New results published this week in the Journal of the American Medical Association say that wearable monitor devices do not help people lose weight. Rather the opposite: People who wore trackers for 18 months of attempted weight loss actually lost less weight than people who went untracked.

In a straightforward clinical trial, researchers at the University of Pittsburgh studied 471 overweight people. Half wore devices that tracked steps and calorie expenditure. At the end of 18 months, the group that did not use the device lost about five pounds more than the people in the self-tracking group. The study’s authors conclude in the journal: “Devices that monitor and provide feedback on physical activity may not offer an advantage over standard behavioral weight-loss approaches.”

Lead researcher John Jakicic, who chairs the University of Pittsburgh’s department of physical activity, said in a publicity video that he expected the devices would help people to lose more weight. “To our surprise, that wasn’t what we found.”

Others seem surprised, too.

NPR said, “Wait. What?”

The New York Times called the finding “fascinating.” There Jakicic reiterated that he was “definitely surprised.”

I think the surprise is surprising. The study is the longest I can find for any of these devices, and it’s important because unlike most other studies of tracking devices, this one tracks actual outcomes—not just whether people exercise more (which past studies have suggested), but whether they actually lose weight.

And it’s becoming clear that counting calories is not an effective approach to weight loss.

Of course, as a problem of thermodynamics, the approach is unquestionably sound. But the psychology is more complicated.

It seems to start with the idea that fitness-tracking devices carry what’s known as a health halo. Wearing fitness trackers gives people a sense that they are doing something good for themselves, even if only subconsciously, by the very act of owning and wearing it.

Then comes bargaining. Someone who has credit for wearing the device might afford themselves more cake. There is even data to support the food decision. One might say, I moved 8,000 steps today, so I’ll have a grilled cheese and an entire bottle of wine.

That’s probably an exaggeration, but it helps make the point.

The same health halo is happening with foods themselves, like those labeled  “natural,” which has no meaning but exudes good vibes. Similarly, buy organic Gatorade and eat “homegrown” candy (or rather “fruit snacks”), and you buy a perception that these products are laying waste to your body less viciously than their nearly biochemically identical counterparts. So we eat, or we eat more. In that way these products stand to make us less healthy.

The fitness tracking devices in this week’s study weren’t little FitBits or iPhone apps, either, but biceps-level monitors that would impose more of a psychic imposition.

Jakicic had a similar read on all of this, suggesting that the devices “might give people a false sense of security, so they don’t pay attention to key behaviors that they otherwise might pay attention to. They’re relying on the device too much.”

So being aware and wary of health halos is a worthwhile health tip. That’s what I take from this. Fitness trackers aren’t bad but should be used with this in mind.

And maybe even more important is the reminder that physical activity can have myriad health benefits, but don’t count on it for losing weight. You can’t exercise away a bad diet, though junk-food manufacturers are set on telling consumers otherwise. If you want to keep a car in good condition, drive carefully, but doing that doesn’t mean you can put apple juice in the gas tank. I don’t see that taking off as an expression, but again, I hope it makes a point.

A more earthly example: Last week on The Dr. Oz Show, Republican presidential nominee Donald Trump perpetuated the age-old argument when he said that his own involvement in sports is what kept him lean in his youth: “I loved to eat and I loved sports, and it worked, because I could do both.” Likewise in approaching childhood obesity today, he proposed, “I’m a big believer in the whole world of sports. I would try and open that up.”

Counting calories also tends to lead to bad decisions. In food, the focus belongs on quality, not quantity.  Focus on quality, and quantity will follow.

Focus on quantity, and expect failure and unhappiness. Which many of us cope with by eating junk. Which we justify because we took a lot of steps. And the snake eats its tail, and we eat the snake.