Weight gain is defined as an increase in body weight. This might be due to an increase in muscle mass, fat deposits, extra fluids like water, or other causes. Weight gain might be a sign of a major medical problem. However, most people will not suffer from serious health issues just because they have gained some weight. The most common reason for weight gain is undereating or eating too much. You may be eating less than usual because you think you are supposed to eat only healthy foods or you may be overeating because of emotional issues (such as stress) or boredom. If you are eating less than 800 calories per day, you are likely to experience some kind of weight gain. Other reasons for weight gain include drinking too much alcohol, taking drugs, having diarrhea, having thyroid problems, and having diabetes. Too much sun exposure can also cause weight gain by causing skin wrinkles and increasing blood volume. Weight loss due to illness or injury is another common cause of weight gain. Being overweight or obese increases your risk of developing health problems such as heart disease, diabetes, cancer, and arthritis. So, weight gain is definitely not good news. But if you are already within a normal range for your height and weight, then you don't need to worry about it.