In a way, Dr. Flier noted, the discovery is a sort of back-to-the-future moment. There was a time a few decades ago, before the current interest in how the brain regulates how much is eaten, when obesity researchers spent all their time studying and discussing fat cells. Investigators discovered that fat people had more fat cells than thin people and that fat cells shrank with weight loss and bulged with weight gain.

Dr. Jules Hirsch of Rockefeller University in New York, who did many of the initial studies with humans, said he started because he could not understand why people who lost weight regained. “They should have been cured,” Dr. Hirsch said. After all, he said, if you cut out a fatty tumor, the fat does not grow back. Why was fat lost from dieting different?

The result was the fat cell hypothesis, a notion that obsessed researchers. Fat cells, the hypothesis said, are laid down early in life and after that, they can change only in size, not in number. When people lose weight and their fat cells shrink, that creates a signal to fill the cells again, making people regain. “We didn’t know a lot about obesity, so that was what we talked about,” Dr. Flier said.

But the discussions stalled. It was not clear what to do about those discoveries or what they meant to efforts to help people lose weight. And no one had a method to ask whether fat cells were being created and destroyed during life. Few even thought to ask that question.

That changed only recently when the new paper’s first author, Kirsty L. Spalding, a neurobiologist at the Karolinska Institute in Sweden, developed a way to ask whether new cells grow in the cortical and cerebellum regions of the human brain. She found no new cells there since birth. One day, she was giving a talk on her brain study when a scientist in the audience, Erik Arner, suggested she use the method to look at fat cells. (Dr. Arner is the second author of Dr. Spalding’s paper.) The method for dating human cells takes advantage of an effect caused by above-ground nuclear bomb testing that took place from 1955 to 1963.

When the bombs were tested, their radioactivity created a spike in the amount of a carbon isotope, C14, in the atmosphere. The C14 made its way into plants and animals that ate the plants. When people ate those plants and meat from the animals, the C14 was incorporated into their human DNA. After the nuclear test ban, C14 levels started to drop. The result is that every cell has a C14 level that reflects the level in the atmosphere at the time the cell was born.

“Each cell is a time capsule of sorts,” Dr. Spalding said.

First the researchers confirmed that the number of fat cells remained constant in adults. Obese people who had weight loss surgery had as many fat cells two years after the surgery as before it, even though they were much thinner.