Carrying around excess pounds is a problem of epidemic proportions in the U.S. In fact, according to the Centers for Disease Control (CDC), over 1/3 of U.S. adults (36.5%) are so overweight they are classified as obese. Because it raises the risk of many health problems – from type 2 diabetes and heart disease to sleep apnea and several cancers – being significantly overweight not only causes suffering on an individual basis but it also takes a significant economic toll, contributing to higher medical and insurance costs, more disability, lost work days and premature death.
But defining who is truly overweight and who isn’t is coming under scrutiny thanks to a reassessment of BMI (body mass index), a mathematical tool long used by physicians to quickly and inexpensively determine whether a person’s weight ranks as normal. Unlike reading pounds on a scale, BMI is calculated by a formula that takes into account a person’s weight in relation to his or her height and is used by doctors to estimate body fat and to help assess the risk for diseases that can occur with excess weight.
Using BMI calculations has long been the “gold standard” for deciding whether someone was “normal weight” for health standards and insurance information. According to the National Heart, Lung and Blood Institute (NHLB), BMI is a useful measure of overweight and obesity – but it has limits. For example, the NHLB recognizes that it may overestimate body fat in athletes and others who have a muscular build. On the other hand BMI may underestimate body fat in older persons and in any individuals who have lost significant muscle mass. But now several studies have revealed BMI has other serious drawbacks, including labeling some people as normal when they have a potentially dangerous distribution of excess fat around their middle.
New Research Suggests BMI Limitations
Some researchers have raised the possibility that people who rank as overweight or even obese based on their BMIs may be healthy if their blood pressure, glucose tolerance tests and cholesterol levels are within normal limits. However, a review by researchers from Mount Sinai Hospital in Toronto, recently published in the Annals of Internal Medicine, analyzed over 1,000 studies about weight and health conducted from 1950 until 2013 and found that the concept a person can be “fat and fit” is most likely a myth. There is no level of “healthy” obesity, the report states.
“This data is suggesting that both patients who are obese who are metabolically unhealthy and patients who are obese who are metabolically healthy are both at increased risk of death from cardiovascular disease, such that benign obesity may indeed be a myth,” study leader Dr. Ravi Retnakaran said in a media statement.
What’s more, a normal BMI doesn’t mean a person is necessarily fit and healthy. Some individuals of normal weight, according to their BMIs, may have adverse metabolic features such as high blood pressure and glucose intolerance. So, the Mount Sinai Hospital researchers concluded, both BMI and metabolic status should be considered when evaluating an individual’s health risks. Can BMI help assess cancer risk? While being overweight has been linked to an increased incidence of several cancers, including breast malignancies, research published in the American Journal of Epidemiology by scientists at Albert Einstein College of Medicine of Yeshiva University suggests BMI is not the best measure for estimating the obesity-linked risk of certain types of cancer.
“It has long been recognized that BMI is an imperfect indicator of body fat because weight does not distinguish between lean body mass (muscle, bones, blood, water) and fat mass,” said lead author Geoffrey C. Kabat, Ph.D., senior epidemiologist at Einstein.
“This means that two individuals can have the same BMI but can have very different percentages of body fat. Furthermore, when using weight and height data, a single BMI formula may not be appropriate for all populations and all diseases.”
“Spare Tires” vs BMIs
A new international collaborative study led by Mayo Clinic researchers concludes that having a “spare tire” (fat that has accumulated around the waist and belly) is detrimental to health, even if your BMI ranks as normal. The research, published in the March 2014 edition of Mayo Clinic Proceedings, found that men and women with large waist circumferences were more likely to die younger from illnesses such as heart disease, respiratory problems, and cancer – even if their BMIs appeared to be in a healthy range.
The study is particularly significant because it included data from eleven different studies involving more than 600,000 people from around the world. The researchers found that men with waists measuring 43 inches or greater in circumference had a 50% higher mortality risk than men with waists less than 35 inches; women with waists that measured 37 inches or greater had about an 80% higher mortality risk than women with a waist circumference of 27 inches or less.
This translates to a three-year lower life expectancy after age 40 for men and a five-year lower life expectancy for women. What’s more, the risk increased as waistlines grew – mortality risk went up about seven percent in men and about 9% in women for every two inches of greater waist circumference. Because of the large size of the study, the researchers believe they’ve definitely shown that a large waist reveals if a person is likely carrying too much health-impacting fat, despite a normal BMI.
“BMI is not a perfect measure,” James Cerhan, M.D., Ph.D., a Mayo Clinic epidemiologist and lead author of the study, said in a press statement. “It doesn’t discriminate lean mass from fat mass, and it also doesn’t say anything about where your weight is located. We worry about that because extra fat in your belly has a metabolic profile that is associated with diseases such as diabetes and heart disease.” The take away message, according to Dr. Cerhan, isn’t that BMI should be abandoned.
However, he says doctors should take both BMI and waist circumference into consideration as part of risk assessment for obesity-related medical conditions. “The primary goal should be preventing both a high BMI and a large waist circumference,” Dr. Cerhan said. “For those patients who have a large waist, trimming down even a few inches through exercise and diet could have important health benefits.”
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