Overweight BMI Linked to Lower Risk of Death

By Jaimie Lazare

Danish researchers analyzing data from three different cohorts find that overweight BMI is associated with the lowest risk for all-cause mortality.

Weight loss is recommended for individuals with a body mass index (BMI) that is defined as overweight (25 to 29.9) and obese (≥30) because of associated adverse health outcomes such as hypertension, diabetes, cancer, and mortality. Various studies, however, show an association between overweight BMI and lowest overall risk for mortality.1-3 In a study conducted by Danish researchers, similar results were found when the investigators analyzed data from three cohorts from the general population enrolled in the Copenhagen City Heart Study in 1976–1978 (n=13,704) and in 1991–1994 (n=9,482) and the Copenhagen General Population Study in 2003–2013 (n=97,362).4 They report that the BMI associated with the lowest risk for all-cause mortality increased over the study period from 24 to 27—a value that falls in the overweight category.

The Limitations of BMI
BMI is a low-cost tool because height and weight are easy to measure, says Tapan Mehta, PhD, an obesity researcher at the University of Alabama in Birmingham. Although BMI is associated with other cardiometabolic conditions, it should not be the only measure used to evaluate health. There's no harm in monitoring BMI, but giving it too much importance from a clinical point of view is impractical. From patient to patient, there can be a lot of variability in factors such as body composition. While BMI provides information about body mass, it doesn't distinguish between lean muscle mass and body fat, Mehta says.

"BMI is not particularly useful for geriatrics, and the older patients get, the less useful it becomes," says Jeffrey D. Schlaudecker, MD, an associate professor of family medicine, the Kautz Family Foundation Endowed Chair of Geriatric Medical Education at the University of Cincinnati, and the medical director of the Acute Care of Elderly Unit at The Christ Hospital. "The impact of cardiovascular disease, diabetes, and other metabolic complications is still much more likely to be germane to the young old as a leading cause of death and especially as a leading cause of sudden death.

"We know from cardiovascular studies that this age group in particular still benefits from lipid-lowering therapy, from really aggressive and interventional cardiology metrics, and from aggressive diabetes care," Schlaudecker continues. "But the older people get, the less likely that they, as a representative group, were included in these big cohort studies or in these even smaller randomized controlled trials."

Some studies discuss healthy obesity and unhealthy obesity and what it means for normal-weight people to be metabolically unhealthy, Mehta says. It's clear that some of those factors such as blood pressure, blood glucose, and lipids play a more important role than BMI alone. Clinical staging systems, such as the cardiometabolic disease and the Edmonton obesity staging systems, which were developed to aid obesity treatment, form the risk profile for patients by an overall clinical assessment of weight-related problems (eg, cardiometabolic health and function). These systems do not rely solely on the use of BMI or waist circumference in determining risk profile for the clinical treatment of obesity, he says. 

Taking Other Factors Into Account
When it comes to factors influencing the Danish study's findings, Mehta says, "One plausible hypothesis is the improvement in treatments and advances in cardiometabolic conditions." Early awareness and treatment has increased over time for conditions such as hypertension, and the fact that these conditions are associated with overweight and obesity implies that if more people are being treated, there maybe some potential benefits, he says.

Since overweight and obese patients are treated more proactively for some metabolic condition, there's a tendency to think of normal-weight patients as healthy; however, they may have an underlying metabolic dysfunction that isn't screened as frequently as it would be in overweight and obese individuals, Mehta says.

Mehta says that in most studies examining the BMI-mortality relationship over time, the results are confounded by unstable study-level factors that have changed over time, which would be a limitation of most of these types of studies. In the Danish study, the waist-to-hip ratio may be considered as an unstable study-level confounder because the distribution shifted, and the leisure-time physical activity also shifted. So it would have been interesting if the Danish study's authors analyzed and presented their findings using waist circumference and waist-to-hip ratio since they did have those measures for two of the cohorts, he says.

Mehta cautions that before exploring whether the overweight cutoff should be adjusted, it's essential to understand why some of these changes occurred and to determine whether the study can be replicated in other populations since ethnicity also introduces differences. A study conducted by Mehta and colleagues that examined Hispanics notes that there was no association found between grade 1 obesity and mortality; whereas studies analyzing Asian populations have found that a high BMI is associated with a high mortality risk.5-6 It's worth noting that the American Diabetes Association has lowered the BMI cut point at which it recommends screening Asian Americans for type 2 diabetes and aligned its guidelines with evidence that many Asian Americans develop type 2 diabetes disease at lower BMI levels than the population at large on the basis of a position statement published in Diabetes Care, he says.7

Mehta stresses that it's crucial to conduct studies in which "generalizability is reserved across different races, ethnicities, and genders," and says that conducting studies across different ethnicities and races and in different settings helps to determine whether study results are generalizable.

A meta-analysis published around the same time as the Danish study found that the lower the BMI, the healthier a person is, Schlaudecker says. The study analyzed 230 cohort studies with more than 30.3 million study participants and had more than 3.74 million deaths and found that among all participants, those with the lowest mortality had a BMI of 25. The BMI for lowest overall mortality was even lower among nonsmokers (23–24), healthy nonsmokers (22–23), and nonsmokers with follow-up periods between 20 and 25 years (20–22).8

Addressing Functional Status, Not BMI
"I think that the two big things geriatricians do well for patients are focusing on functional status by asking what can this person do, determining how active they are and figuring out ways to maximize their activity level and to address malnutrition," Schlaudecker says.

"As age goes up, malnutrition becomes more likely for seniors. As their finances and mobility decrease, their ability to maintain an adequate diet of total calories can really decline," Schlaudecker continues. Older adults may be reliant on whatever groceries a family member can bring over or a bus line that gets them to a convenience store, and that's when nutritional levels really decline. As someone's weight comes down, that's not good because they're not taking in adequate fat and protein to maintain a body that's able to withstand illness and trauma down the road, he says.

While overweight and obesity are associated with myriad metabolic conditions, thinness in older adult patients affects their ability to rebound from health challenges that may come in the form of pneumonia, a fall with a hip fracture, or a stroke, Schlaudecker says.

When older adult patients don't have the nutritional stores or muscle mass needed to be active in therapy in order to recover from any of those medical issues, they have a much higher mortality than if they weighed 20 or 30 lbs more. Therefore, from a nutritional and functional standpoint, they would be better able to withstand the unforeseen stresses that come as people age. One of the reasons BMI is imperfect is because it doesn't tell us anything about this functional status piece, Schlaudecker says.

— Jaimie Lazare is a freelance writer based in Brooklyn, New York.

References
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