While calories are not the only factor in fat loss, they do play a significant role. As a result, people often go to great lengths to reduce calories consumed (diet) and/or increase calories burned (exercise). But losing weight can result in some unintended consequences that impact continued loss or maintenance.
As it turns out, most of the calories we typically burn are not from exercise or even movement; rather, they're burned just keeping the biological machinery running. The most basic measure of this is the Basal Metabolic Rate (BMR). It's measured in kilojoules (kJ) per hour per kilogram (kg) of body mass, but is most commonly expressed as calories per day. True BMR is notoriously hard to measure due to all the possible variables (e.g., movement, temperature, etc.), so it's usually just estimated using a simple calculation based on weight, height, age, and gender. (You can find lots of these calculators online.)
And here is the calorie-counting catch: the more weight you lose (and the older you get trying to lose it), the lower your BMR. For example, a 5'6" 40-year-old 170-pound female who loses 20 pounds in a year goes from burning approximately 1515 calories (resting) per day to 1424 calories. What a gyp!
But wait, there's more! She's probably burning fewer calories moving, too, since she doesn't have as much weight to move (so it takes less energy). So all the incidental walking and stair climbing are "easier." And if her favorite exercise is walking, running, or other weight-bearing cardio, she'll be doing less work (and burning fewer calories) for the same workout. Gyp-tastic!
Now, if she's doing strength training and building more muscle, that will burn more calories, right? Doesn't muscle burn more calories than fat even at rest? Maybe some, but certainly not enough to make up the difference. Gyp-rageous!
Fortunately, it's not all bad.
If she's eating well, reducing caloric intake (a.k.a. eating less) probably won't be too hard since there won't be the blood sugar spikes that tend to drive cravings.
And while building more muscle may not help burn many more calories at rest, it certainly does burn more in action by letting us run faster, jump higher, lift heavier, etc. Yay!
Ultimately, there's the moral of the story: progress your workouts! Lift heavier, walk/run/ride faster, build more muscle and use it! That's the best way to keep a declining BMR from sabotaging continued success.
Be seeing you.
-gary