American adults who prepare their own meals and exercise on the same day are likely spending more time on one of those activities at the expense of the other, a new study suggests.

The research showed that a 10-minute increase in food preparation time was associated with a lower probability of exercising for 10 more minutes -- for both men and women. The finding applied to single and married adults as well as parents and those who have no children.

Researchers analyzed nationally available data on more than 112,000 American adults who had reported their activities for the previous 24 hours. Of those, 16 percent of men and 12 percent of women reported that they had exercised on the previous day. And men spent, on average, almost 17 minutes preparing food, compared to an average of 44 minutes for women.

The average time spent exercising for the entire sample of adults, including those who did not exercise, was 19 minutes for men and nine minutes for women.

This means that the average respondent, male or female, spent less than an hour on both exercise and food preparation on the same day.

By inserting the data into statistical models, the researchers determined that there is a substitution effect for American adults who participate in these two time-consuming health behaviours on the same day.

"As the amount of time men and women spend on food preparation increases, the likelihood that those same people will exercise more decreases," said Rachel Tumin, lead author of the study and a doctoral student in epidemiology in The Ohio State University's College of Public Health. "The data suggest that one behaviour substitutes for the other."

The findings suggest that public health recommendations should not be made in isolation of one another, but should take into account the time available to devote to health-promoting behaviours on a given day, Tumin said.

"If we assume, for example, that adults have 45 minutes of free time to allocate to health-promoting behaviours, maybe we need to look at that holistically and determine the optimal way to use that time," she said.

There is plenty of evidence that time is scarce for most American adults, especially those who work full-time and have children. Previous studies have also shown that time spent preparing food and being physically active have declined in recent years. At the same time, increasingly sophisticated public health recommendations detail the many ways in which Americans can behave to improve their well-being.

Some of those behaviours take little or no time at all, including not smoking, avoiding excessive alcohol intake, reducing fat in the diet and increasing fruit and vegetable intake. Exercise and food preparation, on the other hand, require an investment of time to be most effective.

"For time-intensive behaviours, public health officials may need to triage their recommendations by how much total time they think people have to spend on these activities each day," Tumin and her colleagues concluded. "If adults have a set time budget to devote to healthy behaviours, then recommendations should be tailored to make efficient use of that time budget."