What it is
Iron sits at the center of hemoglobin, carrying oxygen from your lungs to every working muscle. Run low and the symptom set is exactly what you'd predict: fatigue, weak training, poor concentration, feeling cold — iron deficiency is the most common nutrient deficiency worldwide, and it lands hardest on menstruating women, endurance athletes, and plant-based eaters.
How much
References run around 8 mg a day for men and 18 mg for premenopausal women — one of nutrition's biggest sex differences. Iron comes in two forms: heme (animal foods, well absorbed) and non-heme (plants, absorbed at a fraction — which is why plant-based recommendations run higher still). Two practical levers: vitamin C with a meal boosts non-heme absorption; tea and coffee alongside it cut it.
Don't supplement iron on a hunch — excess is genuinely harmful, and a blood test settles the question cheaply.
How well it's tracked
Moderately: a common label line, well covered for fortified foods and meats; some produce data omits it.
Food sources
Red meat and organ meats (heme); lentils, beans, tofu, fortified cereals, pumpkin seeds, and spinach (non-heme — pair with the vitamin C).
In the app
Iron's Insights card with contributors shows where yours comes from, and a custom floor fits anyone working against a diagnosed deficiency.