Somewhere around the age of 30, your calorie-burning machinery slows down and, to compensate, you may have to eat less to stay the same weight. But it's not all about the food.
One of the inevitable truths about aging is that you will start to lose muscle mass as you get older and your lost muscle will be replaced by fat. Your metabolism starts to slow down because fat tissue does not burn calories as efficiently as muscle tissue. In fact, your metabolic rate, or the rate at which you burn calories, takes a two-percent dive every ten years once you're out of your 20's. That means you have to start cutting 30 to 40 calories a day from your usual diet in order to stay the same weight. Or you need to increase the amount of exercise you do to burn that many more calories.
It's hard to imagine that a mere 30 or 40 calories a day-the equivalent of half a slice of bread-would make much difference in your weight. But here's how easily it adds up if you don't cut out that bread (or that extra slice of bacon) or exercise more: Say you consume approximately 2,000 calories a day in your twenties and need to cut or burn 40 calories a day to maintain your weight. If you don't do it, those 40 calories a day add up to an extra 14,600 calories a year. For every additional 3,500 calories you consume, and don't work off, you will gain one pound. So, in one year, you will gain a little over four pounds of weight. That doesn't sound so bad, does it? But over ten years, that adds up to at least 40 pounds of additional weight. And at that point you have to cut calories even more, which becomes harder and harder to do.
It doesn't have to happen, but the best way-and perhaps the only way for most people-to stop age-related weight gain in its tracks, is to start taking action while you're still young. In fact, a study published in a 2010 issue of the Journal of the American Medical Association found that normal-weight women who ate a normal diet and averaged 60 minutes a day of moderate exercise were able to maintain a healthy weight or gain no more than 5 pounds over the 13-year study period. Heavier women in this study had to cut calories and increase exercise in order to prevent further weight gain.
You can't fight your genes, which determine your body type and, to some degree, your weight range, and you can't stop the natural aging process, but you can stay fit and be the best you can be, for decades to come. That much, you can control. Whatever age you are, the best thing you can do, if you're not already doing it, is to start eating balanced, healthful meals, know approximately how many calories you consume and how many calories are in the foods you eat, so that you know how to cut back if the numbers on the scale starts inching up. Equally important, or perhaps even more important, find an exercise or physical activity you enjoy, and make a habit of it.
Sources:
Lee, IM et al. "Physical Activity and Weight Gain Prevention." Journal of the American Medical Association. 24 Mar 2010:303(12) 1173-1179. Web. 25 Jan 2011
http://www.stanford.edu/~kcobb/hrp258/weight%20gain%20and%20physical%20activity.pdf
