Significant weight gain in adulthood is primarily due to fat cell hypertrophy rather than hyperplasia.

“For the the for the average individual that's hypertrophy not a result of hyperplasia.”

“So if a human body is gaining fat mass, it will gain that fat mass through two different mechanisms. It will either be a function of multiplying the fat cells. So the person will have the ability to make new fat cells. That's called hyperplasia. And when the fat is undergoing hyperplasia, the size of the fat cell is staying very modest. So the size of the each individual fat cell is small, there are just a lot more of them. On the other hand, you could have someone who's storing more of their fat through hypertrophy where the number of fat cells is not changing, but the size is the hypertrophic fat cell is a very sick fat cell for two reasons.”