Eat 5,000 calories a day consistently, and you're bound to gain weight. If you're like most people, even if you have a high metabolism or do significant levels of physical activity, 5,000 calories is a calorie surplus, which means you take in more calories than you burn.

Although weight loss is an on-going concern for the average American -- the Centers for Disease Control and Prevention estimates that nearly 70 percent of adults in the United States are either overweight or obese -- gaining a few pounds is a very real goal for the small percentage of people who are underweight.

A special restaurant meal or an indulgent night of drinking that made your total calorie intake for the day come to 3,000 calories might make you feel as if you've gained 10 pounds overnight -- but you haven't. Weight gain requires you to consume a calorie surplus over several days, weeks or months.

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.

If you want your number on the scale to go up, drinking weight-gain shakes without exercising can do it. However, those extra pounds you put on will be more flab than muscle. For healthy weight gain, a little exercise can make a big difference. Consult your doctor before making changes to your diet or exercise routine.