Loss of productivity due to obesity costs as much as medical expenditures for the condition, according to a new study that pegs the cost of obesity among full-time workers in the United States at $73.1 billion per year.

Obesity's hidden costs, the researchers said, stem from the fact that obese people tend to be less productive than normal-weight people while at work – simply accounting for the extra sick days they take misses a big part of the picture.

The study, published in the Journal of Occupational and Environmental Medicine, took into account medical expenses, sick days and health-related productivity costs associated with obesity. The findings suggest employers could save money by investing in health improvement programmes for their employees, the researchers said.

"Now that we've uncovered this sort of hidden cost, I think that it ups the ante for [employers] to think harder about what sort of interventions they want to implement," study author Eric Finkelstein, deputy director for health services and systems research at Duke University and the National University of Singapore, told LiveScience.

Plenty of studies have linked obesity to health-care costs and lost workdays. But fewer have examined "presenteeism", or lost performance while at work. Finkelstein and his co-authors used data from a nationally representative survey on medical expenditures (2006 data) combined with data on absenteeism and presenteeism from the internet-based US National Health and Wellness Survey (2008 data).

Pregnant and underweight individuals were excluded from the analysis. The research was funded by Allergan, Inc., a health-care company that makes LAP-BAND and other devices used in weight-loss surgeries.

After controlling for race and ethnicity, income, education levels, insurance coverage, marital status and smoking, the researchers found significant costs of being obese. These costs increased with body mass index (BMI), a measure of height and weight that researchers use to define obesity. (A BMI over 30 is considered obese.) Presenteeism due to health problems was common in workers regardless of weight, but it doubled with each increase from mild to moderate to extreme obesity.

Female employees with BMIs between 30 and 34.9, for example, experienced 6.3 days of lost time per year (while at work), a number that jumped to 22.7 days in women with BMIs over 40. Men in the lower BMI category lost 2.3 days of at-work productivity per year, while men with BMIs over 40 lost 21.9 – three full weeks.

"As you increase in your BMI, there is just a tremendous increase in the impact of that obesity on work productivity," said Marco daCosta Di Bonaventura, the director of health economics and outcomes research at Kantar Health (a health-care consulting company) and a co-author of the study.

Overall costs also increased along with BMI. Men with BMIs of 30 to 34.9, the low end of the obese range, cost $1,143 more each per year in medical expenditures, missed workdays and lost productivity at work than normal-weight men. Men with BMIs between 35 and 39.9 cost $2,491 more each, and men with BMIs over 40 cost $6,087 more. Women showed a similar pattern.

Having a BMI between 30 and 34.9 cost $2,524 extra each year, while a BMI between 35 and 39.9 cost $4,112. Each woman with a BMI over 40 cost on average $6,694 more than a normal-weight woman. Despite the high prevalence of obesity in America, individuals on the 40-and-over side of the BMI-spectrum are relatively rare.

According to a 2010 study in the Journal of the American Medical Association, about one-third of US adults over age 20 are obese. But only 14.3 percent of American adults have a BMI of 35 or more, and just 5.7 percent have BMIs over 40.

All told, obesity among full-time workers costs $73.1 billion per year, the researchers estimated. That's the equivalent of hiring 1.8 million new workers at annual salaries of $42,000, which is what the average American makes each year.

In comparison, a 2010 report by the American Lung Association estimates that the costs of healthcare, premature death and loss of productivity from smoking tally to $301 billion per year. About 23 percent of Americans smoke.

A 2008 study published in the American Journal of Psychiatry estimates that mental illness, which also affects about a quarter of Americans, costs the economy $317 billion every year in lost wages, healthcare costs and disability benefits.

While 18 percent of the total cost of obesity was because of lost workdays, lost productivity at work due to health troubles contributed 41 percent of the extra cost.

That was the same percentage as the additional cost for medical expenditures.

One reason that presenteeism was so much more influential than absenteeism may reflect a tendency by workers to power through illness instead of taking sick leave, Finkelstein said.

"Especially in a bad economy people want to get paid, so they find a way to go into work even if they're not feeling great," he said.

"I think these results are bearing that out."