Stress can make you fat. And it’s not entirely because you stress eat, a new study suggests.

Researchers found that chronic stress may pump up the rate in which new fat cells are formed, according to a report published Tuesday in Cell Metabolism. It all comes down to levels of hormones called glucocorticoids, which are produced in abundance when we’re stressed.

When glucocorticoids are constantly high, as is the case when we’re chronically stressed out, it can boost the chances for a certain type of cell to morph into fat cells, the study found. And that could bump up our weight, said Mary Teruel, the study’s senior author and an assistant professor of chemical and systems biology at Stanford University.

It's long been known that rises in the stress hormone cortisol can lead to weight gain in humans, but the assumption has been that people were just eating more because the hormone stimulates appetite.

Suspecting something else might be going on, the Stanford researchers studied the effects of glucocorticoids — a steroid hormone produced in the adrenal gland — both in individual cells and in mice.

Under a microscope the researchers saw how the hormone, when kept at constant high levels, caused the development of fat cells. Intriguingly, if the levels rose and fell, there was no impact.

And that was true even if glucocorticoid levels were extremely high, but for a limited period of time.

Most significant — the mice with 24-hour, higher-than-normal glucocorticoid hormone levels saw a doubling of fat.

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“So basically, it’s not about food intake,” she said. “It’s about timing.”

While mice and people are certainly different, both are greatly influenced by circadian rhythms and both produce glucocorticoids in response to stress. Although the research was done in the lab, it's likely human body would react in a similar fashion to the mice to continuous high levels of glucocorticoids, Teruel said.

"However, more experiments would be needed to test this," she said.

The increase in fat is likely related to the fact that under normal circumstances glucocorticoids wax and wane with our circadian clocks, Teruel said. So our bodies are designed to ignore short-term fluctuations of these hormones.

But everything is thrown out of whack when levels stay high — as they will if a person's stress doesn’t diminish, even after the day's work is done.

“So maybe it’s OK to get stressed during the day, but not at night,” Teruel said.

The implication is that if we could find ways to modulate our stress in the evenings and at night, it might not affect our weight.

What helps reduce chronic stress?

That makes sense to Dr. Anthony Heaney, an endocrinologist and an associate professor of medicine and neurosurgery at the University of California, Los Angeles, School of Medicine.

“It would suggest that any method people can use to beat stress could be of benefit,” Heaney said. “I think the challenge is for people who are stressed often. I don’t think jumping into a 30-minute yoga or Pilates class will be enough to address that.”

The kinds of activities that might help are those “that need 100 percent of our attention,” Heaney said.

That could mean a game of tennis over running on a treadmill, for example.

"Certain activities we do are not absorbing enough to distract from stress, Heaney said. "Whereas jogging might be a good healthy sport, you can sometimes still ruminate and be stressed because it doesn't require your sole attention."

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