I’m not saying you’re a bad person if you brag about how much you can eat without gaining weight because of your fast metabolism, but I definitely don’t like you. Saying that you can happily eat fast food all day long without any repercussions is kind of like blabbing about how much money you can burn without worrying about your finances. In both cases, it’s impolite, and you probably just have your parents to thank.
Metabolisms are often treated as this enigmatic concept, something beyond our control. That’s only partially true. By and large, your metabolism is really just the rate at which your body burns calories in order to function. The determinants of this rate are pretty straightforward — your age, weight, height and exercise level are typically the biggest factors. Genetics plays a part, but it’s not destiny. Some people really are blessed with a genetically-inherited metabolism that allows them to eat rather freely, and some are genetically predisposed to weight gain or conditions like hypothyroidism, which causes them to burn calories at a slower rate. Most people, though, are able to exert control over their metabolism simply by monitoring their weight and regularly exercising.
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So while you may indeed have a “slow” metabolism that requires you to put more effort into your diet and fitness, that doesn’t mean you’re fated to have your body look a certain way. Lean muscle mass, for example, is another determinant of your metabolic rate. Generally, people with more lean muscle mass burn calories at a higher rate than those with less, and men have some genetic advantage here. However, men are also genetically predisposed toward gaining weight on their abdomen and developing beer bellies. Men tend to carry weight on their stomachs more easily than women, and men whose fathers had beer bellies may develop them more easily, too. Alternatively, if you’re a man who comes from a long line of men with tree trunks for legs, having a slow metabolism from a lack of exercise or health problem might mean you acquire this trait, as well.
Basically, you’re more likely to have the same body shape as your parents only if you have the same habits as them. Though inheriting a slower metabolism might mean you require more exercise to burn off the calories you consume, you’re still in control of your body shape. It’s definitely unfair that some people’s bodies burn calories faster than others, but the two biggest factors of your weight, diet and exercise, are still day-to-day decisions for you to make.