Accuracy of BMI for Diagnosing Obesity

Body mass index (BMI) is currently widely recommended and used as the best measure of obesity both in population and clinical studies. It dates back to the Belgian statistician Adolphe Quételet, who between 1830 and 1850 described this index as a way to characterize the level of adiposity in sedentary adults.

But how accurate is this index really to identify individuals with excess body fat?

This question was recently addressed by Abel Romero-Corral and colleagues from the Mayo Clinic, MN, USA, who analysed the relationship between BMI and body fat percent (BF%) as measured by bioelectrical impedence in 13,601 subjects (age 20-79.9 years; 49% men) from the Third National Health and Nutrition Examination Survey (Int J Obesity).

In this study, the authors defined obesity based on the World Health Organization (WHO) reference standard for obesity of BF%>25% in men and >35% in women.

BMI-defined obesity (>=30) was present in 19% of men and 25% of women, while BF%-defined obesity was present in 44% of men and 52% of women.

A BMI>=30 had a high specificity (men=95%, women=99%), but a poor sensitivity (men=36%, women=49%) to detect BF%-defined obesity. This means that while the BMI definition does identify the vast majority of men and women who have increased body fat, it also misses a significant number of individuals who have high percent body fat and would be considered obese by the BF% definition.

The diagnostic performance of BMI diminished as age increased and in the intermediate range of BMI (25-29.9), BMI failed to discriminate between BF% and lean mass in both sexes.

The authors conclude that accuracy of BMI in diagnosing obesity is limited, particularly for individuals in the intermediate BMI ranges, in men and in the elderly. Thus, the currently recommended BMI cutoff of >=30 kg for obesity has good specificity but misses more than half the people with excess fat.

The scary part of these results of course is in the fact that based on actual BF% the prevalence of obesity in this population doubled! On the other hand, we know that %body fat or body composition alone is not a particularly reliable measure of health.

I prefer to continue using my operational clinical definition of obesity: the presence of excess body fat that threatens or affects your health.

Given the wide variation in the inter-individual susceptibility to develop adiposity-related health problems, the diagnosis of obesity and the question of whether or not reducing the proportion of body fat will indeed benefit your health will always remain a matter of clinical judgement.

AMS
Duschesnay, Quebec

What is Obesity?

Don't worry - I am not going to take off on a discussion about whether obesity is a disease or "simply" a risk factor. I am also not going to discuss again obesity definitions - anthropometric or otherwise.

Today's post is simply about an analogy that may help sharpen our clinical thinking around excess weight.

Think of someone who has an elevated plasma creatinine level (a marker of kidney failure) - the elevated creatine definitely tells us that there is something "wrong" with the kidneys, but that's about it. From the creatine level alone we can certainly tell that the kidneys are failing in their excretory function, but we cannot tell what is causing the kidneys to fail - is it a pre-renal, intra-renal or post-renal problem? We can probably list a 100 reasons why kidneys could fail and obviously the treatment (apart from some very general principles) will very much depend on the cause, i.e. the actual diagnosis.

In many ways, one can look at excess body fat simply as a sign or symptom of the fact that there is a something "wrong" with energy homeostasis. The excess body fat tells us nothing about what the problem is - sure, it's either excessive food intake or reduced energy expenditure - but that is like saying that the creatinine levels are elevated because the kidney is not excreting properly. I can think of a long list of reasons or factors that would contribute to excessive caloric intake or reduced energy expenditure: sociocultural factors, psychological factors, biomedical factors - figuring out what exactly is causing the energy imbalance is the real problem.

Only when we find what is causing the excessive intake will we have made a diagnosis of what is causing the problem - a few specific examples could include: poor meal planning, peer pressure, hedonic overeating, depression, obesogenic medications, binge eating disorder, defective satiety signaling, etc. The point is that till we know what is causing the overeating, we can't fix it, which means we will have little success in treating the weight problem and will be limited to a "symptomatic" approach - just eat less!

Similarly, when the problem appears to be lack of activity, again the question is what exactly is causing the problem. Obviously if the problem is lack of time our approach will hopefully be very different than if the problem is back pain or lack of motivation (a possible symptom of sleep apnea, exhaustion or depression). A "symptomatic" but useless approach would be to simply recommend 10,000 steps. No better than offering an ice-pack to someone with a fever.

Just as the term "kidney failure" only tells us that there is something "wrong" with the kidneys the term "obesity" only tells us that there is something "wrong" with energy homeostasis.

In itself, neither the term "kidney failure" nor "obesity" is a real diagnosis - they are only helpful if they prompt further investigation into what might have or is still causing the problem. Only when we find the cause will we be on our way to solving the problem.

AMS
Edmonton, Alberta

"Foie Gras" in Kids Risk Factor for Heart Disease

Nonalcoholic fatty liver disease (NAFLD), is now the most common cause of liver disease in children.

Although this condition is associated with insulin resistance its impact on other features of the "metabolic syndrome" is less clear.

In the July 15 issue of Circulation, Jeffrey Schwimmer and colleagues from the University of California, San Diego report on their findings from a case-control study of 150 overweight children with biopsy-proven NAFLD and 150 overweight children without NAFLD.

Children with NAFLD had significantly higher fasting glucose, insulin, total cholesterol, low-density lipoprotein (LDL) cholesterol, triglycerides, systolic blood pressure, and diastolic blood pressure than overweight and obese children without NAFLD.

Not only do these findings support the notion that, as in adults, NAFLD in overweight and obese children is strongly associated with multiple cardiovascular risk factors, they also show that as in adults, weight alone is not a reliable measure of cardiovascular or metabolic risk factors in kids.

Identification of NAFLD in a child should prompt global counseling to address nutrition, physical activity, and avoidance of smoking to prevent the development of cardiovascular disease and type 2 diabetes.

AMS
Edmonton, Alberta

Overeating is a Symptom

Last week, I blogged about the results of a study showing an association between vital exhaustion and weight gain.

This blog posting prompted Sharon Kirkey (award-winning health journalist and diligent reader of my blog) to explore this issue further by actually talking to the researchers themselves. Her article appeared yesterday via the Canwest News Service and contains far more details than I bothered to report in my blog.

For e.g. Sharon lists the actual questions Bryant and colleagues used to explore the presence of vital exhaustion. I found them most interesting so I list them below:

- Do you often feel tired?

- Do you wake up repeatedly during the night?

- Do you have the feeling that you have not been accomplishing much lately?

- Do you believe that you have come to a 'dead end'?

- Do you lately feel more listless than before?

- Do little things irritate you more than they used to?

- Does it take more time to grasp a difficult problem than it did a year ago?

- Do you have increasing difficulty in concentrating on a single subject for long?

While to me none of these questions seem particularly specific and are likely to get a "yes" response from a lot of people (hey, even I have to say "yes" to some), I guess when you have to say "yes" to most or all of them, you are probably not hopping around like the Energizer Bunny or scrambling to follow diet plans.

Sharon quotes me as saying:

"Nobody is claiming all of obesity is exhaustion, but if people are clearly eating more than they need the question is not (just) how to get them to stop, but why, exactly, are they doing that. Maybe you're losing your job. Or you're overworked. Or it's some private issue that needs to be addressed. You don't fix obesity by giving people a copy of Canada's food guide. You have to try to understand what it is that's contributing to the weight gain."

Exactly! Obesity is a symptom of overeating, which in turn is a symptom of something else that's going on: it can be that you simply lack the knowledge about healthy nutrition or caloric content of foods, but it can also be a symptom of emotional (e.g. depression, vital exhaustion) or biological (e.g. MC-4 receptor defect) issue, medications (e.g. clozapine) or any number of other reasons I can think of.

Always remember: overeating (and/or sedentariness) is not a diagnosis - overeating (and/or sedentariness) is a symptom!

AMS
Edmonton, Alberta

Epicardial Fat and Weight Loss

Increased visceral fat or "ectopic" fat deposition is associated with insulin resistance and increased cardiovascular risk.

Traditionally, the term visceral fat has been used to describe the omental and mesenteric fat located inside the abdomen. Over the last several years, this concept has been expanded to include other "ectopic" fat depots including the liver and the heart.

Recognition that the fat located around the heart can be a marker of visceral fat was pioneered by Gianluca Iacobellis, who joined me as a clinical research fellow a few years ago at McMaster University and has since been recruited to their faculty.

In this month's issue of OBESITY, Iacobellis reports on work we collaborated on just before I left McMaster to relocate to the University of Alberta demonstrating that epicardial fat is a sensitive marker of changes in visceral fat associated with weight loss.

In this study in 20 severely obese subjects who underwent a 6-month very low calorie diet weight loss program resulting in a 20% loss of original body weight, waist circumference decreased by 23% whereas epicardial fat thickness decreased by 32% of baseline.

Based on this finding, we suggest that measurement of echocardiographic epicardial fat thickness may provide an additional tool in understanding the metabolic risk associated with variation in fat distribution. Perhaps, more importantly, echocardiographic measurement of epicardial fat can serve as a simple and relatively inexpensive tool to assess changes in visceral fat with weight loss (or gain) in clinical practice.

Certainly beats simply measuring waist circumference (a rather crude surrogate measure of intra-abdominal fat) or the rather more expensive CAT or MRI study.

AMS
Edmonton, Alberta

p.s Thanks to Navneet Singh and Sean Wharton for their great help with this study and to all the patients who volunteered their time for these measurements

Sarcopenic Obesity and Cancer

We know now (although many still do not fully appreciate this) that obesity is a major risk factor for cancers. On the other hand in patients with many chronic diseases, larger patients tend to do better and live longer (the obesity survival paradox).

Last week researchers from the University of Alberta published a study in The Lancet Oncology, that adds another level of complexity to the relationship between obesity and cancer survival. Clarisse MirandaPrado together with other researchers from the UofA, including cancer cachexia researchers Vickie Baracos, studied 2115 patients with solid tumours of the respiratory or gastrointestinal tract, 325 (15%) of who were classified as obese (body-mass index [BMI] >/=30).

With the help of CT images, the researchers found that obese patients had a wide range of muscle mass, with 15% of analysed obese patients meeting criteria for "sarcopenic" obesity (sarcopenia is the medical term for low-muscle mass). By definition, sarcopenic obese patients have more body fat and less lean body mass than non-sarcopenic patients of similar weight.

Not only was sarcopenic obesity associated with poorer functional status compared with non-sarcopenic obese patients but these patients also had a 4-fold hgher risk of dying.

Incidentally, the researchers also used their data to calculate that using conventional dosing criteria for cytotoxic chemotherapeutic drugs, sarcopenic obese patients may be overdosed with a greater likelihood of toxicity.

Overall this study shows that obesity is never just obesity and that BMI in the clinic is a fairly useless concept (a point that I have argued before) and that without proper assessments of body composition rational management of large patients is just not possible.

A patient's size alone proves little in term of health or disease - remember, weight alone is a rather poor measure of health.

AMS
Edmonton, Alberta

The Pap Gap

Previous studies have shown that patients with obesity may not be receiving the same quality of health care as non-obese patients.

Reasons for this are likely to be complicated: yes, there is a provider bias - health professionals are likely to blame most complaints on the presence of obesity and perhaps not order the same tests that they may for the same complaints in a non-obese individual - on the other hand, patients with obesity may be more reluctant to go to their family physician because of embarrassment, frustrations about only being told again and again to simply lose weight, or fear of furniture or equipment that's too small.

How do these circumstances affect the rates of preventive screening?

This was addressed in a study by our own Rebecca Mitchell and colleagues from the University of Alberta, who examined the relationship between body weight and cancer screening in data from the 2003 Canadian Community Health Survey 2003. (The paper will appear in the August issue of the American Journal of Preventive Medicine).

Of the nearly 38,000 women participants, 82.6 percent reported having cervical cancer screening (Pap test) within the past three years. However, women with a BMI of 35 or higher, were nearly 40 percent less likely than others to have had a Pap test.

The findings were not explained by differences in socioeconomic status, health habits, chronic medical conditions or health care access. Reasons for less tests were more likely attributable to fear of pain, embarrassment or of finding something wrong.

Obesity did not alter mammogram or colorectal screening.

This study is only the latest in a number of studies that have looked at this issue before. Thus, Sarah S. Cohen and colleagues from the University of North Carolina in their review of 32 relevant published studies (10 breast cancer studies, 14 cervical cancer studies, and 8 colorectal cancer studies) found that in women obesity most likely is a barrier to screening for breast and cervical cancers whereas the evidence for colorectal cancer screening was inconclusive.

These finding certainly send a message to health care providers to be vigilant that their larger patients receive the same level of screening as their leaner patients - especially since obesity has been noted as a risk factor for both breast and cervical cancers.

AMS
Edmonton, Alberta

Will a Urine Test Predict Obesity?

Urine analysis has long played an important role in diagnosing disease. As most of you may know, the term diabetes mellitus actually refers to the honey-like sweetness of the excessive urine produced by patients with this disorder.

Now Elaine Holmes and colleagues from the Imperial College London, UK, take urine diagnostics to new heights in a recent paper published in Nature.

Holmes and colleagues used a large-scale exploratory analytical approach, based on (1)H NMR spectroscopy, to anlayse the metabolic spectra from two 24-hour urine specimens for each of 4,630 participants from the INTERMAP epidemiological study, involving 17 population samples aged 40-59 in China, Japan, UK and USA.

The metabolic phenotypes are the products of interactions among a variety of factors-dietary, other lifestyle/environmental, gut microbial and genetics.

They were able to show that urinary metabolite excretion patterns for East Asian and western population samples, with contrasting diets, diet-related major risk factors, and coronary heart disease/stroke rates, are significantly differentiated, as are Chinese/Japanese metabolic phenotypes, and subgroups with differences in dietary vegetable/animal protein and blood pressure.

The authors go on to speculate that mean 24-hour urinary excretion of various metabolites, may not only reflect diet and gut microbial activities, but may also help discover novel biomarkers related to cardiovascular disease risk factors including obesity.

Can't wait to see how long it will take before I see a urinal replacing my bathroom scale.

AMS
Edmonton, Alberta