A 2014 study found that the type of fat we consume might make all the difference. Participants in the study were asked to eat 750 extra calories every day for seven weeks. Those having excess calories from saturated fats had activated cells that promoted fat storage in the belly and increased insulin resistance. However, individuals who had a high consumption of polyunsaturated fats found in fatty fish, nuts and seeds, gained less abdominal fat and were more likely to increase muscle mass instead.
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.