- Simple math. To maintain your weight, you need to burn as many calories as you take in. Consuming extra calories will result in weight gain. It's easy enough to exceed your calorie needs on a day-to-day basis, it's especially easy to do so when your routine is altered (like during a vacation).
- A pound is a pound is a pound. To gain a pound you need to consume 3,500 extra calories. Over the course of a special dinner out, an extra serving of dessert, and some extra large portion sizes you can reach that 3,500 calorie mark quite easily. To lose a pound, you need a deficit of 3,500 calories. Reducing the calories you eat and burning extra calories through exercise can give you the deficit you need -- but it takes commitment and effort on your part.
- Exercise is weight discriminating. While 3,500 extra calories will lead to a pound of weight gain no matter what size you are, it's easier to burn 3,500 calories when you weigh more. It may not seem fair, but a higher body weight leads to an increased resting metabolic rate, which helps burn more calories. That's why those last five to 10 pounds can seem particularly stubborn.
- Age contributes to weight gain. As we age, metabolism naturally slows. So it becomes easier to gain weight and a little harder to lose it.
It's also good to remember that, while the simple math is accurate, there are other factors that contribute to weight as well. But don't let the challenge get you down.