You're eating more and you're lifting weights religiously, but you're still not gaining weight. What gives? Here are four possible reasons you're not gaining weight.

Barstool Sports “reporter” decided to ponder if a recent photo of mega-babe Rihanna meant that the singer is getting fat. The Internet was not pleased.

While your fuller-figured friends may think your fast metabolism is a blessing, it doesn’t feel that way to you. You’d love to wear a mini and a fitted top without feeling self-conscious about your petite waist or thigh gap, but you haven’t discovered the trick to putting on pounds.

The media may glorify rail-thin actresses and size 0 models, but being underweight, which means a body mass index below 18.5, is a serious matter with distinct health consequences.

While most kids are struggling to keep from gaining weight, a small percentage are struggling to put weight on. However, when it comes to helping your child gain weight, it's important to follow the same general healthy eating guidelines recommended for all children.

The scale can fluctuate at least 3 to 5 pounds on any given day. To gain a full pound of fat, you must consume 3,500 calories more than you burn. You'd be challenged to eat this much, or more, above and beyond what you normally need to maintain your weight in two days.

As soon as you start lifting weights with the goal of building muscle, you're on track to becoming a bodybuilder. But it will take some time before your gains are noticeable. You can expect large gains as a novice but eventually your muscle growth will begin to slow.

Whole foods and weight training help you pack on pounds in a healthy way. These nutritious foods boost your energy levels and provide you with calories and nutrients that support muscle repair and tissue growth.

Most people struggle to lose weight, but some have trouble putting on pounds. To gain weight healthfully, you need to fill your diet with a mix of nutritious fare from all the food groups, focusing on those that are not only rich in nutrients, but in calories as well.

Bulking up isn't something that happens overnight, even if you're genetically lucky and can build muscle fairly easily. If you're new to exercise, however, you'll see some results quickly -- even in a few weeks -- which can keep you motivated for the months or years it requires to bulk up significantly.

You might be tempted to gain weight as quickly as possible to address the issues of being underweight, such as compromised health and energy as well as possible low self-esteem. Increase your body weight slowly to avoid adding excessive body fat, though.

Gaining healthy weight is a gradual process. You may be able to put on as much as 1 pound in a week by consuming more calories, but it'll take time to make significant changes in your appearance. Consume a greater number of calories than you burn to put on the weight.