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.

哈佛大学公共卫生学院(Harvard T. H. Chan School of Public Health)的营养与流行病学教授胡丙长(Frank B. Hu)博士告诉我，“对饮食建议的过度简化导致了人们对所有脂肪都存在恐惧心理，在很大程度上造成了时下很多美国人面临的营养学困境。

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.

从20世纪70年代起，越来越多来自动物和人类研究的证据表明，高饱和脂肪高胆固醇饮食是心血管疾病的重要风险因素，膳食指南呼吁人们少吃脂肪。

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.

虽然健康建议都侧重于高脂肪动物食品中的饱和脂肪，有许多人都将它们外延到了所有脂肪，并选择代之以从饼干到加糖酸奶等一系列富含碳水化合物的低脂和无脂食品。他们对2种碳水化合物——精制淀粉类和糖类的摄入量增加得尤其明显，这催生了当今肥胖和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.

专家们现在认识到，人们为了避免心脏病和中风而努力纠正既往的饮食误区，结果矫枉过正，走向了另一个错误的方向。

“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.”

“早期膳食指南犯的错误在于过分强调低脂肪，而不重视碳水化合物的质量，让人们误以为所有的脂肪都是坏东西，而所有的碳水化合物都有益于健康，”胡博士说。“区分有益健康的脂肪与坏脂肪，有益健康的碳水化合物与坏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.

他解释说，高脂肪的动物性食物，如肉类和乳制品中的饱和脂肪会提高血液中的胆固醇，不利于健康，“但橄榄油很重要——它对心血管健康和控制体重都有益。”与菜籽油、鳄梨和坚果油一样，橄榄油也是单不饱和脂肪酸，虽然它像肉类和乳制品脂肪同属于高热量食品，但它不会提高血清胆固醇，也不会促进脂肪在血管内的堆积和堵塞。

“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.

“我们必须摆脱恐惧脂肪的心理定势，”胡博士强调，并补充说，我们也必须放弃所有复杂的碳水化合物都有益于健康的想法。

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.

糖类是简单的碳水化合物，而淀粉是复杂的碳水化合物。但它们终将被分解为葡萄糖，作为身体的燃料存在于血液循环当中。糖类的消化速度很快，可以迅速提高血糖，而大多数淀粉需要较长的消化时间。

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.

但精制碳水化合物，如白面包和白米饭是重要的例外。用经过高度加工的谷物制成的淀粉类食物已经丧失了原来的膳食纤维，它们在人体内的消化过程反而与糖类更加接近。它们被迅速地消化和吸收，提高血糖水平，并促进人体分泌胰岛素来应对。当你的摄入量超过了身体即时消耗和能量储存的需要，精制碳水化合物和糖类可能导致胰岛素抵抗，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，美国最受欢迎的蔬菜——土豆也跟糖类和精制碳水化合物差不多。它们都具有较高的升糖指数，也就是迅速提高血糖的能力。胡博士解释说，土豆由长链葡萄糖构成，很容易被口腔和胃部的酶消化，而炸薯条中的脂肪只能稍微减缓这一过程。

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.

升糖指数这一概念是1981年由戴维·詹金斯(David Jenkins)及其同事们在《美国临床营养学杂志》(The American Journal of Clinical Nutrition)上提出的，其后经过反复验证，现已被公认为是区分那些促进健康（或至少也是中性）的碳水化合物与对健康有负面影响的坏碳水化合物的一种好方法。

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，哈佛大学公共卫生学院的教授，波士顿儿童医院(Boston Children’s Hospital)的儿科医生、内分泌学家和营养研究员戴维·S·路德维希(David S. Ludwig)博士发表了一篇关于升糖指数如何影响人体生理的全面综述，明确地阐明了升糖指数对预防和治疗肥胖症、糖尿病以及心血管疾病的重要性。要是当时美国人和他们的医生们能听从这篇综述中的劝告，很有可能我们就可以避免陷入当前的这种困境。

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.

升糖指数可以通过比对标准剂量的碳水化合物与参比食物对葡萄糖的应答而得出，其中参比食物可以是纯葡萄糖（升糖指数为100）或白面包(71)。高升糖指数的食物包括烤土豆(111)、白法式长棍(95)、玉米片(93)、白米饭(89)、咸脆饼干(83)、即食燕麦片(83)、米饼(82)、佳得乐(78)和炸薯条(75)等，它们提升血糖水平的能力都超过了普通的白面包，最好不要经常食用，并注意只能少量食用。

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.

而低升糖指数食物，包括燕麦片(55)、意大利面（细面为46，宽面为32）、苹果(39)、胡萝卜(35)、脱脂牛奶(32)、黑豆(30)、小扁豆(29)、西梅(29)、大麦(28)、鹰嘴豆(10)、葡萄柚(25)、花生(7)和鹰嘴豆泥(6)等对血糖的影响较小，而西兰花等绿色蔬菜对血糖的影响非常小，以至于无法测定。

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.

升糖指数与另一个概念密切相关，那就是升醣负荷。升糖指数衡量的是某一特定食物提升血糖的快慢，而升醣负荷还考虑到了份量因素。比如，西瓜虽然升糖指数很高，但它的升糖负荷较低，因为这种水果的绝大部分成分是水。

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.

对于试图控制体重的人来说，高升糖食物尤其是大麻烦。它们可能导致胰岛素大量释放以降低血糖，但胰岛素一旦释放过量，就会让人很快就重新感到饥饿。相反，低升糖食物就不会造成这种问题。至于那些富含健康脂肪的食物，如花生或鳄梨，其实倒可以延缓人再次饥饿的速度——不过，如果摄入过量，热量也会迅速增加。

“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.

路德维希博士在2002年写道：“近年来，由于美国人对碳水化合物摄入的增加和食品加工技术的改变，人们日常饮食的升糖指数和升糖负荷都有所上升。”在其后的十年里，这种模式仍然持续着，并在很大程度上导致了所有年龄段的美国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，长期以高升糖效应的食物为主食可引发胰岛素抵抗（这也是2型糖尿病的标志），而且，血液中过量的游离脂肪酸还会导致脂肪肝等疾病。近年来这两种疾病的患病率均有所上升，而且，它们都可能导致慢性炎症，继而诱发心血管疾病。

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.

胡博士说，他从小在中国长大，在他的童年时代，大多数人都经常从事体力活动，从而能够应对大量食用白米饭造成的升糖负荷。“然而，现在的中国人越来越爱久坐不动，却仍在食用大量白米饭，肥胖和糖尿病都呈上升趋势，”他说。