Exercise is often cited as the holy grail of weight loss. If you have the discipline to work out for 30 minutes a day, many assume weight loss will be a cinch, but it is not always that easy. Many women who are not used to exercising may actually gain weight when engaging in an exercise regimen. One of the main reasons this can happen is that exercising promotes the formation of muscle mass, which is actually more dense than fat. This means that if you lose some fat but gain some muscle, you can end up gaining weight.
When the muscles are not used to being worked out regularly, they are more likely to become sore and increase in size. After you have been working out for a while, the muscles will stabilize in size and become more toned when doing aerobic exercises. Cardiovascular activities like running, walking and swimming tend to promote lean, toned muscles. Certain activities will yield larger, stronger muscles, which can contribute to weight gain. For example, lifting heavier weights can put on muscle. Some weight gain due to muscle building is common. The upside is that after the muscles are sufficiently strong enough to handle harder workouts, you can burn calories faster.
Another key factor in determining weight gain is changes in appetite and eating habits. When you work out, your body burns calories, and, therefore, must replace those calories by eating more food. When you work out a lot, you will have to eat more to maintain your body. While the need for additional calories does not always lead to increased hunger, it can lead you to eat more than usually without knowing it.
Working out can also lead to detrimental self-rewarding, meaning using exercise to justify bad eating habits. For instance, thinking something like, "Since I just ran for 30 minutes, I will reward myself by eating an ice cream sundae" is common. The problem is, the ice cream sundae will likely negate any calorie burn from the workout or even add more calories than the workout managed to burn.
Another factor that can lead to perceived weight gain is body hydration and normal fluctuations in weight. When you weigh yourself, it is important to do it at the same time every day, such as right when you get up or right before bed. The body can fluctuate up to 5 pounds or more in weight on any given day because of changes in water weight and digestion. If you run in 90-degree heat for an hour, you might shed 5 pounds of water weight. However, weighing yourself after the run is not a useful measure since you are likely to drink a lot of water immediately after the run.