If you’re trying to lose weight, kick off your diet by relaxing. Stress tinkers with your brain chemistry in ways that make it hard to make healthful food choices and maintain self-control, a new study finds.

Volunteers who endured a somewhat stressful experience were 24 percent more likely to choose unhealthful snacks afterward compared with volunteers who hadn’t experienced stress. And researchers think they know why: Brain scans showed that the stressed people had altered neurological connectivity between regions of the brain that process tastiness, make value judgments and plan for long-term goals.

“Being able to regulate and control your decision-making involves this complex interplay between different areas of your brain,” said Todd Hare, senior author of the study. “There’s not just one region or node that turns on and off to establish self-control; they have to all sync up and work in unison. Stress disrupts that synchrony.”

To figure this out, Hare and his colleagues enlisted 51 male volunteers, all of whom had said that they were trying to eat better and stay fit but that they occasionally indulged in some junk food.

Half the men were randomly selected to undergo a moderately stressful experience: immersing their hands in ice water for three minutes while a researcher videotaped them. The other men put their hands in room-temperature water for three minutes, with a researcher in the room but without a video camera rolling. Afterward, all the participants filled out a questionnaire to rate their level of stress.

Although soaking your hands in ice water may not sound like a big deal, previous studies have shown it produces about the same amount of stress as arguing with a co-worker or spouse, having to make a presentation at work, or getting stuck in traffic when you’re already running late.

Next, the volunteers were presented with a series of two food options on a computer screen and asked to choose the one that was more healthful. (Options included cookies, chips, apples and kiwis.) Then the computer presented a pair of foods and randomly selected one of them to recommend to the volunteer to eat. The volunteer could either accept the recommendation or override it and choose the other option, knowing that at the end of the study he would be required to eat the food he chose.

The stressed volunteers went for the tastier, less healthful food more often than their counterparts.

The more stress participants felt, the weaker the connections between the part of the brain that deals with long-term planning and the part that manages value judgments and decision-making.