1 / 9It's one of life's great injustices: Some people must carefully pay attention to everything they put in their mouths in order to maintain their weight, while others can eat doughnuts to their heart's content and achieve the same result.2 / 9The reason for not gaining weight is not just good metabolism. There are many reasons for this. Such as nutrition and mode of behaviour also help maintain body weight. Whether or not you gain weight depends on your individual routine.3 / 9One of the most important factors has nothing to do with body type, metabolism, or performing a spell during the full moon: It's perception. 4 / 9Many people who appear to eat whatever they like without gaining weight aren't actually eating more than the rest of us, Melanson said.5 / 9For example, your friend who eats ice cream on a daily basis might naturally compensate for those extra calories by eating less at another meal, or snacking less throughout the rest of the day. Or perhaps, when they eat pizza, they're eating slowly, getting full, then stopping after just a few slices. 6 / 9Physical activity can also make a difference, but it doesn't have to be a gym workout. 'Some people just move more, even if they're not necessarily athletes,'7 / 9That extra movement can also rev up the body's metabolism, or how much energy your body spends throughout the day, not including exercise. 8 / 9Genetics 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.9 / 9In 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, Melanson said.