（OMNS，2009年12月8日）維他命C通常大量攝取，以改善健康，預防氣喘，過敏，病毒感染與心臟病[1,2]。無毒，無免疫原性，不會像阿司匹靈這類藥物會刺激胃部。然而，維他命C（L-抗壞血酸）是酸性的。所以會有一個常見的問題是，大量攝取的影響是什麼？
抗壞血酸是一種弱酸（pKa = 4.2）[3]，只比醋再略酸一點。當溶於水時，維他命C是酸性的，但比檸檬/萊姆這類的檸檬酸弱。大量的弱酸，如抗壞血酸鈉，吃進身體裡會導致問題嗎？ 答案是，有時在某些情況下，會。但是，經由一些簡單的預防措施，是可以避免的。
口中的酸
首先，任何酸都會侵蝕牙齒的表面。這是牙醫清潔你的牙齒警告會有牙菌斑的原因，因為口腔裡細菌產生的酸會侵蝕牙齒造成蛀牙。可樂等軟性飲料含有磷酸，實際上牙醫在塗抹牙齒密封劑之前會以它來蝕刻牙齒。像軟性飲料一樣，如果只是短暫停留，抗壞血酸不會造成牙齒侵蝕。通常，維他命C錠表面會有硬脂酸鎂的製錠成分，可防止抗壞血酸鹽立即溶解。服用維他命C錠劑不經過咀嚼這種可以防止酸性對牙齒琺瑯質所造成的傷害。
維他命C嚼片
嚼片很受歡迎，因為味道甜甜的，可以讓孩子喜歡服用維他命C [4]。但是有些維他命C嚼片含糖以及抗壞血酸，當在咀嚼時，可能粘在牙齒的縫隙。所以，有個建議是咀嚼完維他命C劑片後，漱口或刷牙。但最好的方法是專門選擇非酸性維他命C嚼片，在商店裡很容易找得到。仔細看一下標籤，確認嚼片完全由非酸性維他命C做成的。
胃酸
胃部敏感的人，當有急性病毒感染，要服用大劑量的維他命C，（每20分鐘1,000-3,000毫克或更多）時。在這種情況下，胃中的抗壞血酸會累積到足夠的酸度導致胃灼熱或類似的反應。
另一方面，很多人一小時服用20,000毫克的劑量也沒問題。胃裡的酸，鹽酸（HCl）非常強，酸性比維他命C高出幾十倍。
當人們吞下大量的抗壞血酸鈉時，消化道會盡快將其吸入到血液中，但也可能花上一段時間。有些人會感覺到吃下去的抗壞血酸錠就“坐在”胃的底部，需要時間來溶解。這個問題很容易解決就是使用緩衝的抗壞血酸鈉，或服用抗壞血酸跟著食物或零食一起服用。當大量攝取維他命C的量超過腸道所能吸收，抗壞血酸鹽會把水吸進腸道，產生便意的作用。這種飽和的攝取稱為腸道耐受性。當發生這種情況時，應該減少用量（20-50％）[1]。
體內酸性平衡
服用大量的酸，甚至像抗壞血酸鈉這樣的弱酸，是否會影響到身體的酸平衡（pH）而造成健康問題呢？不會，因為身體會主動不斷地控制血液的酸鹼值。 腎臟會調節身體的酸鹼值，經過一段長時間，數小時到幾天的時間。經由選擇性排泄出去，不管是裏頭的酸或基本的物質。較短的時間內，數分鐘到數小時，如果血液過酸，自主神經系統會增加呼吸速率，會從血液中移除掉更多的二氧化碳，降低它的酸度。
有些食物會間接導致酸性。例如，當吃了過多的蛋白質，超過要維持與生長所需，會被代謝成酸，必須被腎臟移走，通常是尿酸。這種情況下，鈣和/或鎂會隨著尿液中的酸一起排出，會消耗掉鈣和鎂的供應[6]。不過，因為抗壞血酸是一種弱酸，在它真正影響身體的酸性之前，我們還可忍受。
儘管對維他命C會導致腎結石也些指控，但沒有證據直指這一點，並且其酸度和利尿作用實際上反而傾向會減少腎結石[1,7]。抗壞血酸會溶解磷酸鈣結石並溶解鳥糞石。除此之外，雖然維他命C會增加草酸鹽的排泄，而維他命C同時會抑制鈣和草酸鹽的結合。[1,2]。
維他命C的形式
抗壞血酸鈉有多種形式，每一種都有特定的優點。抗壞血酸是最便宜的，有錠劑長效釋放的錠劑或粉末。較大的錠劑（1000-1500mg）方便且相對便宜。長效釋放片定含有長鏈碳水化合物，會延遲抗壞血酸鈉在胃裡的融解，隨著數小時長效釋放。這有助於在血流裏頭保持高濃度。抗壞血酸的粉末或晶體可以用原料購買相對比較便宜(不確定台灣是否有，只做參考)。純粉末比錠劑更快溶解，更快被身體所吸收。Linus Pauling喜歡服用純抗壞血酸，因為完全不含製錠的其他原料。
緩衝型的抗壞血酸鈉
一小部分一茶匙的碳酸氫鈉（小蘇打粉），長期以來被用來當成安全有效的抗酸劑，可立即降低胃酸。當碳酸氫鈉加入抗壞血酸中時，碳酸氫鈉會發出嘶嘶聲（釋放出二氧化碳），然後釋放鈉以中和抗壞血酸的酸度。
抗壞血酸鈣有粉末劑型，而且容易溶於水或果汁中。在這種緩衝劑型裡，抗壞血酸對於口腔和胃反應敏感的人完全安全，可直接用在牙齦幫助治療感染[8]。 比同等的抗壞血酸和碳酸氫鈉貴一點但更方便。抗壞血酸鈣具有非酸性的優點。有點輕微的金屬味，並具有收斂性，但不像抗壞血酸那麼酸。1000毫克抗壞血酸鈣含有約110毫克的鈣。
其他形式的緩衝型抗壞血酸鈣包括抗壞血酸鈉和抗壞血酸鎂[9]。大多數成年人每天需要800-1200毫克的鈣和400-600毫克的鎂[6]。所有這些緩衝型抗壞血酸鈉瓶裝上的標籤詳細說明了一茶匙中含有多少的『元素』礦物質。它們比抗壞血酸稍貴一點。
長效釋放形的抗壞血酸鈉，通常比抗壞血酸，在更高的劑量下，有更好地耐受性；但在有效預防急性感冒症狀上，不是那麼有效。這可能是因為它們被吸收後，需要從體內吸收電子才能變得像天然的抗壞血酸鈉一樣有效[1]。有些劑型的維他命C聲稱是專利配方，比標準的維他命C更有效[9]。
酯化維他命C
最近有一種革命性的抗壞血酸鈉出現。這種形式的維他命C，被包裝在奈米級的磷脂質球體裡(磷脂囊)內，就像細胞膜保護其內容物一樣。磷脂質球體保護維他命C，免受環境的降解，而且更快被吸收到血液中。磷脂囊能促進細胞間的內容物得攝取，這能在傳遞諸如維他命C時，引起額外的臨床影響。這個劑型的吸收率，被認為比直接抗壞血酸高出5-10倍。但比抗壞血酸錠劑或粉末更貴。
抗壞血酸棕櫚酸酯
抗壞血酸棕櫚酸酯，是由抗血酸鈉綁在棕櫚酸分子結合而成。它是兩親性的，代表可溶解在水或脂肪中，就像脂肪酸在細胞膜一樣。被當成抗氧化劑廣泛運用在加工食品裡，或外用乳膏，被認為比維他命C更穩定。不過，當要吃進肚子裡時，抗壞血酸棕櫚酸酯的抗壞血酸鈉成分被認為會分解成抗壞血酸鈉跟棕櫚酸。分子因此會失去特殊的兩親性質。也比抗壞血酸貴。
天然的抗壞血酸鈉
抗壞血酸鈉的天然形式，來自既有的植物。西印度櫻桃，含有大量的維他命C，取決於它的成熟度，傳統用於治療感冒。從西印度櫻桃或玫瑰實中，提煉純化的維他命C錠劑市面上有，但通常劑量不高，而且比抗壞血酸更貴。雖然有些人強烈打這款的劑型，但鮑林和很多人都說，這樣的天然萃取的維他命C，不會比純商用抗壞血酸鈉更好[2,9]。
生物類黃酮，是在柑橘類水果或玫瑰實中發現的抗氧化劑，被認為能提高維他命C的攝取率與利用率。通常，營養補充品的錠劑，含有生物類黃酮，沒有很高含量不會產生很大的差異。對於有預算考量的消費者而言，最好的方法可能是買不貴的維他命C，不管是否有生物類黃酮。柑橘類水果，辣椒和很多其他水果、蔬菜，含有大量的生物類黃酮。這也是吃對食物跟吃對營養補充品一樣重要的原因。
FOR IMMEDIATE RELEASE
Orthomolecular Medicine News Service, December 8, 2009
VITAMIN C AND ACIDITY
What Form is Best?
(OMNS, December 8, 2009) Vitamin C is commonly taken in large quantities to improve health and prevent asthma, allergies, viral infection, and heart disease [1,2]. It is non-toxic and non-immunogenic, and does not irritate the stomach as drugs like aspirin can. Yet vitamin C (L-ascorbic acid) is acidic. So, a common question is, what are the effects from taking large quantities?
Ascorbic acid is a weak acid (pKa= 4.2) [3], only slightly stronger than vinegar. When dissolved in water, vitamin C is sour but less so than citric acid found in lemons and limes. Can large quantities of a weak acid such as ascorbate cause problems in the body? The answer is, sometimes, in some situations. However, with some simple precautions they can be avoided.
Acid in the Mouth
First of all, any acid can etch the surfaces of your teeth. This is the reason the dentist cleans your teeth and warns about plaque, for acid generated by bacteria in the mouth can etch your teeth to cause cavities. Cola soft drinks contain phosphoric acid, actually used by dentists to etch teeth before tooth sealants are applied. Like soft drinks, ascorbic acid will not cause etching of teeth if only briefly present. Often, vitamin C tablets are coated with a tableting ingredient such as magnesium stearate which prevents the ascorbate from dissolving immediately. Swallowing a vitamin C tablet without chewing it prevents its acid from harming tooth enamel.
Chewable Vitamin C Tablets
Chewables are popular because they taste sweet and so are good for encouraging children to take their vitamin C [4]. However, some chewable vitamin C tablets can contain sugar and ascorbic acid which, when chewed, is likely to stick in the crevices of your teeth. So, after chewing a vitamin C tablet, a good bit of advice is to rinse with water or brush your teeth. But the best way is to specifically select non-acidic vitamin C chewables, readily available in stores. Read the label to verify that the chewable is made entirely with non-acidic vitamin C.
Stomach Acidity
People with sensitive stomachs may report discomfort when large doses of vitamin C are taken at levels to prevent an acute viral infection (1,000-3,000 milligrams or more every 20 minutes) [1, 5]. In this case the ascorbic acid in the stomach can build up enough acidity to cause heartburn or a similar reaction. On the other hand, many people report no problems with acidity even when taking 20,000 mg in an hour. The acid normally present in the stomach, hydrochloric acid (HCl), is very strong: dozens of times more acidic than vitamin C.
When one has swallowed a huge amount of ascorbate, the digestive tract is sucking it up into the bloodstream as fast as it can, but it may still take a while to do so. Some people report that they seem to sense ascorbic acid tablets "sitting" at the bottom of the stomach as they take time to dissolve. It is fairly easy to fix the problem by using buffered ascorbate, or taking ascorbic acid with food or liquids in a meal or snack. When the amount of vitamin C ingested is more than the gut can absorb, the ascorbate attracts water into the intestines creating a laxative effect. This saturation intake is called bowel tolerance. One should reduce the amount (by 20-50%) when this occurs [1].
Acid Balance in the Body
Does taking large quantities of an acid, even a weak acid like ascorbate, tip the bodys acid balance (pH) causing health problems? No, because the body actively and constantly controls the pH of the bloodstream. The kidneys regulate the acid in the body over a long time period, hours to days, by selectively excreting either acid or basic components in urine. Over a shorter time period, minutes to hours, if the blood is too acid, the autonomic nervous system increases the rate of breathing, thereby removing more carbon dioxide from the blood, reducing its acidity.
Some foods can indirectly cause acidity. For example, when more protein is eaten than necessary for maintenance and growth, it is metabolized into acid, which must be removed by the kidneys, generally as uric acid. In this case, calcium and/or magnesium are excreted along with the acid in the urine which can deplete our supplies of calcium and magnesium [6]. However, because ascorbic acid is a weak acid, we can tolerate a lot before it will much affect the bodys acidity.
Although there have been allegations about vitamin C supposedly causing kidney stones, there is no evidence for this, and its acidity and diuretic tendency actually tends to reduce kidney stones in most people who are prone to them [1,7]. Ascorbic acid dissolves calcium phosphate stones and dissolves struvite stones. Additionally, while vitamin C does increase oxalate excretion, vitamin C simultaneously inhibits the union of calcium and oxalate. [1,2].
Forms of Vitamin C
Ascorbate comes in many forms, each with a particular advantage. Ascorbic acid is the least expensive and can be purchased as tablets, timed release tablets, or powder. The larger tablets (1000-1500 mg) are convenient and relatively inexpensive. Timed-release tablets contain a long-chain carbohydrate which delays the stomach in dissolving the ascorbate, which is then released over a period of hours. This may have an advantage for maintaining a high level in the bloodstream. Ascorbic acid powder or crystals can be purchased in bulk relatively inexpensively. Pure powder is more quickly dissolved than tablets and therefore can be absorbed somewhat faster by the body. Linus Pauling favored taking pure ascorbic acid, as it is entirely free of tableting excipients.
Buffered Ascorbate
A fraction of a teaspoon of sodium bicarbonate (baking soda) has long been used as a safe and effective antacid which immediately lowers stomach acidity. When sodium bicarbonate is added to ascorbic acid, the bicarbonate fizzes (emitting carbon dioxide) which then releases the sodium to neutralize the acidity of the ascorbate.
Calcium ascorbate can be purchased as a powder and readily dissolves in water or juice. In this buffered form ascorbate is completely safe for the mouth and sensitive stomach and can be applied directly to the gums to help heal infections [8]. It is a little more expensive than the equivalent ascorbic acid and bicarbonate but more convenient. Calcium ascorbate has the advantage of being non-acidic. It has a slightly metallic taste and is astringent but not sour like ascorbic acid. 1000 mg of calcium ascorbate contains about 110 mg of calcium.
Other forms of buffered ascorbate include sodium ascorbate and magnesium ascorbate [9]. Most adults need 800 - 1200 mg of calcium and 400-600 mg of magnesium daily [6]. The label on the bottle of all these buffered ascorbates details how much "elemental" mineral is contained in a teaspoonful. They cost a little more than ascorbic acid.
Buffered forms of ascorbate are often better tolerated at higher doses than ascorbic acid, but they appear not to be as effective for preventing the acute symptoms of a cold. This may be because after they are absorbed they require absorbing an electron from the body to become effective as native ascorbate [1]. Some of types of vitamin C are proprietary formulas that claim benefits over standard vitamin C [9].
Liposomal Vitamin C
Recently a revolutionary form of ascorbate has become available. This form of vitamin C is packaged inside nano-scale phospholipid spheres ("liposomes"), much like a cell membrane protects its contents. The lipid spheres protect the vitamin C from degradation by the environment and are absorbed more quickly into the bloodstream. Liposomes are also known to facilitate intracellular uptake of their contents, which can cause an added clinical impact when delivering something such as vitamin C. This form is supposed to be 5-10 fold more absorbable than straight ascorbic acid. It is more expensive than ascorbic acid tablets or powder.
Ascorbyl Palmitate
Ascorbyl palmitate is composed of an ascorbate molecule bound to a palmitic acid molecule. It is amphipathic, meaning that it can dissolve in either water or fat, like the fatty acids in cell membranes. It is widely used as an antioxidant in processed foods, and used in topical creams where it is thought to be more stable than vitamin C. However, when ingested, the ascorbate component of ascorbyl palmitate is thought to be decomposed into the ascorbate and palmitic acid molecules so its special amphipathic quality is lost. It is also more expensive than ascorbic acid.
Natural Ascorbate
Natural forms of ascorbate derived from plants are available. Acerola, the "Barbados cherry," contains a large amount of vitamin C, depending on its ripeness, and was traditionally used to fight off colds. Tablets of vitamin C purified from acerola or rose hips are available but are generally low-dose and considerably more expensive than ascorbic acid. Although some people strongly advocate this type, Pauling and many others have stated that such naturally-derived vitamin C is no better than pure commercial ascorbate [2,9].
Bioflavonoids are antioxidants found in citrus fruits or rose hips and are thought to improve uptake and utilization of vitamin C. Generally, supplement tablets that contain bioflavonoids do not have enough to make much difference. For consumers on a budget, the best policy may be to buy vitamin C inexpensively whether or not it also contains bioflavonoids. Citrus fruits, peppers, and a number of other fruits and vegetables contain large quantities of bioflavinoids. This is one more reason to eat right as well as supplement.
References:
[1] Hickey S, Saul AW (2008) Vitamin C: The Real Story, the Remarkable and Controversial Healing Factor. ISBN-13: 9781591202233
[2] Pauling L (1986) How to Live Longer and Feel Better, by Linus Pauling (Revised version, 2006) ISBN-13: 9780870710964
[3] Handbook of Chemistry and Physics (2004), CRC Press, ISBN-13: 978-0849304859
[4] http://www.doctoryourself.com/megakid.html (Ideas on how to get children to take vitamin C.)
[5] Cathcart RF (1981) Vitamin C, titrating to bowel tolerance, anascorbemia, and acute induced scurvy. Med Hypotheses. 7:1359-1376.
[6] Dean C (2006) The Magnesium Miracle. (2006) ISBN-13: 9780345494580
[7] http://www.doctoryourself.com/kidney.html
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