The micronutrients hard-training people are most likely to run low on

Training itself increases demand for a handful of specific vitamins and minerals

Chasing protein and calorie targets can leave micronutrient status as an afterthought, but intense training genuinely raises the requirement for several specific nutrients beyond what a sedentary person needs. Iron is a common one, especially for women and endurance athletes - heavy training increases red blood cell turnover and iron losses through sweat, and low iron directly limits oxygen delivery and endurance performance well before it shows up as diagnosed anemia.

Vitamin D and magnesium are two more frequent gaps. Vitamin D affects muscle function and recovery and is easy to run low on for anyone training indoors or living somewhere with limited sun exposure for much of the year. Magnesium is lost through sweat and plays a role in muscle contraction and relaxation, energy production, and sleep quality - all relevant to someone training hard and trying to recover from it.

None of this means blindly supplementing everything; excess of some of these nutrients carries its own risks. The practical move is getting key markers checked periodically (especially iron and vitamin D) rather than guessing, and prioritizing a varied diet with iron-rich foods, some sun exposure, and magnesium-rich foods like leafy greens, nuts, and whole grains before reaching for a supplement.