Caffeine is in a number of foods, including tea, coffee, chocolate, energy drinks and some types of soft drinks. It's even an ingredient in certain medications.

When you start working out, you typically expect to lose weight, not gain it. But don't worry if you're not dropping pounds on the scale -- chances are, your workout routine is making you gain lean mass, not fat.

Rapid weight gain has many causes, including hormonal changes, medication side effects and lifestyle changes. Even when trying to gain weight, a gain of more than one to two pounds a week is considered rapid and may be cause for concern.

Lithium, also known as Eskalith, is a prescription medication used to treat bipolar disorder. Bipolar disorder, which is also known as manic depression, is characterized by extreme manic or depressive moods. Weight gain is a known side effect for lithium.

People with fast metabolisms may have a lean build, small joints, long limbs, stringy muscles and seem unable to gain weight. Short, intense weight-lifting workouts that focus on big muscle groups and increasing the number of calories in your diet can help you gain weight if you have a fast metabolism.

You're doing your best to lose weight by watching what you eat and reducing portions, but the pounds continue to pile on. Decreasing your calories doesn't bring about weight loss unless you're consuming fewer calories than you burn daily.

Most weight gain in adulthood results when you consistently consume more calories than you burn. Even 100 extra calories per day can lead to a 10-pound increase in weight over the course of a year. Where you store that fat depends on your genetic body type, which is usually dictated by your gender and hormones.

Increasing your caloric intake helps you gain weight. To help you put on quality pounds in the form of lean muscle, make the calories come from healthy sources. If you're underweight because of injury, surgery or illness, adding nutrient-rich calories will help you heal.

Weight gain can help improve your performance in certain sports or fill you out so your physique matches that of other guys at school. Take in more calories than you burn daily to put on pounds -- but not just any calories.

Many combat sports are divided into separate weight classes to keep fights fair, as a size deficit between two skilled athletes can make a huge difference in terms of power and reach.

There are a number of reasons why you may discover that your favorite pair of jeans no longer fit the way they used to. While it can be shocking and saddening to noticed that you have gained weight around your waist, in most cases weight gain is reversible.

Periactin is a brand name version of the prescription drug cyproheptadine. This drug is usually used to treat the symptoms of allergic reactions and seasonal allergies.