With so many people struggling to lose weight, your friends may be surprised that you have a hard time gaining. But if you have lean genes or a very active lifestyle, it can be a challenge to add pounds to your frame.

Bipolar disorder is a psychiatric illness that is also known as manic depression. It most commonly appears in the late teen or early adult years and is characterized by a mood state that alternates between depression and mania.

For men and women who are looking to put on lean mass, the key is to combine resistance training with a high-calorie diet. The goal is to eat more than your body burns so you gain weight. This doesn't mean having a free-for-all diet where you eat a lot of pizza and burgers.

You can't choose from where you lose weight, and you can't really pick where you gain it either. If you have long, thin legs, you may desire a bigger, more solid-looking base.

Gastroparesis, also known as delayed gastric emptying, is a health condition in which ingested food does not empty from the stomach at the normal rate, causing uncomfortable feelings of fullness and possibly more severe symptoms.

Sure, bulking up can make you look stronger and fitter, but if you don't gain weight the right way, you might end up adding more fat than muscle to your frame.

Unexplained or sudden weight gain can be an alarming occurrence for women -- and it does not always signify you have been eating too many high-fat foods. A hormonal imbalance can result in excess testosterone production and subsequent weight gain in women.

If your stomach appears bigger even though your weight stays the same, you might be concerned. Abdominal bloating can enlarge your waist and make your pants fit tighter. Many issues, from not at all serious but quite uncomfortable to potentially life-threatening, can cause abdominal bloating.

Ear infections commonly result from mucus accumulation in the ear, according to Mayo Clinic. This health issue causes inflammation and pain. Mucus accumulation in the ears most frequently occurs during a viral or bacterial infection.

You naturally begin to lose muscle with age, but resistance training can help prevent or reverse this loss. The exercise can also help promote bone strength, boost your metabolism, reduce your risk of injury and help prevent or relieve conditions such as diabetes, back pain, arthritis and osteoporosis.