It’s usually not just due to losing muscle mass over time (more on that later). "It's multifactorial," says Fatima Stanford, MD, MPH, obesity medicine physician scientist at Massachusetts General Hospital and Harvard Medical School. Numerous factors can sneak up on you and help you gain or hold onto weight, including your lifestyle, your food, your biology, and your sleep habits. The good news in that is that there are also numerous ways to tackle getting your weight to where you want it—you don't have to force yourself into one approach, and you get to choose what works for you.
Between desk jobs, commutes, and family activities, many 40-somethings don’t have a lot of free time to work out. But it’s important -- for your weight and your overall health -- to fit in at least 2 1/2 hours of moderate physical activity (like brisk walking or light yard work) every week. Pencil times in to your calendar, and make them a priority.