文章出處 http://longevity.about.com/od/whyweage/a/Understanding-The-Aging-Process.htm
翻譯人員 http://myhealthbiosystems.blogspot.tw/
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你該認識的老化過程

當你想到老化，腦海中可能浮現的是白髮蒼蒼的畫面，或是長滿皺紋的皮膚。你想到的這些都是跟著你全身細胞和整個身體一起老化的外在表徵。
老化是所有人都會共同經歷的一個過程，美國國家衛生研究院表示，在所有自然過程中，老化是我們了解最少的一種過程。下面是一些關於時間如何影響你個人生理老化的理論。

老化是生物預設好的過程 :

• 細胞生理時鐘理論 :人類細胞的複製次數是有限的，而老化就是當細胞複製的次數快到達極限所發生的現象。
• 內分泌學說 : 荷爾蒙分泌水平的下降導致老化，女性停經期便是與老化相關的內分泌學說。雌激素的下降導致某些老化疾病如骨質疏鬆。
• 免疫系統衰退 :人類特定的免疫細胞數量會隨著年齡增長而減少，使得身體暴露於危急生命疾病的風險增高如癌症。

老化是細胞受損的結果 :

• 部分的細胞損耗 : 端粒的縮短就是一個例子。細胞每複製一次，染色體上的保護尾端，也就是端粒的長度就會縮短一點，直到細胞無法複製為止。
• 氧化(自由基)壓力 : 在1950年代被科學家提出，細胞代謝和氧化反應的副產物自由基，被視為是造成DNA損害的物質。細胞內雖存在著清除自由基的機制，但DNA的傷害卻因不斷累積進而導致細胞死亡。這個理論也是人們相信多吃些抗氧化的食物，可以保護細胞少受到自由基的傷害。
• 交聯(糖化)作用 : 細胞中的蛋白質和DNA或是遺傳物質，會產生干擾細胞的不必要附著物或交聯物質。
• 錯誤的遺傳訊息 : 環境中的有害物質或是偶發性的突變所導致不可修復的DNA損害與錯誤的DNA訊息，進而造成細胞死亡和器官功能失常，加速老化過程。

老化相關的身體變化

老化的程度會隨著遺傳，飲食，文化，運動，環境因子而不同，然而某些長期的人口觀察研究還是可以告訴我們一些可能的老化相關身體變化。以下是 Baltimore Longitudinal Study of Aging的研究人員自1958年成立以來針對1400多人的追踪觀察，年齡涵蓋從20歲到90歲：

心血管系統 : 隨著年齡增長，心臟肌肉增厚與血管硬化使得輸送到大腦與全身的氧氣量減少。肺活量從20歲到70歲約下降40%。

改善的方式 : 規律持續地運動在任何年齡都可以促進與強化心肺功能。

腦神經系統 : 某些神經細胞的功能因結構改變而喪失。成人的神經細胞具有再生複製的功能，但再生的程度尚不清楚。大部份嚴重的心智功能衰退像是阿茲海默症或老人癡呆症都不是症常的老化現象。
改善的方式 : 藉由腦力激盪和學習新的技能(像是跳舞或樂器)可以促進記憶功能。

腎臟膀胱及泌尿系統 : 隨著年齡腎臟清除血液中廢棄物的效率下降，膀胱容量縮小與受尿失禁所擾，這些症狀好發於女性。

改善的方式 : 藥物可以改善膀胱容量。

骨頭和關節 : 35歲後骨密度開始下降，而邁入更年期的女性流失速度更快。

改善的方式 : 負重訓練，競走或慢跑都是維持本的方法。

肌肉 : 缺乏規律地運動的人，肌肉質量從30歲到70歲約下降20%。

改善的方式 : 規律持續地運動可以減少骨本流失及維持肌肉質量。

眼:到了40歲後，眼睛的lens開始硬化，導致無法聚焦在近物上的老花眼。另外，老化疾病還包含黃斑部病變，青光眼，以及一半以上的80歲老人都會罹患的白內障。

改善的方式 : 預防白內障的方法包含減少讓眼睛曝光在紫外線下以及減少吸菸。若已經有白內障的症狀，便只能選擇白內障手術來改善。

耳: 一般而言聽力，男性聽力衰退的程度較女性為快。隨著年齡增長常患有的兩種聽力退化問題包含耳聾以及耳鳴。耳鳴也有可能是其他問題所造成如高血壓。

改善的方式 : 平時降低手機與電視的音量，若有較嚴重重聽者，便只能帶助聽器改善。

下方為原文

Understanding the Aging Process

When you think of how your body is aging, it's probably the most visible changes that come to mind. Maybe you've noticed more gray hair, or your skin doesn't seem as smooth as it used to be. These are just external signs of a series of processes going on within your cells and bodily systems that together constitute normal aging.

While this is one of the few experiences all humans have in common, the National Institutes of Health say aging is actually one of nature's least understood processes. Here are some of the leading theories about how time affects your personal biology.

Aging is pre-programmed:

• Cellular clock theory: Since many human cells do not continue to reproduce indefinitely, this theory suggests that aging is a result of cells hitting their programmed reproductive limit.

• Neuroendrocrine hypothesis: Shifting hormone levels determine rate of aging. Menopause is an example of an age-related hormone change, which in turn results in aging (less estrogen, greater susceptibility to conditions like osteoporosis).

• Immune system decline: Specific immunity cells decline in number and how well they function with age. This leaves the body more vulnerable to life-threatening diseases like cancer.

Aging is a result of cell damage:

• Parts of cells just wear out: An example of this is the shortening of telomeres, the caps on cell chromosomes. Each time a cell reproduces, the chromosome or genetic material loses a bit of the protective cap until the cell can no longer divide.

• Oxidative (free radical) stress: The theory that free radicals -- the toxic byproducts of cell metabolism or oxidation -- are responsible for damaging DNA and therefore a cause of cell death. Though cleanup systems within cells exist, the damage is cumulative over time. Mitochondria, the engines of cell metabolism, are thought to play a central role because of the disproportionate free radicals they produce. First proposed in the 1950s, this theory has led to the belief that consuming antioxidants in food would counteract this process.
• Cross-linking (glycation): Over time, cells' proteins and DNA or genetic material develop unnecessary attachments, or cross-linkages, which can interfere with cell function.
• Genome error: Caused by environmental toxins or spontaneous mutations, damage to our DNA that isn't repaired on a cellular level can lead to cell death and organ malfunction, accelerating the aging process.

Over time, gerontologists will determine which of the above theories, or combination of theories, is the greatest contributor to aging.

Age-Related Changes in Your Body

While people age at different rates depending on genetics, diet, culture, activity levels and environmental exposure, certain long-term population studies give us clues about what changes we can expect. Here's what researchers in the Baltimore Longitudinal Study of Aging, in which more than 1,400 people from age 20 to 90 have been tracked since its inception in 1958, have observed:

Cardiovascular System: Heart muscle thickens and blood vessels stiffen with age, reducing the amount of oxygen available to the brain and body. Breathing capacity declines by as much as 40 percent between 20 and 70 years of age.

What you can do: Regular, sustained exercise can improve heart and lung function at any age.

Brain and Nervous System: Some nerve cell structure is lost, along with some function of individual nerve cells. Adult nerve cells may be reproduced, but the extent of this regeneration is not fully understood. Most severe decline in mental function is a result of diseases like Alzheimer's and dementia, not normal aging.

What you can do: Scientists are just beginning to learn how plastic, or adaptable, the brain is. You can improve memory and other brain functions by trying brain exercises and learning new skills (like dancing or playing a musical instrument).

Kidneys, Bladder and Urinary Systems: Kidneys lose efficiency at clearing waste from the blood. Bladder capacity declines and urinary incontinence can occur, especially in women.

What you can do: Medication and behavioral techniques can improve bladder capacity.

Bones and joints: Bone density diminishes on average beginning at age 35, with an accelerated rate of loss in menopausal women.

What you can do: Weight-bearing exercise, like strength training, in addition to walking and running, has been shown to help maintain bone density.

Muscles: Between the ages of 30 and 70, muscle mass declines more than 20 percent in men and women, in the absence of regular exercise.

What you can do: The same regular exercise that slows bone loss will help maintain muscle mass.

Eyes: By age 40, the lens begins to stiffen, resulting in difficulty focusing on near objects.Age-related eye conditions, such as macular degeneration and glaucoma, can develop. Cloudy areas on the lens will become cataracts in more than half of people by the age of 80, according to the National Eye Institute.

What you can do: It's believed that limiting tobacco use and ultraviolet light exposure will help prevent cataracts. Cataract surgery is a safe and effective remedy.

Ears: Two types of hearing loss that commonly occur with advancing age are presbycusis, which tends to run in families, and tinnitus, a ringing or hissing in the ears that can also be a sign of other problems, like high blood pressure. Hearing declines with age more rapidly in men than in women.

What you can do: An audiologist or hearing specialist may recommend a hearing aid. Other devices, like phone or television amplifying devices, may make declining hearing ability easier to manage.

Sources:

Aging Under the Microscope. Public Information Booklet. U.S. National Institutes of Health, National Institute on Aging. Accessed October 17, 2011. http://www.nia.nih.gov/health/publication/biology-aging/aging-under-microscope

Florian L. Mullera, Michael S. Lustgartenb, Youngmok Janga, Arlan Richardson, Holly Van Remmen. Free Radical Biology; Medicine 43 (2007) 477-503.http://www.rand.org/content/dam/rand/www/external/labor/aging/rsi/rsi_papers/2009/benz5.pdf

Hearing Loss and Older Adults. Public Information Sheet. U.S.National Institute on Deafness and Other Communications Disorders. Accessed October 17, 2011. http://www.nidcd.nih.gov/health/hearing/Pages/older.aspx

The Beginning of Something New-The Baltimore Longitudinal Study of Aging. Public Information Sheet. U.S. National Institutes of Health, National Institute on Aging. Accessed October 14, 2011.http://www.grc.nia.nih.gov/branches/blsa/blsafindings.pdf

Why and How do we Age? Public Information Sheet. American Federation for Aging Research. Accessed October 17, 2011. http://www.afar.org/pdfs/AFAR-Guide-to-Theories-of-Agingsm.pdf