In fact, men in the study who fit that profile had almost twice the risk of premature death as men who were overweight or obese only. For women, the increased risk was lower, but still significant: Those with a normal BMI but a high waist-to-hip ratio were found to have an almost 50 percent increased risk of premature death when compared to women with a similar BMI and a “normal” waist-to-hip ratio.
The findings add to a growing body of research that suggests that waist-to-hip ratio should be included in the assessment toolbox when determining an individual’s risk of deteriorating health, especially heart health.
“Waist size matters, particularly in people who are a normal weight,” Dr. Francisco Lopez-Jimenez, the study’s senior author and a cardiologist at the Mayo Clinic, told Reuters reporter Lisa Rapaport. “The lack of recognition of this leads people with abnormal distribution of fat to have a false sense of safety or reassurance that they don’t need to exercise or they can eat whatever they want because they are ‘skinny’ when in reality, if a person has a normal BMI and an abnormal waist size the risk is worse than if they have a high BMI.”
The study was published Monday in the Annals of Internal Medicine.
Waist-to-hip ratio is calculated by measuring your hips and your waist and dividing the waist measurement by the hip one. A ratio of 1.0 or greater (if you’re a man) and of 0.85 or greater (if you’re a woman) indicates that you’re carrying too much adipose tissue (“belly fat”) around your abdomen.
Having a high waist-to-hip ratio has been associated in other research with an increased risk of type 2 diabetes and heart disease. But as belly fat is usually accompanied by a high BMI, doctors do not always measure it, particularly in people with normal BMIs.
For this study, Dr. Lopez-Jimenez and his colleagues used data collected from 12,785 people aged 18 and older who had participated in the Third National Health and Nutrition Examination Survey, which was conducted from 1988 to 1994. (They had to go back to those years because the survey has since stopped measuring hip size.) The data included not only body measurements, but also demographic and health information.
The researchers then searched through the National Death Index to determine which of the 12,785 participants had died before the end of December 2006 — an average follow-up period of slightly more than 14 years. They found 2,562 deaths, of which 1,138 were related to cardiovascular disease.
They then calculated the chances of dying within five- to 10-year time frames for people with different combinations of BMI (normal, overweight or obese) and waist-to-hip-ratios (normal or with “central obesity”).
They found that the risk of dying from heart disease was 2.75 times higher, and the risk of death from all causes was 2.08 times higher, among people who had normal BMIs but also central obesity.
Here’s how the experts at the U.K.’s National Health Service put those numbers into perspective: “At age 50, a man with a normal BMI and a normal [waist-to-hip ratio] had a 5.7% chance of dying within the next 10 years, but that rose to 10.3% chance for men with normal BMI, but a high [waist-to-hip ratio]. … A woman aged 50 of normal weight and normal [waist-to-hip ratio] had a 3.3% chance of dying within 10 years, rising to 4.8% for women of the same weight, but a high [waist-to-hip ratio].”
Limitations and implications
Of course, this is an observational study. It does not prove that excess belly fat contributes to heart disease and premature death. Other factors, not examined in the analysis, might explain the study’s results.
In addition, the study was not designed to examine why people with excess belly fat — even when they have a normal BMI — are more likely to die earlier than their peers with smaller waistlines.
Lopez-Jimenez and his colleagues suggest several possibilities: The high risk of death may be related to the fact that the visceral fat (the type that accumulates around the internal organs in the abdomen) is associated with insulin resistence, higher cholesterol levels and inflammation of the blood vessels. Also, a higher waist-to-hip ratio is associated with decreased muscle mass in the legs, which is a risk factor for heart disease.
“Future studies should focus on identifying factors associated with the development of normal-weight central obesity and better understanding the effect of normal-weight central obesity on health outcomes,” urge the researchers.
No matter what the reason, all of us — whether we are “officially” overweight or not — should probably be taking a closer look at where our fat is distributed on our body by getting a waist-to-hip measurement. The National Heart, Lung, and Blood Institute offers online help for how to do that.
The Mayo study is, unfortunately, behind a paywall, but you’ll find an abstract on the Annals of Internal Medicine website.