little radiation exposure are also used, and magnetic resonance imaging (MRI) is a potential, though very expensive, candidate.

Many studies have shown that the waist circumference-to-BMI ratio was correlated with the greatest increase in the risk of death. Alternatively, the National Heart, Lung, and Blood Institute (NHLBI) suggest using three measures to assess the health risks and the weight of an individual, namely, the BMI, waist circumference, and risk factors for any conditions related to obesity.

Some studies demonstrate that a high waist circumference explains the increased risk for type 2 diabetes in the USA compared to the UK, rather than the BMI. Again, the risk of death at any given BMI increases by 17% and 13% per 5 cm increase in waist circumference. This lends urgency to the task of standardizing the protocol for waist circumference measurements to ensure comparability between studies. Reassuringly, available evidence suggests that by either the NIH or WHO protocol, the estimates of obesity prevalence appear similar, though different between men and women.

Similarly, the reliability of self- or technician-measured waist circumference should be evaluated further, though current evidence suggests good agreement between the two. Ethnic differences in the risk threshold for waist circumference must also be defined by prospective studies for each BMI category. These methods are better than the BMI to determine fat mass. Moreover, researchers say, “It now is generally accepted that a relationship between BMI and mortality risk should be applied only to large populations. It should not be applied to an individual in an unqualified fashion. A segment of the population is now considered to be “fat” by any criteria but “fit” and not at risk for early mortality.”

Oxford mathematician Nick Trefethen believes that using weight/height^2.5 would provide a better measure of relative body mass compared to the BMI. And even apart from the obsession with body fat mass, many experts think that diagnostic tests such as an electrocardiogram, blood sugar, blood lipids, and blood pressure would provide a better-rounded picture of overall health.

In conclusion, it may safely be said that most people with a BMI in the “overweight” or “pre-obese” category are not at higher risk for disease and death compared to those with “normal” or “underweight” BMI. Novel measurements such as the Trefethen Index, or established ones such as the waist circumference, can be tested using the massive amounts of data available from earlier long-term prospective studies for their correlation with health risks over time.

Obesity researcher Dr. Ahima of the Perelman School of Medicine at the University of Pennsylvania said, “There is an urgent need for accurate, practical and affordable tools to better predict the risks of diseases and mortality. Advances to improve the measurement of obesity and related factors will help determine the optimal weight for an individual, taking into account factors such as age, sex, genetics, fitness, pre-existing diseases, as well novel blood markers and metabolic parameters altered by obesity.”