It helps produce a slew of hormones crucial to your health, including reproductive hormones such as oestrogen, appetite-regulating hormones like leptin, blood-pressure-controlling hormones like angiotensin, and tumour necrosis factor-alpha (TNF), which helps keep cells healthy.Another fun fact: We tend to talk about fat like it’s something you shed and gain, but in reality, the number of fat cells you have is set during childhood and adolescence, and that remains pretty much constant throughout adulthood. Research shows about 10% of your fat cells die each year and are automatically replaced. When you gain or lose weight, what happens instead is that these fat cells simply expand or shrink, Dr. Lavie says.Over the past few years, there’s been more and more evidence that a little fat can actually do the body good. “Being slightly overweight actually seems to increase longevity: Study after study shows that overweight people with certain chronic diseases like high cholesterol or diabetes actually often live longer and do better than normal-weight patients with the same conditions,” Dr. Lavie says.People who are classified as overweight actually have a 6% lower risk of dying from any cause, according to one 2013 study . “One theory is that fat helps guard the body from damage, particularly as we age,” Dr. Lavie says. “Anytime your body is fighting an illness or dealing with a chronic disease, it requires more energy, so it makes sense that that extra fat is helpful.” This seems particularly true if your fat’s located around your butt and thighs, according to one Oxford University study . “The thought is this fat traps the potentially harmful fatty acids that can travel through your bloodstream to your heart,” Dr. Lavie says.So knowing all this, why do many health experts continue to wag their fingers about obesity? “When it comes to being overweight or obese, there’s a myriad of factors at play, including how fit you are, if you have other health conditions, and what your attitude is about your weight [that determines your health],” explains Dr. Lavie. But there does often come a point at which weight gain can begin to affect your ability to stick with the healthy habits (like exercise) that are really important for your health.“If you’re overweight or slightly obese but fit and metabolically healthy — normal blood pressure, normal cholesterol levels, normal blood sugar — then it’s hard in that situation to show that being thinner would make you even healthier,” Dr. Lavie says. “But the picture changes once you really become obese.”Simply put, obese people have a harder time with exercise, both because it’s physically harder if you’re heavier and because weight stigma can keep people from wanting to be seen at the gym. If your weight interferes with your mobility, or even just your motivation, then that’s probably going to affect your heart health as you age.The bottom line: You absolutely can be fat and fit — your level of health has everything to do with the way you're treating your body, and not its size.