マグネシウムの腸の吸収過程
マグネシウムの腸の吸収過程 2012/12/07
今回は前回記事『予防と治療のマグネシウム』の続編です。
マグネシウムの腸からの吸収・排出機能・構造の説明です。
難しい用語などが出てきますが、それを無視してざっと読んでいただければ、概要は把握できると思います。
マグネシウムがいかに健康な身体維持に必然であるかを多少なりとも理解できると思います。
他方、食事によるマグネシウムの吸収率は30～40パーセント前後と言われており、更にはJTの精製塩やカルシウムを摂りすぎるとマグネシウムはそれに比例し尿中から自動排泄（はいせつ）されますから、どうしても不足がちになるのです。
（前々回記事）
『突然死対策：マグネシウム』 2018/11/28(水)
（前回記事）
『予防と治療のマグネシウム』 2018/12/5(水)
Magnesium in Prevention and Therapy
3. Magnesium and Nutrition
Dietary surveys of people in Europe and in the United States still reveal that intakes of magnesium are lower than the recommended amounts . Epidemiological studies in Europe and North America have shown that people consuming Western-type diets are low in magnesium content, i.e. <30%–50% of the RDA for magnesium.
It is suggested that the dietary intakes of magnesium in the United States have been declining over the last 100 years from about 500 mg/day to 175–225 mg/day. This is likely a result of the increasing use of fertilizers and processed foods . In 1997, the Food and Nutrition Board (FNB) of the Institute of Medicine had increased the dietary references intakes (RDA) for magnesium, based on the results of controlled balance studies.
米国におけるマグネシウムの食物摂取量は、過去100年間で約500mg /日から175-225mg /日に減少していることが示唆されています。
これは、化学肥料や加工食品の使用が増加したことによる可能性が高いことです。 1997年、医学研究所の食物栄養委員会（FNB）は、管理されたバランス研究の結果に基づいて、マグネシウムの1日あたりの推奨摂取量（RDA）を増加させました。
The new RDA ranges from 80 mg/day for children 1–3 year of age to 130 mg/day for children 4–8 year of age. For older males, the RDA for magnesium ranges from as low as 240 mg/day (range, 9–13 year of age) and increases to 420 mg/day for males 31–70 year of age and older. For females, the RDA for magnesium ranges from 240 mg/day (9–13 year of age) to 360 mg/day for females 14–18 year of age. The RDA for females 31–70 year of age and older is 320 mg/day .
新しい1日あたりの推奨摂取量（RDA）は、1〜3歳の子供の80mg /日から4〜8歳の子供の130mg /日の範囲です。 高齢の男性の場合、マグネシウムのRDAは240mg /日（範囲は9-13歳）の範囲で、31-70歳の男性の場合は420mg /日に増加します。
女性の場合、マグネシウムの1日あたりの推奨摂取量（RDA）は、14〜18歳の女性の場合、240mg /日（9〜13歳）から360mg /日の範囲です。 31-70歳の女性の1日あたりの推奨摂取量（RDA）は320mg /日です。
Water accounts for ~10% of daily magnesium intake , chlorophyll (and thus green vegetables such as spinach) is the major source of magnesium. Nuts, seeds and unprocessed cereals are also rich in magnesium. Legumes, fruit, fish and meat have an intermediate magnesium concentration. Some types of food processing, such as refining grains in ways that remove the nutrient-rich germ and bran, lower magnesium content substantially. Low magnesium concentrations are found in dairy products, except milk .
The United States NHANES 2005–2006 survey reported that nearly one half of all American adults have an inadequate intake from food and water of magnesium and do not consume the estimated average requirements (EAR) (set at 255–350 mg depending on gender and age group) . A chronic magnesium deficiency (serum magnesium <0.75 mmol/L) is associated with an increased risk of numerous preclinical and clinical outcomes, including atherosclerosis, hypertension, cardiac arrhythmias, stroke, alterations in lipid metabolism, insulin resistance, metabolic syndrome, type 2 diabetes mellitus, osteoporosis as well as depression and other neuropsychiatric disorders. Furthermore, magnesium deficiency may be at least one of the pathophysiological links that may help to explain the interactions between inflammation and oxidative stress with the aging process and many age-related diseases .
米国の国民健康栄養調査（NHANES）2005-2006の調査によれば、全米の成人のほぼ半数が食物とマグネシウムの摂取量が不十分で、推定平均所要量（EAR）を消費していないと報告されています（性別および年齢によって255-350 mgに設定されている グループ）。
慢性マグネシウム欠乏（血清マグネシウム<0.75ミリモル/ L）は、アテローム性動脈硬化症、高血圧、心臓不整脈、脳卒中、脂質代謝の変化、インスリン抵抗性、メタボリックシンドローム、２型糖尿病を含む多くの前臨床および臨床転帰 骨粗しょう症、ならびにうつ病および他の神経精神障害が挙げられます。
さらに、マグネシウム欠乏症は、老化過程および多くの加齢性疾患と炎症と酸化ストレスとの間の相互作用を説明するのに役立つ病態生理学的リンクの少なくとも1つであり得る。
4. Magnesium Absorption and Excretion
Magnesium homeostasis is maintained by the intestine, the bone and the kidneys. Magnesium is mainly absorbed in the small intestine, which was shown by 28Mg isotope measurements, although some is also taken up via the large intestine . Of the total dietary magnesium consumed, only about 24%–76% is absorbed in the gut the rest is eliminated in the faeces .
4.マグネシウム吸収および排泄
マグネシウムの一定の状態に保ち続けようとする恒常性は、腸、骨および腎臓によって維持されます。
消費される食事中のマグネシウムの総量のうち、約24％〜76％のみが腸に吸収され、残りは糞便中で排泄されます。
The majority of magnesium is absorbed in the small intestine by a passive paracellular mechanism, which is driven by an electrochemical gradient and solvent drag (see Figure 1). Paracellular magnesium absorption is responsible for 80%–90% of intestinal magnesium uptake.
（註１）傍細胞（ぼうさいぼう）
壁細胞（へきさいぼう）ともいう。胃の内壁に存在する外分泌細胞で塩酸や内因子を分泌する。
A minor, yet important, regulatory fraction of magnesium is transported via the transcellular transporter transient receptor potential channel melastatin member TRPM 6 and TRPM 7—members of the long transient receptor potential channel family—which also play an important role in intestinal calcium absorption .
軽度ではあるが、マグネシウムの調節のほんの一部は、腸内のカルシウム吸収に重要な役割を果たします。（説明ヶ所省略）
It is worth noting that intestinal absorption is not directly proportional to magnesium intake but is dependent mainly on magnesium status. The lower the magnesium level, the more of the mineral is absorbed in the gut, thus relative magnesium absorption is high when intake is low and vice versa. The kidneys are crucial in magnesium homeostasis as serum magnesium concentration is primarily controlled by its excretion in urine.
Under physiological conditions, ~2400 mg of magnesium in plasma is filtered by the glomeruli. Of the filtered load, ~2300 mg is immediately reabsorbed and only 3%–5% is excreted in the urine, i.e., ~100 mg . Only little magnesium is reabsorbed in the proximal tubule.Most of the filtered magnesium is reabsorbed in the loop of Henle, mostly in the thick ascending limb (up to 70% of total magnesium reabsorption). The reabsorption and excretion of magnesium is influenced by several not yet classified mechanisms.
生理学的条件下では、血漿中の約2400mgのマグネシウムが糸球体によって濾過されます。 ろ過された負荷のうち、〜2300mgは直ちに再吸収され、3％〜5％のみが尿中に排出され、〜100mgとなります。 近位尿細管ではわずかなマグネシウムしか再吸収されません。
Most of the filtered magnesium is reabsorbed in the loop of Henle, mostly in the thick ascending limb (up to 70% of total magnesium reabsorption). The reabsorption and excretion of magnesium is influenced by several not yet classified mechanisms.
（註２）ヘンレ係蹄上行脚（ヘンレけいていじょうこうきゃく）
フィルターのバックアップとして機能し、髄質部の濃度を減少させる部分。水分は透過せず、イオンのみが透過する。
In this context, we could show that an overload of blood cells with magnesium in renal insufficiency can be avoided by a special cell membrane buffering system for magnesium. In severe forms of renal insufficiency, this buffering system for magnesium is destroyed and an overload with magnesium in human cells is observed . Furthermore, the exchange time for magnesium between intra- and extracellular pools is relatively long . Hypomagnesaemia is frequently linked with hypokalemia owing to disturbances in renal secretion of potassium in the connecting tubule and collecting duct .
Magnesium absorption and excretion is influenced by different hormones. It has been shown that 1,25-dihydroxyvitamin D [1,25(OH)2D] can stimulate intestinal magnesium absorption.On the other hand, magnesium is a cofactor that is required for the binding of vitamin D to its transport protein, vitamin D binding protein (VDBP). Moreover, conversion of vitamin D by hepatic 25-hydroxlation and renal 1α-hydroxylation into the active, hormonal form 1,25(OH)2D is magnesium-dependent. Magnesium deficiency, which leads to reduced 1,25(OH)2D and impaired parathyroid hormone response, has been implicated in “magnesium-dependent vitamin-D-resistant rickets”. Magnesium supplementation substantially reversed the resistance to vitamin D treatment .
Next to 1,25(OH)2D, several other factors, such as oestrogen or parathyroid hormone (PTH), are involved in the magnesium excretion. Oestrogen is known to stimulate TRPM6 expression . Thus, oestrogen substitution therapy can normalize hypermagnesuria, which occurs frequently in postmenopausal women. Interestingly, TRPM6 expression appears to be regulated by serum magnesium levels and oestrogens, but not by 1,25(OH)2D or PTH action .
Of special importance is PTH. Absorption of both magnesium and calcium appears to be inter-related, with concomitant deficiencies of both ions well described.For example, the stimulation of PTH secretion in response to hypocalcemia acts to restore the serum calcium concentration to normal. Hypomagnesemia impairs hypocalcemic-induced PTH release, which is corrected within in minutes after infusion of magnesium. The rapidity of correction of PTH concentrations suggests that the mechanism of action of magnesium is enhanced release of PTH. Magnesium is also required for the sensitivity of the target tissues to PTH.
Calciotrophic hormones, such as PTH, have profound effects on magnesium homeostasis. PTH release enhances magnesium reabsorption in the kidney, absorption in the gut and release from the bone . PTH influences magnesium absorption, however, hypercalcemia antagonizes this effect. In this context, different findings have often been described in primary hyperparathyroidism. Also in Addison’s disease as well as in spironolactone treated patients, magnesium excretion is slightly decreased .
（註４）アジソン病
In recent years, gene-linkage studies in families with hypomagnesemia have been performed. Some of these diseases are familial hypomagnesemia with hypercalciuria and nephrocalcinosis.
つづく
（注）以上は機械語訳による。
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糖尿病ネットワーク
マグネシウムを多く含む食品
栄養＆カロリー計算（マグネシウム）
生化学 第８５巻 第７号 P５７４～
腎尿細管におけるマグネシウム輸送の分子制御
