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維他命E與相關抗氧化劑: 對抗癌症的武器 Vitamin E and other Antioxidants: Weapons against cancer 列印
作者是 4S Pharmacists   

科學界普遍認為,飲食營養與癌症有莫大關係。當前的研究表明,癌症是外在因素與遺傳因素相結合而產生的疾病。估計75至80%的癌症病例是由外在因素引起,包括吸煙、酷酒、飲食習性和生育行為、輻射、空氣污染和傳染病等。當中大概有三至四成病例歸因於飲食習慣。

抗氧化營養素,包括維他命A 、C 、E 和礦物質硒等,是重要的營養素,能降低患上癌症的風險。這些抗癌作用是根據動物測試和人類流行病學研究的反覆認證歸納出的結果。

自由基與其損害

我們經常暴露於一種稱為自由基的可致癌物質,自由基介入了許多癌症的始發和發展過程 。自由基是一種非常不穩定的物質,能攻擊身體細胞,導致基因變化和損壞細胞蛋白質。有部分自由基的產生是由於外在刺激、空氣污染和輻射等,但新陳代謝中氧氣的使用過程也會產生這種副產物。其實體內某程度的自由基水平是有利用價值的。但是,受生活方式、飲食習慣和健康狀況等影響,自由基大幅超過了合理水平的時候,它們便開始攻擊身體細胞。

自由基所攻擊的一個主要目標,是組成細胞膜的主要成分多元非飽和脂肪酸(PUFA) 。自由基的攻擊導致PUFA變質,對細胞造成一連串損害和導致癌症的發生。隨年齡增長而增加的癌症發病率,從某程度看與不斷增加的自由基活動有非常密切關係,因為免疫系統消除自由基和被損壞細胞的能力隨年齡增長而減弱。

身體的天然防禦系統: 剷除自由基

身體擁有一套完整的抗氧化防禦系統保護我們的細胞免受自由基損害。這個防禦系統是互動的,透過維他命A 、C 、E 和含硒的酵素組成(表1) 。

抗氧化劑 溶解性 剷除自由基 詳情
維他命 E 脂溶性 主要脂溶性抗氧化劑。保護細胞膜結構和功能。降低體內維他命A耗損。減少硒質流失。
水溶性 為維他命E的保護作用提供後援。
胡蘿蔔素 脂溶性 在細胞膜沒有特殊作用。
維他命 C 水溶性 在血液系統和其他水溶系統工作。當缺少維他命E是幫助維他命E 重生。

表1: 主要抗氧化天然防禦系統

抗氧化劑提供的額外保護

抗氧化劑除了有效中和有害的自由基外,研究顯示它們也有減少癌症發病率和阻礙癌細胞生長的作用:

‧服用高劑量維他命E能促進身體的免疫反應,保衛身體免受癌細胞侵害。
‧亞硝胺是致癌物質,胃部消化某些食物(例如煙薰和醃製食物)時會產生這種有害物質,維他命E能防止亞硝酸鹽轉化成亞硝胺。
‧維他命C 能提高免疫反應,防止亞硝酸鹽轉換成亞硝胺。
‧維他命A 能促進免疫反應,控制細胞組織的生長、改變和其作用。
‧硒能促進免疫反應。


Vitamin E and other Antioxidants: Weapons against cancer

The scientific community now recognises that nutrition plays a large part in the development of cancer. Current research indicates that cancer is the result of external factors combined with an hereditary disposition for cancer. The external factors, which include smoking, heavy alcohol consumption, dietary habits and reproductive behaviour, radiation, air pollution, and infections, are estimated to be responsible for 75 to 80% of all human cancers. Of these 30 to 40% are attributed to diet.
The antioxidant nutrients - vitamins A, C, E and the mineral, selenium - are important diet components that are associated with lowering cancer risk. Evidence of this anti-cancer effect is based upon animal studies and human epidemiological studies.

Free radicals and free radical damage


We are constantly exposed to cancer-causing substances called "free radicals" which are thought to be involved in the initiation and promotion of many cancers. Free radicals are highly unstable chemicals that attack the cells of the body, produce genetic mutations, and damage cell proteins. Some result from external stimuli, air pollutants and radiation, for example, and others are produced internally when oxygen is used in metabolism. A certain level of free radicals is needed in the functioning of the human body. However, when the level of free radicals becomes excessive, as a result of life-style, diet and medical conditions, they attack the body's cells.

A specific target of the free radicals are the polyunsaturated fats (PUFA) that are a major part of cell membranes. Free radical attack causes the PUFA to go rancid, starting a chain reaction that causes cell damage and gives cancer a place to start. Increased incidence of cancer with advancing age may be due, at least in part, to the increasing level of free radical reactions with age, along with the diminishing ability of the immune system to eliminate the damaged cells.

Fighting free radicals: the body's natural defences.


The body has an antioxidant defence system that protects against free radical damage. This defence system is interactive and includes vitamins A, C, E and a selenium-containing enzyme.

Added protection by the antioxidants.

In addition to the role of the antioxidants in neutralising free radicals, there is research data that suggests the antioxidants alter cancer incidence and growth by acting directly as anticarcinogens:
‧ High intakes of vitamin E increase the body's immune response, which may help defend the body from cancer.
‧ Vitamin E inhibits the conversion of nitrites found in smoked, cured, and pickled foods into nitrosamines in the stomach. Nitrosamines are strong cancer promoters.
‧ Vitamin C enhances the immune response and also prevents conversion of nitrites to nitrosamines.
‧ Vitamin A stimulates the immune response and controls growth, changes, and functions of body tissues.
‧ Selenium enhances immune response.

 

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