Yes, it reduces strength and energy when you don’t have enough nutrients to support your body. I’m actually underweight myself, and I feel tired from simple activities like doing my homework or jogging. Your body eventually turns on itself, and tries using your fat and muscles as energy so you can live. That is why a lot of people are malnourished, especially the ones with their ribs sticking out of their skin. You must eat healthy and exercise daily to improve muscle and fat tissue. Some people will tell you to eat more, but don’t listen to them. Eating more will just result in more calories, and you have to burn them or else you will become overweight . And if asking me if being overweight is bad, then yes it is. The perfect size is the right weight in your age group, and feeling refreshed and strong in your daily life.