When the going gets tough, the tough (and not-so-tough) often get hungry. Why that happens has been a mystery.

Although researchers have had clues that there might be some scientific basis for the notion of “comfort food,” the precise link between stress and eating has been fuzzy.

Now, scientists have developed a model for a biological link between stress and the drive to eat: Food with lots of sugar, fat and calories appears literally to calm the body’s response to chronic stress.

In addition, research indicates stress hormones encourage formation of fat cells, particularly the kind that are the most dangerous to health. That may be at least one reason why obesity rates are skyrocketing in the United States and many other modern societies.

“In highly industrialized countries, people do apparently seem to feel more stressed — more under the gun,” said Mary Dallman, a professor of physiology at the University of California at San Francisco, who outlined her theory in a paper to be published in an upcoming issue of the Proceedings of the National Academy of Sciences. “And they certainly are eating a lot more.”

The new theory has been drawing praise from other scientists since it was posted on the Internet earlier this month.

“It’s an important new model,” said Alan Watts, a professor of neuroscience at the University of Southern California. “She’s brought together under one roof two parallel processes. This is the first time anybody’s been able to put together a united theory on stress and energy metabolism. It presents a new way of thinking about this.”

Scientists have long known that during times of stress, parts of the brain emit a chemical signal called corticotropin-releasing factor (CRF), which in turn causes the adrenal gland to pump out large amounts of hormones known as adrenal corticosteroids, including cortisol. These “stress hormones” flood the body, producing a wide array of effects designed to get ready to flee or fight: The immune system gets damped down. Alertness increases. Heart rate quickens.

Dallman’s study, performed on rats, induced stress on the subjects and then provided high-calorie, fatty food for them to intake.

The fat cells, in turn, appear to send signals back to the brain, shutting down the production of stress hormones, which makes animals — and people — feel better and relax until they burn off those fat deposits. After ingesting high-sugar, high-fat diets, and developing fat deposits, the levels of CRF in the laboratory rats dropped.