A 1995 metabolic ward study by Levine et al. observed that individuals who overate by 1,000 calories per day for six weeks showed significant variability in weight and fat gain, attributed to spontaneous increases in physical activity.

“So I believe they had people overeat. And I think it was by like 1,000 calories a day and I think for six weeks. And I mean, this is the metabolic ward. So this is very tightly controlled. It's as tight as you get. And what was interesting is, of course, on average, people gained weight and gained fat mass. But some people gained more than expected, and there was one person, in particular, who only gained like just over 1/2 a kilo. They should have gained like-- I think it was something like 3 to 4 kilos. It was predicted. And what they found is this individual just spontaneously increased their physical activity. He didn't purposefully do it, it just happened.”