鐵吸收的生物化學

basic knowledge

There are two types of absorbable dietary iron: heme and non-heme iron.

  • Heme iron from hemoglobin and myoglobin from animal food sources (meat, seafood, poultry) is the most easily absorbed form (15% to 35%) and accounts for 10% or more of our total absorbed iron.
  • Non-heme iron comes from plants and iron-fortified foods and is poorly absorbed.

Despite its relative abundance in the environment and relatively low human daily iron requirements, iron is often a growth-limiting nutrient in the human diet. Low iron intake is responsible for most anemia in developed countries and nearly half of anemia in non-industrialized countries. One reason for insufficient iron absorption is that upon exposure to oxygen, iron forms highly insoluble oxides that cannot be absorbed in the human gastrointestinal tract. Human intestinal epithelial cells contain apical membrane-bound enzymes whose activity can be regulated and which reduce insoluble iron (Fe3+) to absorbable ferrous iron (Fe2+) ions.

While iron deficiency is a relatively common problem, it's not the only extreme on the iron balance spectrum that must be avoided. Iron overload is particularly harmful to the heart, liver, and endocrine organs. Excess ferrous iron forms free radicals through the Fenton reaction, causing damage to tissues through oxidative reactions with lipids, proteins and nucleic acids. Therefore, where possible, dietary iron absorption and factors affecting bioavailability in the body are tightly controlled.

cellular level

Most dietary iron absorption occurs in the duodenum and proximal jejunum and depends largely on the physical state of the iron atoms. At physiological pH, iron exists in the oxidized ferric (Fe3+) state. To be absorbed, iron must be in the ferrous (Fe2+) state or bound by proteins such as heme. The low pH of gastric acid in the proximal duodenum allows the ferric reductase duodenal cytochrome B (Dcytb) to convert insoluble iron (Fe3+) into absorbable ferrous (Fe2+) ions at the brush border of enterocytes. . Gastric acid production plays a key role in plasma iron homeostasis. Iron absorption is greatly reduced when proton pump inhibitors such as omeprazole are used. Once ferric iron is reduced to ferrous iron in the intestinal lumen, a protein on the apical membrane of the enterocyte called divalent metal cation transporter 1 (DMT1) transports the iron across the apical membrane into the cell. Hypoxia-inducible factor 2 (HIF-2α) upregulates DMT1 and Dcytb levels in the hypoxic environment of the intestinal mucosa.

Certain dietary compounds inhibit or enhance the duodenal pH-dependent iron absorption process.

  • Inhibitors of iron absorption include phytate, a compound found in plant-based diets that exhibits dose-dependent effects on iron absorption. Polyphenols are found in black and herbal teas, coffee, wine, legumes, grains, fruits and vegetables, and have been shown to inhibit iron absorption. Unlike other inhibitors, such as polyphenols and phytates, which only block non-heme iron absorption, calcium inhibits both heme and non-heme iron when initially absorbed by enterocytes. Animal proteins such as casein, whey, egg whites, and plant proteins have been shown to inhibit the body's absorption of iron. Oxalic acid, found in spinach, beets, legumes and nuts, binds and inhibits iron absorption.
  • The enhancer of iron absorption is primarily the effect of vitamin C, which can overcome the effects of all dietary inhibitors when included in a diet high in non-heme iron (usually a diet high in vegetables). Ascorbic acid forms a chelate with ferric iron (Fe3+) in the low pH of the stomach, which persists and remains soluble in the alkaline environment of the duodenum.

molecular level

Once inside enterocytes, iron can be stored as ferritin or transported across the basolateral membrane into the circulation bound to ferroportin.

Ferritin is a hollow, globular protein composed of 24 subunits that enhances the storage and regulation of iron levels in the body. Iron is stored in the interior of the ferritin globules in the Fe3+ state by incorporation into a solid crystalline mineral called ferrihydrite [FeO(OH)]8[FeO(H2PO4)].

The monomer of the ferritin molecule has ferroxidase activity (Fe3+ ↔ Fe2+), which causes the Fe2+ ions to migrate out of the ferrihydrite lattice structure, allowing them to subsequently flow out of the intestinal epithelial cells through ferroportin and cross the basolateral membrane of the intestinal epithelial cells. Enter the loop. The transmembrane protein ferroportin is the only efflux pathway for cellular iron and is almost entirely regulated by hepcidin levels. High levels of iron, inflammatory cytokines, and oxygen lead to increased levels of the peptide hormone hepcidin. Hepcidin binds to ferroportin, causing its internalization and degradation, and effectively shunts cellular iron into ferritin stores and prevents its absorption into the blood. thereby,

If hepcidin levels are low and ferroportin is not downregulated, ferrous iron (Fe2+) can be released from the enterocyte, where it is again oxidized to ferric iron (Fe3+) to bind to transferrin, which is present in Carrier proteins in plasma. Two copper-containing enzymes, ceruloplasmin in plasma and hephaestin on the basolateral membrane of enterocytes, catalyze the oxidation of ferrous iron and subsequently bind to transferrin in plasma. The main function of transferrin is to chelate iron to make it soluble, prevent the formation of reactive oxygen species, and facilitate its transport into the cell.

clinical significance

Enterocyte DMT1 and Dcytb levels are upregulated in the setting of iron deficiency anemia, and mutations in DMT1 have been shown to cause microcytic anemia and hepatic iron overload.

Conditions that degrade the duodenal mucosa that reduce iron absorption include:

  • Celiac disease
  • tropical sprue
  • Crohn's disease
  • duodenal cancer
  • duodenal ulcer
  • familial adenomatous polyposis

Anemia of chronic disease is a normocytic, normocytic anemia characterized by elevated ferritin stores but reduced systemic iron levels. Inflammatory states increase cytokine release (IL-6), which stimulates hepcidin expression in the liver. Hepcidin degradation through ferroportin leads to reduced iron absorption and reduced iron release from macrophages. The iron accumulated in the cells of anemia of chronic disease is stored in the form of ferritin.

Iron deficiency anemia is a hypochromic microcytic anemia caused by bleeding, reduced dietary iron, or reduced iron absorption. Menstruating women of childbearing age need twice the amount of iron as men of the same age. Pregnancy and breastfeeding also significantly increase a woman's iron requirements, helping to make iron deficiency the most common dietary deficiency in the world.

Review

All comments are moderated before being published

HealthyPIG Magazine

View all
Transform Your Home with the Philips Smart 1000i Air Purifier: Allergy Relief Meets Smart Living

Transform Your Home with the Philips Smart 1000i Air Purifier: Allergy Relief Meets Smart Living

In today’s fast-paced world, where indoor air quality often goes unnoticed, the Philips Air Purifier Smart 1000i Series offers a breath of fresh ai...
皮質醇管理:如何控制皮質醇?我們能夠自行管理或調節劑量嗎?

皮質醇管理:如何控制皮質醇?我們能夠自行管理或調節劑量嗎?

皮質醇是一種在壓力反應中發揮重要作用的激素,適量的皮質醇可以幫助我們應對壓力和維持健康。然而,過量或長期的高皮質醇水平可能會對身體帶來負面影響。以下我們將探討如何控制和管理皮質醇,包括自然方法、藥物干預、以及測量皮質醇的方式。 1. 自然方法調節皮質醇 壓力管理技術:研究顯示,冥想、深呼吸...
皮質醇是什麼?它如何影響我們的身體與日常生活?

皮質醇是什麼?它如何影響我們的身體與日常生活?

皮質醇(Cortisol)是一種由腎上腺分泌的激素,通常被稱為「壓力荷爾蒙」。它的主要功能是幫助身體應對壓力情境,並且在多種生理過程中扮演重要角色。皮質醇的釋放受腦部下丘腦-垂體-腎上腺軸(HPA軸)控制,這是一個調節人體反應於壓力的系統。 皮質醇對身體的影響 當我們處於壓力下時,皮質...
為什麼我們在緊張時總是忍不住吃零食?科學解密壓力性飲食行為

為什麼我們在緊張時總是忍不住吃零食?科學解密壓力性飲食行為

當人們感到緊張或壓力時,經常會無意識地吃零食,這種行為主要涉及大腦的多巴胺系統、情緒反應以及身體的生理需求。以下是背後的幾個主要原因: 壓力荷爾蒙的影響:壓力會觸發皮質醇的釋放,這種壓力激素會引發人們對高糖和高脂肪食物的渴望。這些食物能帶來短暫的愉悅感,因為它們能刺激大腦分泌多巴胺,讓人感...
光學治療濕疹 - 全面總結

光學治療濕疹 - 全面總結

簡介 光療使用光波來治療某些皮膚問題。皮膚會暴露於紫外線 (UV) 光下一段設定的時間。光療利用人造的紫外線光源,紫外線也來自陽光。當與一種叫做甲氧補骨脂素的藥物一起使用時,這個程序稱為 PUVA 光療。 紫外線光能夠抑制皮膚中的免疫系統細胞,對於因免疫系統過度反應引起的皮膚問題有幫助。可以使...
什麼是「操縱者」?

什麼是「操縱者」?

操縱者,也可以說成「擅用手段的人」,「心機重的人」。操縱者利用欺騙、影響或者其他形式的心理操控來控制或影響他人,以達到自己的目標。他們的行為通常包含使用隱蔽、間接或偷偷摸摸的手法來獲得他們想要的東西,往往是以犧牲他人為代價。以下是一些常見的特徵和手段: 欺騙: 他們可能會說謊或扭曲事實來誤...
什麼是肌肉抽搐?你需要去看醫生嗎?

什麼是肌肉抽搐?你需要去看醫生嗎?

肌肉抽搐,也稱為肌束顫動,是指身體各部分出現不自主的肌肉收縮。以下是肌肉抽搐的原因、症狀及管理方法的詳細介紹: 肌肉抽搐的原因 壓力和焦慮 高水平的壓力和焦慮會導致肌肉緊張和抽搐。身體對壓力的反應會觸發神經系統,導致肌肉不自主地收縮。 疲勞 過度使用或劇烈運動後的肌肉疲勞會導致肌...
蘋果與牙齒健康:保護牙齒的小技巧

Apples and Dental Health: Tips to Protect Your Teeth

Apples are widely loved for their rich nutritional content and refreshing taste. However, apples' acidic and sugary content may also have an impact...
蘋果籽的毒性:它們真的有毒嗎?

Apple Seed Toxicity: Are They Really Poisonous?

Apple seeds contain cyanogenic glycosides, compounds that break down in the body to produce cyanide, which has raised concerns about the toxicity o...