由於血紅素鐵非常有利於人體吸收，而素食中的鐵不利於人體吸收。因此，當人體戒斷血紅素鐵後－吃素後，需要有適應期。
很多人剛吃素後會感到全身沒有勁等其他缺鐵的症狀，因而產生種種疑惑。但其實這是人體對戒斷血紅素鐵(肉食中的鐵)的正常反應。由於血紅素鐵非常有利於人體吸收，而素食中的鐵不利於人體吸收。因此，當人體戒斷血紅素鐵後——吃素後，需要有適應期。
首先，我們需要知道，過量的鐵對人體有很大的毒性，一些研究顯示，血紅素鐵顯著增加心血管死亡風險，及第2型糖尿病和某些癌症風險。因此人體對鐵吸收的管理非常嚴格，當人體監測到體內鐵過量時可在幾小時內做出反應，透過肝臟產生的肽激素 hepcidin 來迅速抑制鐵的吸收。但是，當人體缺鐵時，通常需要3-5天才做出反應，以提高鐵的吸收。這是人體的保護機制，需要確認人體在持續缺鐵時，才會做出反應。顯然，人體寧願冒缺鐵的風險也不願意冒鐵過量的風險。
這提示我們，血紅素鐵只是肉食的營養優勢，並非健康優勢，或健康劣勢。如果營養是為健康服務，而非為了營養而營養(顯而易見)，那麼，肉食中的血紅素鐵其實是肉食的一大缺陷。實際上，我們更應該對鐵過量感到恐懼，而不是對缺鐵感到恐懼
此外，人體對鐵吸收的調整能力很強。例如，當人體鐵的儲存較低時，透過降低勝肽激素 hepcidin 可以使鐵的吸收增加10-20倍[21]。因此，我們對缺鐵也不必過度擔憂，除非你的調節能力有問題。
人體除了透過肽激素 hepcidin 來系統性地調整鐵的吸收以外，還需要調整許多其他生物機制來使鐵的吸收最終適應，包括腸道鐵轉運蛋白、腸道環境等。當人體缺鐵時，例如，剛放棄肉食轉為素食者，視不同的機制可能需要較長的適應期，從幾天(如肽激素 hepcidin)到幾週[11]，十週[33](吸收率初步提升)，到六個月或更長[3](最終穩定優化)。因此，在上述適應期內，可能會有相當多的人出現缺鐵的症狀，需要積極調整飲食結構來應對，部分人可能需要在醫生指導下服用鐵補充劑。
但是，即使在適應期內出現缺鐵症狀，依然不建議盲目服用鐵補充劑。因為鐵補充劑本身會抑制身體對鐵的吸收(原理同戒斷肉食類似)。最好的方法是積極調整飲食結構，讓身體順利度過適應期。除非有很高的貧血風險的，或高強度的運動員等，可以在醫生指導下服用，必要時可以採用間隔服用(如每週2-3次)的策略。
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