It’s no secret: many people tend to start adding on the pounds as they get older. They often chalk it up to their waning metabolism — after all, the same pattern seems apparent in everyone around them, and it seems to be a natural progression with age. They hit somewhere around 30, start gaining weight as other body functions and parts begin to head downhill, and so it stands to reason that one’s metabolism starts to take a dive as well.

What is metabolism? It’s the body’s ability to take food and turn it into energy. The theory among a lot of these people is that with age, metabolic rates decline and so weight goes up — and it’s technically true — nutritionist Rebecca Mohning told the Washington Post, “This is because after the age of 25 — which is the age where we stop growing bone — the metabolic rate goes down by 2 percent or more per decade.” However, it’s the severity that is more of a myth (the exception being with metabolic disorders or other rare cases). A lot of people seem unwilling to admit that these changes might actually be preventable.

So what causes weight gain as one gets older? The short answer: life gets in the way.

When most people are in their 20s, they are more active than they realize. A lot more 20-somethings are inclined to lift weights, go for runs, or hit a long bike ride than their 30-something-year-old counterparts. Even if they aren’t exercise fanatics, they tend to be out and about more often. College students are generally walking from class to class and are using public transportation, and a server at a restaurant is constantly on their feet, moving from table to table. These things might seem minuscule, and of course, actually running or lifting weights is going to translate much better, but it actually matters quite a bit.

There is a significant difference between jobs that don’t really require a whole lot of bodily activity and an actual sedentary job. A teacher isn’t exactly at the level of a construction worker, but it’s disingenuous to say they aren’t constantly on their feet working their body. Contrast that to a job that requires someone to sit at a desk for seven hours in a day, almost completely sedentary the entire time — just try both for a few months each, and you’ll notice the difference.

Alan Fogel of Psychology Today wrote that, “In 1960, about half of the jobs available required at least moderate physical activity. In the US today, only 20% of jobs require moderate activity. The rest are either sedentary or require only minimal activity.” As modern folks grow up, they tend to exercise less, and their jobs rarely make up for any significant fraction of that loss. They often have kids, which further complicates things — the lifestyle change for one, not to mention the massive bodily changes for women. Work hours become increasingly sedentary and extend further into the day (and/or night).

Another example of this is among military veterans, especially ones who get out at a young age. Many veterans will experience quite a bit of weight gain as soon as they depart from the service. They are eating the same amount as back when they were burning an obscene amount of calories, and yet they stop exercising altogether. Even on days when you aren’t conducting physically demanding training exercises or doing daily exercise, the military still has you on your feet all the time. Once all of these vanish with the DD-214, many veterans (who don’t continue to work out) gain significant weight as they transition into the civilian world, even if they are in their mid-20s, well before the alleged decline in metabolism.

So what do you do?