Runner’s World this month (Feb. 2022, available online here) had an excellent article “The Dangerous Lie of the Perfect Running Weight”, which echoes a lot of what was discussed here previously under the topic focus on health, not weight. It profiles four runners of various weights. The first is a young woman who was a competitive runner through college. She was told to stay at a certain weight by her high school coach, who had no training in exercise physiology, because it was her “perfect running weight”. This happened to be her weight when she was 13! So at an age when girls should be going through a major growth period, she was trying not to let her weight go up. Fortunately, later on her better-trained college coach told her there is no perfect running weight and got her to eat more. But the damage was already done, she had gone for years with Relative Energy Deficiency in Sport (RED-S), and now at 38, still suffers from shin problems. To prevent RED-S, you definitely need to take in enough nourishment to support your training (and your growth if you are still growing). RED-S used to be called the “female athlete triad” of symptoms, but it was found that guys suffer from it too so the name has been updated.
But even non-competitive athletes can suffer from not taking in enough calories. This is especially true because people try to lose weight with exercise. The other three people profiled were in this category. All of them did manage, at least for a time, to lose weight through running and restricting calories. But they all ended up feeling better and enjoying their running more when they stopped trying to use it for weight loss, and instead ate enough to fuel their exercise.
The article has good nutritional advice on how to make sure you are getting enough food for the amount of exercise you are doing. I highly recommend it. There was only one thing I wasn’t sure about: it is recommended to eat a “preworkout snack that’s rich in simple carbohydrates (which are easy to digest), 30 to 40 minutes before every run, no matter the distance” [I added the emphasis]. Maybe this is useful for harder workouts, but I think it is good to do easy exercise sessions without eating first. This helps adapt the body to be better at fat burning, discussed here.