Q: I breathed into a device to get my metabolism tested at my gym two years ago; it showed I was burning about 1,600 calories a day. Recently I was tested again, and it was only 1,250 calories. I'm 42—could my rate have changed that much in two years? Is there anything I can do to increase it? — Lora Butler, Frisco, Texas
A: Metabolism definitely slows with age, but not at that pace. The difference is more likely due to the way your rate was tested. Many gyms use a handheld device that measures the ratio of oxygen to carbon dioxide in your exhalations. This reveals how efficiently you use oxygen, which in turn provides a window into your resting metabolic rate—how many calories you need just to maintain your body's functions. The number can be valuable because resting metabolism accounts for about 70 percent of a person's calorie burn. (Assuming you don't take in more calories than your body requires, exercise and activity use up another 15 to 20 percent of calories, and the rest help fuel digestion.)
Unfortunately, any number of factors can skew the results. If you ate or worked out within a few hours of taking the test, the measurement will be off. Being on pain medication or lying down instead of sitting could also alter the outcome.
The gold standard for measuring metabolism is a metabolic chamber—a sealed room in which a person stays for a day while every bit of energy consumed is tracked and the oxygen and carbon dioxide content of the room is continuously monitored. A 10-minute measurement in your trainer's office won't be as exacting in terms of accuracy.
The good news is that it's very unlikely your calorie burn has plummeted. Still, if you have gained body fat or lost muscle over the past two years, your metabolism may well have slowed. With each passing year, alas, our energy requirements do tend to decrease slightly. You can fight that decline by continuing to exercise. Building muscle, which requires more calories to maintain than fat, will also help shore up your metabolic rate. Aim for three sessions of resistance training each week.