It happens regularly in each section of High School Health classes that an athletic girl mentions having infrequent or absent periods, and some of them are not able to bring the energy to their sport that they would like. They tap out halfway through practice and have to grit their teeth through the second hour because they are wiped.
We all tend to eat less when it's the same day after day or week after week. In my family, a huge increase in appetite happens around age 10 or 11. And since they eat a large amount of what they like and only basic amounts of what they find less exciting, it seems that more variety helps teens get the nutrition they need. We rotate recipes and repeat winners and try old favorites that might become favorites again. It seems to be a byproduct of so much variety. While me and my siblings filled up on the same 10 meals and 5 snacks for years, my kids routinely eat foods that I rarely or never had eaten growing up. Goat cheese, dried mangoes, seaweed snacks, salami, brie, nuts, pesto.... it's excellent for them and, for better or worse, raises the bar overall for getting them to eat the quantities they need.
When we discuss food and nutrition in health classes, it often becomes clear that these girls aren't eating enough to support a cycle even if they weren't having sports practice several days a week. They might be bored with the choices at home; they don't tolerate some common foods like dairy; they aren't hungry in the morning and their classes run straight through til lunch; their family doesn't snack; they don't like to take the time to make themselves food even when ingredients are there; they are in the process of moving or some other schedule upheaval; the parents rarely buy treats because they want to focus on meals; their work or sports practice schedule makes it challenging to eat dinner. So many reasons, and often several at a time.
I encourage them to talk with their parents about the shopping routines, meal timing, and nutritious snacks, and to find a few easy things that they like and that their parents are happy to stock. Also, there may be a time of day when it's more convenient to prepare food, so they could make yogurt parfaits or sandwiches in the afternoon or evening for the next day, chop vegetables for a few days of snacks, and use a time of day when they are less productive or less scheduled to make the busy parts of the day feel easier.
I've had nutrition consultations with teen girls about more protein for muscle tone, how to eat enough to restock their glycogen after an intense workout, and how to get enough calories for a normal cycle. In every case there were factors (sometimes their own pickiness or indecisiveness!) preventing them from eating enough, and they felt much better when eating more, and more frequently. This is not to say that every active girl who skips breakfast will miss her period - some girls eat an ample lunch and dinner and have no problem going on two meals a day. But most moms I know are not such amazing cooks, and many kids don't have the stomach space, to load up all their calories in two hot meals per day. If that's your family, good for you! My moderately active teen girls, doing non-cardio activities, have needed 2-3 meals per day, plus a snack, to feel good emotionally and physically.
In health class, I encourage athletic girls to eat a solid breakfast and lunch to ensure that their muscles are fully stocked for soccer or swimming, and to eat dried or fresh fruit or a smoothie after practice, before they arrive at home too tired to eat. It's not ideal to get in the habit of making up all their missed calories from the day after 9 pm and starting the cycle of can't-fall-asleep-til-midnight and too-tired-and-not-hungry-for-breakfast again. We also do a reality check with some sample meals the girls provide, so they can see by how much they might be under eating. The numbers can be a real eye opener. They say "Oh wow! I need to eat a lot more to improve my stamina."
Finally, we cover iron intake and its contribution to sports performance through muscle iron and hemoglobin in the blood. Eating more meat, fish, eggs, beans, nuts, seeds, fruits and vegetables, means more muscle strength and better oxygen levels throughout the body. I discourage them from filling up on dairy since that tends to crowd out iron-rich foods and contribute to anemia if their appetite is already not enough for their activity level. And we review that for most people, sufficient carbs are important for sports performance if speed matters; they don't need to agonize over a scoop of sorbet or a package of trail mix as "too much sugar" or "not healthy enough". One girl described how much better she played hockey after eating "junk food" - a big muffin - just before going on the ice. I assured her there's no metabolic surprise here for a lean, muscular girl. Just before an intense practice is exactly the time to enjoy a muffin.
While we want girls to choose nutritious foods and build good habits, it's easy for the pickiness and busyness of teens to combine with perfectionism and lead to active girls not eating enough. If your daughter isn't enjoying her sport, feels that she isn't improving, or has lost her cycle as she increased her activity, it might be time to review how she can eat more to improve her hormones and her athletic experience.