The Highs and Lows of BMI and Mortality
Almost all biological variables are detrimental to health at the extremes. A blood pressure that is too high can kill you – so can a blood pressure that is too low. A blood sugar that is too high can kill you – so can a blood sugar that is too low. It turns out that BMI is no different – too high and too low both carry a risk – a risk, however, that is substantially confounded by actual body fat%, which is not reliably measured by BMI. This is basically the message in a paper by my colleagues Raj Padwal and co from the University of Alberta in a paper published in the Annals of Internal Medicine. The researchers looked at data from about 50,000 women and 5,000 men (mean age, 63.5 years; mean BMI, 27.0 kg/m2) referred for bone mineral density (BMD) testing with dual-energy x-ray absorptiometry (DXA), which they linked to administrative databases. Given the size and demographics of the cohort, death occurred in almost 5000 women over a median of 6.7 years and 1000 men over a median of 4.5 years. Women in the lowest BMI and body fat% quintiles had a 40% higher risk of dying (compared to quintile 3). Risk of dying were also about 20% greater in the highest body fat% quintile for women. Similarly in men, both low BMI (HR, 1.45 for quintile 1) and high body fat percentage (HR, 1.59 for quintile 5) were associated with increased mortality. The exciting bit about this study is that the researchers had both BMI and body fat% available to them and were able to show that both variables independently of each other contribute to mortality risk. Thus, the worst possible combination in both men and women was low BMI and high body fat%. Or, as the authors put it, “Low BMI and high body fat percentage were both associated with increased all-cause mortality. Mortality increased as BMI decreased and body fat percentage increased…..Thus, our results suggest that BMI may be an inappropriate surrogate for adiposity, and this limitation may explain the presence of the obesity paradox in many studies.” As the authors discuss, these finding should have clinical implications as they clearly demonstrate the limitations of BMI as a measure of health risk. “..our findings underscore that the risk for all-cause mortality increases with both increasing adiposity and decreasing BMI in a general population of middle-aged and… Read More »
BMI Versus Fitness In Hypertension Risk
A higher BMI has long been associated with increased risk of hypertension (and there are many important physiological mechanisms at play to account for this relationship). Now a study by Crump and colleagues published in JAMA Intern Medicine suggests that some of this risk may be mitigated by increased physical fitness. The cohort study involving over 1.5 million Swedish young men in Sweden, who underwent standardized aerobic capacity, muscular strength, and BMI measurements obtained at a military conscription examination and were followed for up to 40 years. Almost 100,000 men went on to develop hypertension, whereby both high BMI and low aerobic capacity (but not muscular strength) were associated with increased risk of hypertension, independent of family history or socioeconomic factors. A combination of high BMI (overweight or obese vs normal) and low aerobic capacity (lowest vs highest tertile) was associated with the highest risk of hypertension. The association with aerobic fitness was apparent at every level of BMI. Form this study the authors conclude that high BMI and low aerobic capacity in late adolescence are associated with higher risk of hypertension in adulthood. Although one must also be cautious in assuming causality with regard to associations found in such studies, the observations are certainly compatible with the notion that increased cardiorespiratory fitness may well mitigate some of the impact of increased BMI on hypertension risk. @DrSharma Edmonton, AB
Severity Of Sleep Apnea Is Related To Distance From Sleep Centre
Although sleep apnea is one of the most common and devastating complications of obesity, it remains woefully under-diagnosed and under-treated. One factor accounting for this may well be the lack of timely access to sleep testing. Now, a study by Hirsch Allen and colleagues from the University of British Columbia Hospital Sleep Clinic, published in the Annals of the American Thoracic Society, examined the relationship between severity of sleep apnea and travel times to the clinic in 1275 patients referred for suspected sleep apnea. After controlling for a number of confounders including gender, age, obesity and education, travel time was a significant predictor of OSA severity with each 10 minute increase in travel time associated with an apnea-hypopnea-index increase of 1.4 events per hour. The most likely explanation for these findings is probably related to the fact that the more severe the symptoms, the more likely patients are to travel longer distances to undergo a sleep study. Thus, travel distance may well be a significant barrier for many patients accounting for a large proportion of undiagnosed sleep apnea – at least for milder forms. Given the often vast distances in Canada one can only wonder about just how much sleep apnea goes under diagnosed because of this issue. @DrSharma Edmonton, AB
Risks Of Severe Obesity In Children And Young Adults
Yes, body fat is not a measure of health and it is possible to be healthy across a wide range of body weights. However, this may become harder and harder, the more weight you gain. Thus, a study by Asheley Skinner and colleagues, published in the New England Journal of Medicine, shows that increased cardiometabolic risk is tightly linked with severe obesity both in children and young adults. The study looks at cross-sectional data from overweight or obese children and young adults (3-19 yrs) who were included in the US National Health and Nutrition Examination Survey (NHANES) from 1999 through 2012. Among 8579 children and young adults with a body-mass index at the 85th percentile or higher (according to the Centers for Disease Control and Prevention growth charts), 46.9% were overweight, 36.4% had class I obesity, 11.9% had class II obesity, and 4.8% had class III obesity. Overall, for a given weight, males tended to have higher cardiometabolic risk than females. Even after controlling for age, race or ethnic group, more severe obesity maps more likely to be associated with low HDL cholesterol level, high systolic and diastolic blood pressures, and high triglyceride and glycated hemoglobin levels. Importantly, while this relationship was constantly present in males, the there were fewer significant differences in these variables according to weight category among female participants, suggesting that for a given body weight, girls were less likely to be at cardiometabolic risk compared to boys. Thus, while body weight (or body fat) may not be a precise measure of individual health, the risk for having one or more cardiometabolic risk factor increases substantially with increasing severity of obesity. However, it is also important to note that even in kids and youth with class III obesity, 70% of participants had normal lipids and about 90% of participants did not have elevated blood pressure or glycated hemoglobin. This points to the fact that for a given body weight there is indeed wide variability in whether or not someone actually has cardiometabolic risk factors. Thus, whether or not it makes sense to target every kid that presents with an elevated BMI for intervention, remains to be shown – most likely such an approach would probably not be cost-effective. As in adults, it seems that interventions in kids are probably best targeted by global risk rather than simply by numbers on a scale. @DrSharma Edmonton, AB
What Are The Health Benefits of Intentional Weight Loss?
To conclude this brief series on our new exhaustive review of the putative health benefits of long-term weight-loss maintenance, published in Annual Reviews of Nutrition, here is the summary paragraph of our findings: “Obesity is well recognized as a risk factor for a wide range of health issues affecting virtually every organ system. There is now considerable evidence that intentional weight loss is associated with clinically relevant benefits for the majority of these health issues. However, the degree of weight loss that must be achieved and sustained to reap these benefits varies widely between comorbidities. Downsides of weight loss that is too rapid and/or extreme may occur, as in the increased risk of gallbladder disease, the presence of excess residual skin, or deterioration in liver histology. Uncertainty also remains about the potential benefit or harm of intentional weight loss on patients presenting with some chronic diseases and on overall mortality. Clearly, well- controlled prospective studies are needed to better understand the natural history of obesity and the impact of weight-management interventions on morbidity, quality of life, and mortality in people living with obesity.” The is much left to be done and answering some of these questions will become progressively easier as better treatments for obesity become available. @DrSharma Kananaskis
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