I’ve discussed the concept of overtraining and “how much exercise is too much?” previously. Another harmful phenomenon among exercisers is undereating, now called “relative energy deficiency in sport (RED-S)”, which I discussed here. It used to be called the female athlete triad, because it appeared to be more common in female athletes, but is now known to affect males as well. A lot of the discussion of RED-S has been about issues like trying to achieve an ideal training weight.
But a recent study  has shown that undereating can occur unintentionally for people undergoing a high training volume. It seems that your appetite cues become unreliable and can lead to undereating, and especially inadequate carbohydrate consumption. This inadequate fueling can exacerbate the symptoms of overtraining.
I think the key takeaway is to not exceed the amount of training that is right for you. Then this unintentional undereating phenomenon won’t kick in. I think the important issue might be those exercising to lose weight quickly. Keeping to the common-sense suggestion from many authors of not trying to lose much more than 1 lb per week will help avoid this.
- Stellingwerff, T, et al, “Overtraining Syndrome (OTS) and Relative Energy Deficiency in Sport (RED-S): Shared Pathways, Symptoms and Complexities”, Sports Med, 2021. link here.