Abdominal obesity, sometimes referred to colloquially as "belly fat," is a risk factor for coronary heart disease.
These findings contrast with past research that used data from the National Health and Nutrition Examination Survey (NHANES).
Previous analyses suggested that the prevalence of obesity had not changed significantly from 2003-2012.
However, the authors of the new study state that "in contrast, our analyses using data from the same surveys indicate that the prevalence of abdominal obesity is still increasing."
"The reasons for increases in waist circumference in excess of what would be expected from changes in BMI remain speculative, but several factors, including sleep deprivation, endocrine disruptors, and certain medications, have been proposed as potential explanations."
The dangers of abdominal obesity
Abdominal obesity - sometimes referred to as "belly fat" - is excess fat comprised of both the fat just below the skin (subcutaneous fat) and fat inside the abdomen surrounding the organs within (visceral fat).
The National Heart, Lung and Blood Institute (NHLBI) identify abdominal obesity as a metabolic risk factor - a condition that raises the chances of developing health problems such as heart disease. They state that excess fat in the area of the stomach is a greater risk factor for heart disease than excess fat in any other area of the body.
Abdominal obesity is measured by the waist circumference. While obesity is often calculated using body mass index (BMI) measurements, some studies have found that individuals with a normal BMI score have had a large waist circumference, thus increasing their risk of cardiovascular disease.
Measuring waist circumferences
The study, conducted by Dr. Earl S. Ford and colleagues from the Centers for Disease Control and Prevention (CDC), involved analyzing data from seven 2-year cycles of the NHANES. They began with 1999-2000 and observed waist circumferences and rates of abdominal obesity up to the 2011-2012 cycle.
The authors examined data from 32,816 men and non-pregnant women aged 20 years and older. Abdominal obesity was defined by the authors as a waist circumference greater than 40.2 inches (102 cm) for men and greater than 34.6 inches (88 cm) for women.
They found that, over time, the average waist circumference increased significantly, from 37.6 inches at the beginning of the study to 38.8 inches at the end. The overall prevalence of abdominal obesity also rose, from 46.4% to 54.2%. Significant increases were noted in the following groups of adults:
- Men: 0.8-inch waist circumference increase, 6.4% abdominal obesity increase.
- Women: 1.5-inch waist circumference increase, 9.3% abdominal obesity increase.
- Non-Hispanic white people: 1.2-inch waist circumference increase, 8% abdominal obesity increase.
- Non-Hispanic black people: 1.6-inch waist circumference increase, 8.5% abdominal obesity increase.
- Mexican Americans: 1.8-inch waist circumference increase, 9.3% abdominal obesity increase.
The authors acknowledge that one limitation of this research is that their analysis does not include data for people of Asian origin prior to 2011-2012, nor does it distinguish the data from specific Asian or Hispanic populations.
"At a time when the prevalence of obesity may have reached a plateau, the waistlines of US adults continue to expand," say the authors in their study, published in JAMA.
As abdominal obesity is a risk factor for coronary heart disease - the number one cause of death for men and women in the US - addressing these increases could be high on the agenda for health officials.
The NHLBI recommend that clinicians assess the waist circumference of their patients. The researchers state that their results "support the routine measurement of waist circumference in clinical care consistent with current recommendations as a key step in initiating the prevention, control, and management of obesity among patients."
The best ways to battle belly fat and reduce the waistline are to eat a healthy diet full of fruits, vegetables and whole grains, keep portion sizes in moderation and get the appropriate amount of physical activity every day.
While it provides physical health benefits by combating abdominal obesity, physical activity can also improve psychological health. Recently, Medical News Today reported on a study that found cycling or walking to work is better for people's mental health than driving.