You don't have to have an eating disorder to be an under-eater. I just posted those links as an example to show what is common knowledge that women under-eat far more than men do. Most weightloss advice says to calorie-restrict. Busy people can easily under-eat accidentally. Some people just don't realize they are under-eating.
You only need more fat and protein when you're not eating enough food in general, but then you also need more of the third macronutrient; carbs! I think people who under-eat particularly crave fats and high-protein foods, like meat, because pro-longed under-eating causes your body to start craving the most calorie-dense foods you once used to eat. Fat has 9 calories per gram. Protein and carbs both have 4 calories per gram, so you can see why high-protein foods, like meat, are craved for and recommended a lot of the times because it's one of the most calorie-dense foods from being so high in both protein and fat.
The best thing you can do for yourself is start tracking what your average calorie-intake is to make sure you're not under-eating for what's recommended for your age and gender. If you are, then that's probably what's causing your symptoms some people mistakenly refer to as "AF".