鎂 (Magnesium)

鎂在人體內的含量僅次於鈣﹑磷﹑鈉，是體內礦物質含量第四豐富的陽離子。

一般在正常的生理狀況下，不易產生鎂的缺乏，主要是因鎂的吸收率與攝取量成反比，鎂之攝取量低時 (＜24 mg/d)，其吸收率可高達75 %，而在高鎂攝取時( 600 m g/d)，其吸收率會降低至25 %。

鎂是人體中所有合成過程（biosynthesis process）、葡萄糖分解、形成cyclic-AMP和傳遞遺傳物質的必需因子。

臨床上，低血鎂或鎂缺乏的情況常見於酗酒者﹑營養不良﹑腸胃道或腎臟疾病的患者。

理化性質

鎂是所有活細胞所需，原子量為 24.3，是典型二價金屬，具有金屬的共同特性。鎂與一價陰離子形成的碳酸鹽和鹵化物易溶解；但與二價陰離子形成的硫酸鹽、硝酸鹽、磷酸鹽與草酸鹽則不易溶解。鎂最重要的聚合物為葉綠素，普遍存在綠葉蔬果中，因此綠葉蔬菜為鎂的重要食物來源。

建議食物： 發芽糙米、小麥草、綠花椰苗、黃豆芽、綠豆芽(苗)、紅豆芽(苗)。

代謝

(一)吸收

食物中的鎂在整個腸道均可吸收，但主要是在空腸末端與迴腸吸收，鎂在腸道的吸收機轉，尚不清楚，推測可能與細胞膜、離子間的運輸有關。

飲食中鎂之吸收率約為 40–60 %，但受膳食中鎂含量所影響，攝入多則吸收少，反之亦然；而胺基酸、乳糖等會促進鎂的吸收；過多的磷、草酸、植酸和膳食纖維等則會降低鎂吸收。

鎂、鈣在腸道中會有競爭性的抑制作用；大多數之研究認為鈣對鎂的吸收無影響。但每天鈣攝取量若超過 2600 mg，則會降低鎂的吸收。

(二) 分佈

人體含鎂量共約 25 g ，其中約50 – 60 %存於骨骼中，其餘約39 %分布在肌肉及軟體組織，僅1%在細胞外液。

人體正常紅血球細胞中鎂約 2.3 - 3.1 m mol/L，而血清鎂約0.75 - 0.95 m mole/L (1.8 - 2.3 m g/dl)。但是，除非發生很嚴重的缺乏，血清中鎂的濃度並不會下降，而且，紅血球中鎂的濃度也會維持在同樣的基準下。血鎂能維持衡定之機制迄今未明；可能藉由腸道之吸收、腎臟之排泄、膜上陽離子的流通及荷爾蒙的調節，來維持恆定。

在血漿中大約 30 %的鎂會與蛋白質結合，其餘的鎂則分為離子態鎂(55 %)和非離子態鎂(15 %)，並與碳酸鹽、檸檬酸鹽、磷酸鹽和硫酸鹽結合。

(三) 排泄

健康成年人從食物攝取的鎂約有 60–70 %隨糞便排出，有些會由流汗和皮膚脫落流失，其餘由尿中排出。

腎臟是維持鎂體內恆定及排泄的主要器官，過濾的鎂 65 %在腎亨利氏環的上升支（ascending limb of Henle loop）和20 - 30 %近側彎曲小管（proximal tubule）重吸收。當飲食中鎂的攝食量很低時，腎臟對鎂再吸收的能力增加，以維持體內鎂平衡，一般正常的飲食狀態下，每日尿鎂的排泄量約在 2 - 5 m mol/L左右。除了飲食因子外，尚有一些會影響尿鎂的排泄，如：大量的酒精、葡萄糖攝取，利尿劑使用等皆會促進尿鎂的流失；反之，當鎂缺乏，低鎂飲食攝取及副甲狀腺素濃度過高時，則會促進鎂之再吸收。血清鎂濃度降低時，副甲狀腺分泌副甲狀腺素（PTH），增加腎臟對鎂的重吸收；血清鎂增加時，腎臟過濾率會增加。除此外升糖激素（glucagon），抑鈣素（calcitonin）,和抗利尿激素（ADH）皆與副甲狀腺素有相似有類似功能，但其機轉尚不清楚。

生理功能及缺乏症

(一) 生理功能

鎂為維持骨骼之結構及功能重要元素，鎂可影響骨吸收，使骨骼生長與維持。

鎂做為體內超過 300種以上酵素作用之輔因子，其間接以Mg-ATP聚合體或直接活化激素參與有氧或無氧代謝途徑，產生能量及糖解作用。而粒線體酵素利用鎂螯合ATP和ADP做轉磷反應（Mg DRI）。

鎂可做為腺甘酸環化 ? 的激活劑，而許多荷爾蒙，神經傳導物質及細胞效應物（ effectors），則藉由腺甘酸環化?系統控制細胞活動。

鎂可維持適當嘌呤與嘧啶供應細胞分化所需的 DNA與RNA。鎂亦參與蛋白質合成作用，對細胞之分裂非常重要 (6) 。

鎂激活 Na + -K + -ATPase之活性，維持細胞內外鈉、鉀之平衡。鎂控制心肌細胞內鉀往外移出作用，維持細胞內鉀恆定，避免產生心率不整。

鎂能調節鈣之恆定，維持神經、肌肉之正常功能。當鎂缺乏時，細胞內的鈣會上升。由於鈣濃度會影響骨骼肌與平滑肌的收縮，鎂缺乏時會造成肌肉痙癵，高血壓，冠狀及腦血管痙攣。

(二) 缺乏症

一般正常狀況下，不易產生鎂的缺乏，但多發生於酗酒者、營養不良者、腎病患者、胃腸及病患者及使用利尿劑導致尿鎂流失增加者。

研究亦顯示，慢性鎂缺乏會造成：（1）細胞內鉀流失、鈣增加。（2）肌漿網無法釋放鈣。（3）細胞膜上Ca-ATPase酵素活性失全，使細胞無法正常將鈣移出細胞。（4）鈣流入粒線體內使其結構受損功能失全。鎂缺乏症狀包括：肌肉顫抖、神經過敏、共濟失調，嚴重時會出現譫妄（ delirium）、神經錯亂、定向力失常等。因血鎂濃度降低時，會影響血鈣及血鉀濃度，因此部分鎂缺乏症狀，可能是因為其他礦物質濃度改變所引起。

除此外，研究顯示，低鎂攝取與許多慢性疾病有關：

1．鎂與骨質疏鬆

研究顯示補充鎂可改善停經後婦女骨質疏鬆之情況，會增加骨質密度及降低骨折之機率，推測鎂與骨質疏鬆有相關。

2．鎂與心血管疾病

流行病學資料顯示，血鎂濃度正常者，其動脈粥狀硬化症之危險性較低，補充鎂可改善血脂濃度，飲用硬水地區之居民，因為鈣、鎂含量較高，有較低心血管疾病之死亡率。同時，研究發現飲食之鎂含量與血壓呈負相關。

3．鎂與糖尿病

研究顯示胰島素阻抗與鎂攝入不足有關。流病資料顯示 25﹪的糖尿病患者有低血鎂之情形。鎂的補充可改善非胰島素糖尿病患者之葡萄糖耐量。

 鎂能幫助脂肪燃燒並產生能量，這點對男女同樣重要。如果您老是覺得疲勞，而且體重逐漸增加，可能是缺鎂的徵兆。

停經後的婦女特別容易缺鎂，因此常發生血液不當凝結，導致心臟病及中風。體內鎂含量不足除了容易增加女性心臟病的罹患率之外，還會引起另外一個問題，那就是骨質疏鬆症。

孕婦如果缺鎂，易得毒血症而導致早產（毒血症是一種對母體和胎兒都有害的高血壓）。缺鎂也是引起經前症候群的因素之一。

鎂會轉化成一種酵素，有助於維生素B1、B2、B6的運用，因此缺鎂可能會引起缺乏維生素B的症狀，通常是痙攣。

Although magnesium may not be at the top of the list of minerals that first come to your mind as necessary for good health, it is in fact very important to your body and is required to maintain wellness. Magnesium is actually the fourth most abundant mineral in your body, which demonstrates how much your body depends on its presence. More than half of the magnesium found in your body is located in your bones, and the remaining quantity is primarily located in cells of tissues, muscles, organs, and bodily fluids. Like other minerals found in your body, magnesium cannot be produced within the body and therefore must be received through outside sources, such as diet and supplements.
 

You can find magnesium in spinach, broccoli and certain other foods, but the majority of Americans do not get enough of this healthy mineral from diet alone.
 

There are multiple benefits to keeping a healthy level of magnesium present in your body, including effective temperature regulation, energy production, transmission of nerve impulses, and the healthy formation of bones and teeth. Like other important vitamins and minerals, magnesium is essential for good health.
 
Why is Magnesium So Important?
 
Magnesium is not just responsible for a few of your bodily functions. Rather, it is necessary for more than 300 biochemical reactions in all different parts of your body. Magnesium contains an enzyme that is required to help trigger chemical reactions within bodily functions, which helps in metabolizing proteins, fats, and carbohydrates, along with assisting in the proper function of genes. Another important reason to have healthy levels of magnesium present is due to muscle retention. There are certain fuels that your muscles store in their cells, but without an available supply of magnesium, this storage is not possible.
 
Magnesium helps to relax your nerves and muscles, build and strengthen your bones, as well as assists with the healthy, smooth circulation of blood flow. Since magnesium is associated with so many diverse actions in your body, having a magnesium deficiency can have a negative impact. So many organs and systems in your body require magnesium in order to function properly including your:
 •
Cardiovascular system

•
Nervous system

•
Kidneys

•
Digestive system

•
Hormone-secreting glands

•
Brain

 
As mentioned, your muscles are also affected by magnesium. Your liver is also a vital organ that requires magnesium to metabolize. With all of these vital organs and systems requiring magnesium to operate, maintaining a healthy balance of the magnesium mineral is essential to proper growth and overall health.
 
How to Know if You Need More Magnesium
 
Since it is so important to keep healthy levels of magnesium in your system, knowing the symptoms of a magnesium deficiency is imperative. As there are multiple systems affected, the symptoms for a magnesium deficiency are just as diverse. Some of the symptoms include:
 •
Muscle weakness, cramps, twitches

•
Lack of appetite

•
Insomnia

•
Weakening of bone structures

•
Imbalanced blood sugar levels

•
Seizures

•
Elevated fats in the bloodstream

 
An additional magnesium deficiency “clue” for some people can be the "difficulty in phasing out background noise."
 
Magnesium deficiencies can also have very negative effects on your heart muscle. The resulting symptoms associated with your heart include:
 •
Arrhythmia

•
Irregular contraction

•
Increased heart rate

 
If you are experiencing some of these symptoms, you may want to consult your healthcare practitioner for information on whether or not a magnesium deficiency may be the cause.
 
How to Increase Your Magnesium Intake
 
There are different ways that you can increase the amount of magnesium that you consume. As stated earlier, your body cannot naturally produce magnesium because it is a mineral; therefore, you must receive it from outside sources. There are certain foods that are known to be high in magnesium levels, which is one way that you can be certain you are consuming the needed mineral. Some of these foods include:
 •
Spinach

•
Cooked turnip greens

•
Steamed broccoli

•
Flax seeds

•
Raw celery

•
Ripe tomato

•
Almonds and cashew nuts

 
There are many more foods that contain high levels of magnesium, and you can determine which ones work best for you by discussing your magnesium intake with your healthcare provider.
 
However, keep in mind that the majority of Americans are not getting enough magnesium in their daily diets. As many as 68 percent of Americans do not consume the daily recommended amount of magnesium, according to a government study, and 19 percent do not consume even half of it.
 
Even the National Institutes of Health Office of Dietary Supplements (ODS) points out that:
 
"For many people, dietary intake may not be high enough to promote an optimal magnesium status, which may be protective against disorders such as cardiovascular disease and immune dysfunction."
 
With that in mind, another way to increase your intake of magnesium is through supplementation. You can purchase magnesium as a dietary supplement in two different forms. One form is chelated, and the other is non-chelated. Chelated magnesium means that the magnesium molecule is joined with another molecule, which is normally a form of amino acids, which serve as foundations for proteins. Names of these supplements that are widely used include:
 •
Magnesium glycinate

•
Magnesium aspartate

•
Magnesium taurate

 
The other form of magnesium that you can purchase, the non-chelated form, involves the magnesium being attached to an organic acid or a fatty acid. These forms include magnesium sulfate, magnesium oxide, and magnesium carbonate.
 
For determining which form of magnesium would best suit your bodily needs, discuss these options with your healthcare practitioner.
 
Ask your health care practitioner help you understand the differences in the magnesium supplements available, as well as recommend the option most suited for your needs.
 
Your Medications Could Lower Your Magnesium Levels
 

Certain medications, including diuretics, antibiotics and birth control pills, may impact your magnesium levels.
 

There are many medicines that affect your levels of magnesium as well. Some of these medicines include thiazide diuretics and loop diuretics that are used to lower blood pressure, and antibiotics, including neomycin, tetracycline, erythromycin, sulfamathoxazole, and sulfonamides. Other medications that can affect magnesium levels are birth control pills, warfarin, and cyclosporine. If you take prescription medication, ask your healthcare provider or pharmacist if they could be causing a deficiency of magnesium in your body, along with the associated side effects.
 
Numerous Health Conditions Associated with Magnesium
 
There is a long list of health conditions that can be improved with proper magnesium levels. Some of the conditions that magnesium can help prevent include:
 •
Heart attack

•
Alcoholism

•
Autism

•
Congestive heart failure

•
Asthma

•
Diabetes

 
Other negative health conditions associated with a lack of magnesium include but are not limited to coronary heart disease, glaucoma, epilepsy, hypertension, inflammatory bowel disease, migraines, and multiple sclerosis.
 
One recent study published in Magnesium Research even found that, in magnesium-deficient people, taking a magnesium supplement not only increased magnesium levels but also reduced levels of C- reactive protein (CRP). CRP is a special type of protein produced by your liver. When your body experiences systemic inflammation, levels of this protein go up. Many researchers and doctors now believe that CRP may be as important as -- or more important than -- cholesterol levels in determining risk of heart disease. The researchers noted:
 
"The findings show that many individuals have a low magnesium status associated with increased chronic inflammatory stress that could be alleviated by increased magnesium intake."
 
Current Issues and Misconceptions about Magnesium: Does it Help Your Heart, Blood Pressure and Diabetes?
 
Magnesium and its relation to potassium and blood pressure has been a debated issue within the medical field; however, there have been studies that have shown the positive relationship between healthy magnesium levels and healthy blood pressure levels. Green vegetables and foods high in magnesium are also typically high in levels of potassium and calcium, and are low in sodium and fat. Because of this, it can be difficult at times to evaluate the independent effect of magnesium on blood pressure and other health conditions because it can simply be hard to eliminate other nutritional factors.
 
However, new scientific evidence by Dietary Approaches to Stop Hypertension (DASH) is strong in proving that diets containing plentiful amounts of magnesium are very effective in individuals working to modify their hypertension. This group also suggests that even people with “prehypertension” who want to avoid developing full-blown high blood pressure should also make sure they eat a magnesium-rich diet.
 
Magnesium also decreases the chance of developing diabetes in older women. The Iowa Women’s Health Study followed a group of women beginning in 1986, and the study determined that the risk is lowered based solely on the dietary intake of whole grains, dietary fiber, and magnesium. Also, several studies have tested the option of using supplemental magnesium for controlling type 2 diabetes. This study concluded that the participants who received the magnesium supplement had higher blood levels of magnesium and had improved control of their diabetes.
 
Another common topic of discussion around magnesium deals with its relation to cardiovascular disease. Some studies have shown that having higher blood levels of magnesium can lower your risk of having coronary heart disease as well as reduce your risk of having a stroke. These studies recommend consuming healthy amounts of magnesium to benefit your entire cardiovascular system. These findings are also leading to new research on how magnesium levels may be associated with cardiovascular disease.
 
Consider This...
 
Magnesium has been proven to affect multiple important bodily functions, and your risk for certain unhealthy conditions could be significantly lowered by maintaining healthy levels of this pertinent mineral. Whether you simply want to relieve muscle aches, PMS symptoms, digestive irregularities, or address more serious issues such as diabetes and hypertension control, your healthcare professional can give you more information on how you can achieve healthy levels of the magnesium mineral through both dietary and supplement options.
 
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Sources
 
National Institutes of Health, Office of Dietary Supplements: Magnesium
 
World’s Healthiest Foods: Magnesium
 
Magnesium Research December 2010 Dec: 158- 68.
 
NewsMax.com January 20, 2011