Scientists from Stanford University in California found that in the past 20 years there has been a sharp decrease in physical exercise and an increase in average body mass index (BMI), while caloric intake has remained steady. They theorized that a nationwide drop in leisure-time physical activity, especially among young women, may be responsible for the upward trend in obesity rates.
By analysing National Health and Nutrition Examination Survey (NHANES) data from the last 20 years, scientists from Stanford University discovered that the number of US adult women who reported no physical activity jumped from 19.1 per cent in 1994 to 51.7 per cent in 2010.
For men, the number increased from 11.4 per cent in 1994 to 43.5 per cent in 2010. During the period, average BMI has increased across the board, with the most dramatic rise found among young women aged 18-39.
“These changes have occurred in the context of substantial increases in the proportion of adults reporting no leisure-time physical activity, but in the absence of any significant population-level changes in average daily caloric intake,” said lead investigator Uri Ladabaum, Associate Professor of Medicine, Stanford University School of Medicine.
“At the population level, we found a significant association between the level of leisure-time physical activity, but not daily caloric intake, and the increases in both BMI and waist circumference,” said Ladabaum.
The study looked at the escalation of obesity in terms of both exercise and caloric intake.
While investigators did not examine what types of foods were consumed, they did observe that total daily calorie, fat, carbohydrate, and protein consumption have not changed significantly over the last 20 years, yet the obesity rate among Americans is continuing to rise.
Researchers also tracked the rise in abdominal obesity, which is an independent indicator of mortality even among people with normal BMIs.
Abdominal obesity is defined by waist circumference of 88 cm or greater for women and 102 cm or greater for men.
Data showed that average waist circumference increased by 0.37 per cent per year for women and 0.27 per cent per year for men. Just like the rise in average BMIs, the group most affected by increased rates of abdominal obesity was women.
“The prevalence of abdominal obesity has increased among normal-weight women and overweight women and men,” said Ladabaum.