First, the bad news. We lose about 2% of our basal metabolic rate (the amount of calories burned per day by doing absolutely nothing) per decade after our 20s. That means even if you ate the same and exercised the same in your 30s as in your 20s, you will gain weight and body fat. By your 50s, this process accelerates even faster. To put it in perspective, you are about 40 calories per day less efficient in your 30s vs your 20s at using energy if you baseline needs are 2000 calories per day. That means 80 calories less efficient by your 40s and so on. While 40 calories per day may not sound like much, it equates to a 4 lb weight gain per year, and about 20 lbs in 5 years by having the exact same diet and activity level in your 30s vs 20s. Or 40s vs 30s, etc. This inevitable change means that either calorie intake has to be decreased or activity increased just to maintain your current weight.
There is some good news, though. Total energy expenditure, how active you are, is in your control. We generally see about 150 fewer calories utilized per day each decade, and if you think about it, that makes sense. Whether you participated in sports or marching band or cheer or even just riding your bicycle to your friend’s house, chances are good that you were more active as a kid. Then as jobs start or babies come, the time you previously had to yourself and also the previous motivations for fitness (making the team, impressing a cute boy, etc.) can begin to slip away. As your activity level declines, the amount of calories burned each day does as well, but by finding time to keep up with exercise, your body can continue to burn nearly the same amount of calories as before.
Need some added motivation to get back into an exercise routine? In addition to burning the calories during exercise, activity also boosts the resting metabolic rate (RMR). For instance, women who were active for 9 hours per week had a RMR about 70 calories per day higher, about 10% less body fat and a little over 15 lbs less body fat than their sedentary counterparts. There are a number of factors that contribute to the improved body composition, including increased muscle mass (muscle burns more calories than fat, even at rest), increased production of growth hormone (this helps build muscle and reduce fat), and also the increased calorie utilization to help your body recover from the exercise.
Staying fit is hard, and avoiding weight gain is an active process. As an unavoidable part of the aging process, nutritional needs change as we get older, but we do have some control over how our metabolism changes affect us. By being proactive and monitoring changes in our weight, we can make some small tweaks in diet and exercise to closely maintain our fitness, or if needed, bigger changes to work toward a healthier lifestyle. Good luck in making the second half of 2019 your healthiest year yet!