If being obese is bad for health, so is being underweight. Having a skinny and lean body does not always mean that you are healthy. It actually makes you more prone to health concerns like osteoporosis, fractures, fertility issues and weak immunity. A person is considered underweight when their body mass index (BMI) is below 18.5. The number is estimated to be less than the body mass needed to sustain optimal health. If you are underweight, you need to gain a balanced amount of muscle mass and subcutaneous fat, which can only be achieved by eating healthy foods and following a healthy lifestyle. Here are some foods that may help to increase muscle mass and boost overall health.