不要對脂肪敬而遠之 碳水化合物纔是更可怕

The nutritional pickle so many Americans are now in is largely a result of “an oversimplification of dietary recommendations that created a fat phobia,” Dr. Frank B. Hu of the Harvard T. H. Chan School of Public Health told me.

哈佛大學公共衛生學院(Harvard T. H. Chan School of Public Health)的營養與流行病學教授胡丙長(Frank B. Hu)博士告訴我，“對飲食建議的過度簡化導致了人們對所有脂肪都存在恐懼心理，在很大程度上造成了時下很多美國人面臨的營養學困境。

Starting in the 1970s, when accumulating evidence from animal and human studies showed that a diet high in saturated fats and cholesterol was an important risk factor for cardiovascular disease, dietary guidelines urged people to eat less fat.

從20世紀70年代起，越來越多來自動物和人類研究的證據表明，高飽和脂肪高膽固醇飲食是心血管疾病的重要風險因素，膳食指南呼籲人們少吃脂肪。

Although health advice focused on saturated fats from high-fat animal foods, many people generalized the advice to mean all fats, choosing in their stead a panoply of reduced-fat and fat-free foods rich in carbohydrates, from crackers to sweetened yogurts. They especially increased their consumption of two kinds of carbohydrates, refined starches and sugars, that have helped to spawn the current epidemic of obesity and Type 2 diabetes.

雖然健康建議都側重於高脂肪動物食品中的飽和脂肪，有許多人都將它們外延到了所有脂肪，並選擇代之以從餅乾到加糖酸奶等一系列富含碳水化合物的低脂和無脂食品。他們對2種碳水化合物——精製澱粉類和糖類的攝入量增加得尤其明顯，這催生了當今肥胖和2型糖尿病的流行。

Experts now realize that efforts to correct past dietary sins that made heart disease and stroke runaway killers have caused the pendulum to swing too far in the wrong direction.

專家們現在認識到，人們爲了避免心臟病和中風而努力糾正既往的飲食誤區，結果矯枉過正，走向了另一個錯誤的方向。

“The mistake made in earlier dietary guidelines was an emphasis on low-fat without emphasizing the quality of carbohydrates, creating the impression that all fats are bad and all carbs are good,” Dr. Hu, a professor of nutrition and epidemiology, said. “It’s really important to distinguish between healthy fats and bad fats, healthy carbs and bad carbs.”

“早期膳食指南犯的錯誤在於過分強調低脂肪，而不重視碳水化合物的質量，讓人們誤以爲所有的脂肪都是壞東西，而所有的碳水化合物都有益於健康，”胡博士說。“區分有益健康的脂肪與壞脂肪，有益健康的碳水化合物與壞碳水化合物非常重要。”

He explained that saturated fat, found in fatty animal foods like meats and dairy products, raises blood levels of cholesterol and is not healthy, “but olive oil is important — it’s beneficial for cardiovascular health and body weight.” Olive oil, like canola, avocado and nut oils, is monounsaturated, and while it has as many calories as meat and dairy fat, it does not raise serum cholesterol or foster fat-clogging deposits in blood vessels.

他解釋說，高脂肪的動物性食物，如肉類和乳製品中的飽和脂肪會提高血液中的膽固醇，不利於健康，“但橄欖油很重要——它對心血管健康和控制體重都有益。”與菜籽油、鱷梨和堅果油一樣，橄欖油也是單不飽和脂肪酸，雖然它像肉類和乳製品脂肪同屬於高熱量食品，但它不會提高血清膽固醇，也不會促進脂肪在血管內的堆積和堵塞。

“We have to get out of the fat phobia mind-set,” Dr. Hu stressed, adding that we also have to abandon the idea that all complex carbohydrates are good.

“我們必須擺脫恐懼脂肪的心理定勢，”胡博士強調，並補充說，我們也必須放棄所有複雜的碳水化合物都有益於健康的想法。

Sugars are simple carbohydrates and starches are complex carbohydrates; all are ultimately broken down into glucose, the body fuel that circulates in blood. Sugars are digested rapidly, quickly raising blood glucose, but most starches take longer to digest.

糖類是簡單的碳水化合物，而澱粉是複雜的碳水化合物。但它們終將被分解爲葡萄糖，作爲身體的燃料存在於血液循環當中。糖類的消化速度很快，可以迅速提高血糖，而大多數澱粉需要較長的消化時間。

Important exceptions are refined carbohydrates, like white bread and white rice. Starchy foods with highly processed grains that have been stripped of dietary fiber act more like sugar in the body. They are rapidly digested and absorbed, raising blood levels of glucose and prompting the secretion of insulin to process it. When consumed in excess of the body’s need for immediate and stored energy, refined carbs and sugars can result in insulin resistance and contribute to fatty liver disease.

但精製碳水化合物，如白麪包和白米飯是重要的例外。用經過高度加工的穀物製成的澱粉類食物已經喪失了原來的膳食纖維，它們在人體內的消化過程反而與糖類更加接近。它們被迅速地消化和吸收，提高血糖水平，並促進人體分泌胰島素來應對。當你的攝入量超過了身體即時消耗和能量儲存的需要，精製碳水化合物和糖類可能導致胰島素抵抗，引起脂肪肝等疾病。

Alas, potatoes, the nation’s most popular vegetable, act like sugars and refined carbohydrates. They have what is called a high glycemic index, the ability to raise blood glucose rapidly. Potatoes, Dr. Hu explained, are made of long chains of glucose easily digested by enzymes in the mouth and stomach, and the fat in French fries slows the process only slightly.

可惜，美國最受歡迎的蔬菜——土豆也跟糖類和精製碳水化合物差不多。它們都具有較高的升糖指數，也就是迅速提高血糖的能力。胡博士解釋說，土豆由長鏈葡萄糖構成，很容易被口腔和胃部的酶消化，而炸薯條中的脂肪只能稍微減緩這一過程。

The concept of a glycemic index, proposed in 1981 by David Jenkins and his colleagues in The American Journal of Clinical Nutrition, has since been validated repeatedly and is now accepted as a good way to distinguish between the kinds of carbohydrates that are health-promoting or at least neutral and those that have negative health effects.

升糖指數這一概念是1981年由戴維·詹金斯(David Jenkins)及其同事們在《美國臨牀營養學雜誌》(The American Journal of Clinical Nutrition)上提出的，其後經過反覆驗證，現已被公認爲是區分那些促進健康（或至少也是中性）的碳水化合物與對健康有負面影響的壞碳水化合物的一種好方法。

In 2002, Dr. David S. Ludwig, a pediatrician, endocrinologist and nutrition researcher at Boston Children’s Hospital and professor at the Harvard T. H. Chan School of Public Health, published a comprehensive review of how glycemic index influences human physiology, clearly demonstrating its importance to preventing and treating obesity, diabetes and cardiovascular disease. Had Americans and their physicians heeded it then, we might have been largely spared the fix we’re now in.

2002年，哈佛大學公共衛生學院的教授，波士頓兒童醫院(Boston Children’s Hospital)的兒科醫生、內分泌學家和營養研究員戴維·S·路德維希(David S. Ludwig)博士發表了一篇關於升糖指數如何影響人體生理的全面綜述，明確地闡明瞭升糖指數對預防和治療肥胖症、糖尿病以及心血管疾病的重要性。要是當時美國人和他們的醫生們能聽從這篇綜述中的勸告，很有可能我們就可以避免陷入當前的這種困境。

The index was developed by testing the glucose response to a standard amount of carbohydrate against a reference food, either pure glucose (index number 100) or white bread (71). High-glycemic foods like baked Russet potatoes (111), white baguette (95), cornflakes (93), white rice (89), pretzels (83), instant oatmeal (83), rice cakes (82), Gatorade (78) and French fries (75) induce higher blood glucose levels than ordinary white bread and are best consumed infrequently and in small quantities.

升糖指數可以通過比對標準劑量的碳水化合物與參比食物對葡萄糖的應答而得出，其中參比食物可以是純葡萄糖（升糖指數爲100）或白麪包(71)。高升糖指數的食物包括烤土豆(111)、白法式長棍(95)、玉米片(93)、白米飯(89)、鹹脆餅乾(83)、即食燕麥片(83)、米餅(82)、佳得樂(78)和炸薯條(75)等，它們提升血糖水平的能力都超過了普通的白麪包，最好不要經常食用，並注意只能少量食用。

At the other end of the glycemic spectrum, oatmeal (55), pasta (46 for spaghetti, 32 for fettuccine), apples (39), carrots (35), skim milk (32), black beans (30), lentils (29), prunes (29), barley (28), chickpeas (10), grapefruit (25), peanuts (7) and hummus (6) have a smaller effect on blood glucose, and green vegetables like broccoli have too little an effect to be measured.

而低升糖指數食物，包括燕麥片(55)、意大利麪（細面爲46，寬面爲32）、蘋果(39)、胡蘿蔔(35)、脫脂牛奶(32)、黑豆(30)、小扁豆(29)、西梅(29)、大麥(28)、鷹嘴豆(10)、葡萄柚(25)、花生(7)和鷹嘴豆泥(6)等對血糖的影響較小，而西蘭花等綠色蔬菜對血糖的影響非常小，以至於無法測定。

Closely related to the glycemic index is the glycemic load. While the glycemic index measures how quickly a particular food raises blood sugar, the glycemic load takes portion sizes into account. Hence a food like watermelon, with a high glycemic index, has a low glycemic load, since much of the fruit is water.

升糖指數與另一個概念密切相關，那就是升醣負荷。升糖指數衡量的是某一特定食物提升血糖的快慢，而升醣負荷還考慮到了份量因素。比如，西瓜雖然升糖指數很高，但它的升糖負荷較低，因爲這種水果的絕大部分成分是水。

High-glycemic foods are a particular problem for people trying to control their weight. The amount of insulin released to lower blood glucose can overshoot the mark and result in a rapid return of hunger. A low-glycemic food, on the other hand, has no such effect. And those that are rich in wholesome fats, like peanuts or avocado, can actually delay the return of hunger, though the calories can add up quickly if consumed to excess.

對於試圖控制體重的人來說，高升糖食物尤其是大麻煩。它們可能導致胰島素大量釋放以降低血糖，但胰島素一旦釋放過量，就會讓人很快就重新感到飢餓。相反，低升糖食物就不會造成這種問題。至於那些富含健康脂肪的食物，如花生或鱷梨，其實倒可以延緩人再次飢餓的速度——不過，如果攝入過量，熱量也會迅速增加。

“The glycemic index and glycemic load of the average diet in the United States appear to have risen in recent years because of increases in carbohydrate consumption and changes in food-processing technology,” Dr. Ludwig wrote in 2002. The pattern persisted in the decade that followed, and can largely explain the rise in overweight and obesity among Americans of all ages.

路德維希博士在2002年寫道：“近年來，由於美國人對碳水化合物攝入的增加和食品加工技術的改變，人們日常飲食的升糖指數和升糖負荷都有所上升。”在其後的十年裏，這種模式仍然持續着，並在很大程度上導致了所有年齡段的美國人中超重和肥胖的增加。

In addition, chronic consumption of meals with a high-glycemic effect can induce insulin resistance, the hallmark of Type 2 diabetes, and an excess of free fatty acids in the blood, resulting in fatty liver disease. The prevalence of both these disorders has risen in recent years, and both can lead to chronic inflammation, a promoter of cardiovascular disease.

此外，長期以高升糖效應的食物爲主食可引發胰島素抵抗（這也是2型糖尿病的標誌），而且，血液中過量的遊離脂肪酸還會導致脂肪肝等疾病。近年來這兩種疾病的患病率均有所上升，而且，它們都可能導致慢性炎症，繼而誘發心血管疾病。

Dr. Hu said that when he was growing up in China, most people were physically active and thus able to handle the glycemic load of large amounts of white rice consumed. “Now, however, the Chinese have become more sedentary but still consume large amounts of white rice, and both obesity and diabetes are on the rise,” he said.

胡博士說，他從小在中國長大，在他的童年時代，大多數人都經常從事體力活動，從而能夠應對大量食用白米飯造成的升糖負荷。“然而，現在的中國人越來越愛久坐不動，卻仍在食用大量白米飯，肥胖和糖尿病都呈上升趨勢，”他說。