Two iron forms, two absorption routes
Heme iron comes bound inside the hemoglobin and myoglobin of animal tissues (meat, poultry, fish) and is taken up largely intact through its own pathway, giving relatively high and stable absorption, often cited around 15 to 35 percent. Non-heme iron is the inorganic form found in plants, eggs, dairy, and fortified foods, and its absorption is far more variable, roughly 2 to 20 percent.
Most iron in a mixed diet, and essentially all iron in a plant-based diet, is non-heme, so the factors that modulate it carry real weight.
What pushes non-heme absorption up or down
Non-heme iron is sensitive to its meal company. Vitamin C and the so-called meat factor enhance uptake, while tea and coffee tannins, calcium, and phytates in whole grains and legumes inhibit it. The body also self-regulates: when iron stores are low, the gut absorbs a larger fraction of what arrives.
This is general educational information, not medical advice. People at higher risk of deficiency, such as menstruating or pregnant individuals, and anyone considering iron supplements should be guided by a clinician, since excess iron can be harmful.