Iron carries oxygen. When it's low, everything feels heavier: workouts, stairs, focus, mood. Women lose iron monthly, and anyone eating mostly plant-based, training hard, or eating a lot less than they used to is at higher risk.
There are two forms. Heme iron — red meat, poultry, fish — absorbs easily. Non-heme iron — beans, lentils, tofu, spinach, fortified grains — absorbs much less well on its own, but vitamin C alongside it helps a lot. Squeeze lemon on the lentils; have fruit with the fortified cereal.
Coffee and tea with a meal cut iron absorption noticeably. If you're low, move the coffee an hour away from your iron-heavy meals rather than giving it up.
Persistent fatigue is worth a blood test, not a guess. Iron supplements are genuinely helpful when you're deficient and unhelpful — sometimes harmful — when you're not, so get numbers before you supplement.