文章來源：Harvard School of Public Health
文章連結：http://www.hsph.harvard.edu/nutritionsource/calcium.html
常聽人家說，牛奶是很好的鈣質補充來源；在奇摩知識搜尋一下，可以看到也是有人會持對立的看法，牛奶裡的鈣質並不容易被人體吸收，甚至會讓骨頭的鈣質加速分解在血液中。到底孰是孰非？在好奇心驅使之下，用google一下子就找到第二種說法的來源。它是源自於Harvard School of Public Health的文章。以下就重點部分做中文解釋，有興趣的人可以詳讀原文。
P.S. 個人的專長不在生物，故若有名詞的中譯不恰當的地方，還請多指教。
What's Best For Your Bones?
Those advertisements pushing milk as the answer to strong bones are almost inescapable. But does "got milk" really translate into "got strong bones?"
The pro-milk faction believes that increased calcium intake - particularly in the form of the currently recommended three glasses of milk per day - will help prevent osteoporosis, the weakening of bones. Each year, osteoporosis leads to more than 1.5 million fractures, including 300,000 broken hips.
On the other side are those who believe that consuming a lot of milk and other dairy products will have little effect on the rate of fractures but may contribute to problems such as heart disease or prostate cancer.
Which view is right? The final answers aren't in. But here is a summary of what's currently known about calcium and its effects on the body.
What is calcium? 什麼是鈣？
Calcium is a mineral that the body needs for numerous functions, including building and maintaining bones and teeth, blood clotting, the transmission of nerve impulses, and the regulation of the heart's rhythm. Ninety-nine percent of the calcium in the human body is stored in the bones and teeth. The remaining 1 percent is found in the blood and other tissues.
鈣在人體中扮演很重要的角色。包括骨骼、牙齒的成長和維護、神經脈衝的傳輸、調節心臟的跳動和血液凝結。人體99%的鈣，都儲存在骨頭和牙齒。只有1%的鈣是在血液和其他的組織裡。
Where do we get calcium? 鈣的來源
The body gets the calcium it needs in two ways. One is by eating foods that contain calcium. good sources include dairy products, which have the highest concentration per serving of highly absorbable calcium, and dark leafy greens or dried beans, which have varying amounts of absorbable calcium.
人體鈣的來源主要有兩個。第一個是從飲食中吸收。好的鈣質來源有富含可吸收鈣質(absorbable calcium)的奶製品以及深綠色蔬菜和豆子。
The other way the body gets calcium is by pulling it from bones. This happens when blood levels of calcium drop too low, usually when it's been a while since having eaten a meal containing calcium. Ideally, the calcium that is "borrowed" from the bones will be replaced at a later point. But, this doesn't always happen. Most important, this payback can't be accomplished simply by eating more calcium.
第二個來源是從骨頭釋放。當人體血液中鈣的量降的太低時，骨頭的鈣質便會釋放出來以維持血液中鈣的濃度。通常在剛吃完飯後，血液的鈣濃度會暫時地降 低，這時人體會從骨頭借一些些鈣。理想的情況下，被借出來的鈣會從食物中在補回去。不過，並不是吃進越多鈣，回到骨頭的鈣就越多。
Growing healthy bones 健康的骨頭
Bone is living tissue that is always in flux. Throughout the lifespan, bones are constantly being broken down and built up in a process known as remodeling. Bone cells called osteoblasts build bone, while other bone cells called osteoclasts break down bone.
所以在人的一生當中，骨頭是維持一個動態的平衡的。骨頭一直被消耗，同時也一直在成長。這個過程就叫”remodeling”。負責成長的細胞就叫成骨細胞(osteoblast build)，負責相反過程的細胞叫osteoclasts break down bone.
In healthy individuals who get enough calcium and physical activity, bone production exceeds bone destruction up to about age 30. After that, destruction typically exceeds production.
透過運動和吸收足夠的鈣質，一個健康的人在30歲以前，骨頭成長的速度是比消耗的速度快的。在這之後，通常消耗的速度會大於成長的速度。
What is osteoporosis? 什麼是骨質疏鬆症
Osteoporosis, or "porous bones," is the weakening of bones caused by an imbalance between bone building and bone destruction. People typically lose bone as they age, despite consuming the recommended intake of calcium necessary to maintain optimal bone health. An estimated 10 million Americans - 8 million women and 2 million men - have osteoporosis. Another 34 million have low bone mass, placing them at increased risk for osteoporosis.(1)
骨頭的消耗和成長如果不平衡會造成骨質疏鬆症(osteoporosis)。從字面上來看，骨質疏鬆症就是指有孔(porous)的骨頭(bones)。骨頭有孔，當然就不堅固，容易骨折或變形。
Achieving adequate calcium intake and maximizing bone stores during the time when bone is rapidly deposited (up to age 30) provides an important foundation for the future. But it will not prevent bone loss later in life. The loss of bone with aging is due to several reasons, including genetic factors, physical inactivity, and lower levels of circulating hormones (estrogen in women and testosterone in men).
所以在30歲以前，在骨頭還能持續成長前，儘可能的增加鈣的吸吸，可說是在儲存骨本。隨著年紀增加的骨質流失有許多原因，包括不常運動、新陳代謝的降低等等。
Postmenopausal women account for 80 percent of all cases of osteoporosis because estrogen production declines rapidly at menopause. Of course, men are also at risk of developing osteoporosis, but they tend to do so 5-10 years later than women, since testosterone levels do not fall abruptly the way estrogen does in women. It is estimated that osteoporosis will cause half of all women over age 50 to suffer a fracture of the hip, wrist, or vertebra.
更年期的女性占了80%骨質疏鬆症的人口，因為雌性激素(estrogen)分泌性在更年期後會急速的降低。當然男性也會有骨質疏鬆症的危險，但是會比女性發生的晚約5至10年。根據估計，50歲以上的女性有一半有髖部、腕和脊椎骨的裂縫(fracture)。
How can osteoporosis be slowed down?要如何降低骨質疏鬆發生的速度呢？
Preventing osteoporosis depends on two things: making the strongest, densest bones possible during the first 30 years of life and limiting the amount of bone loss in adulthood.
要避免骨質疏鬆可從兩方面著手。第一：在30歲以前，儘可能地使骨頭強壯；骨密度儘可能地提高。第二：減緩骨質的流失速度。
There are a number of lifestyle factors that can help with the latter:
關於第二個原因，以下幾點是能有幫助的
- Getting regular exercise, especially weight-bearing and muscle strengthening exercise.
- Getting adequate vitamin D, whether through diet, exposure to sunshine, or supplements.
- Consuming enough calcium to reduce the amount the body has to borrow from bone.
- Consuming adequate vitamin K, found in green-leafy vegetables.
- Not getting too much preformed vitamin A.
- 規律的運動，特別是重量軸承鍛煉(weight bearing)，以及加強肌肉的耐力。
- 充足的維他命D的攝取，可以從飲食、曬太陽或補給品。
- 消化足夠的鈣來降低身體向骨頭借鈣的量。
- 消化足夠的維他命K(深綠植物)。
- 不要攝取過多的既成維生素A
Preventing bone loss in adulthood
Several complementary strategies can help prevent or minimize bone loss during adulthood and old age. These include:
Getting regular exercise
Physical activity that puts some strain or stress on bones causes the bones to retain and possibly even gain density throughout life. Cells within the bone sense this stress and respond by making the bone stronger and denser. Such "weight-bearing" exercises include walking, dancing, jogging, weightlifting, stair-climbing, racquet sports, and hiking.
經由運動的訓練中，成骨細胞會感受到外來的壓力，為了和外力抗衡，骨頭就會長地比較強壯，密度也較過。重量軸承鍛煉包括走路、跳舞、慢跑、舉重、爬樓梯、羽球、網球和健走(hiking)等等。
(to be continued)
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