One important hormone involved in this system is leptin. It helps regulate how much food we want to eat over longer periods of time, not just for our next meal. So a person with a more sensitive system might go back for seconds and thirds at a party, then feel full for the next few days and eat less. "They just automatically can kind of recalibrate their energy balance because their appetite signaling system can say, 'Okay, we got enough energy,
Genetics can play a role in a person's tendency to gain or lose weight. Researchers have identified over 250 different regions of DNA that are associated with obesity, according to a 2019 study published in PLOS Genetics. For this study, researchers compared 1,622 healthy people with low body mass index (BMI) against 1,985 people with severe obesity and 10,433 control people of normal weight. They found that the thin participants had fewer genes associated with obesity. But according to Barroso, who was a co-author on the study, genes alone don't determine your weight. "We didn't find genes that were exclusively either protecting from obesity or predisposing someone to obesity. It seemed like a continuum," Barroso said, "You also have people who have the genetic determinants for obesity yet they're not [obese].
In the end, the answer is complex: our tendency to gain weight or maintain our weight isn't pre-determined, but it's also not entirely under our control. There's no genetic on-off switch that allows some people to eat all they want without gaining weight; at the same time, a tendency to gain weight isn't necessarily due to a lack of self control