Eating Too Much Protein Makes Pee Pollute the U.S.蛋白摄入多，尿液污美国

In the U.S.， people eat more protein than they need to. And though it might not be bad for human health， this excess does pose a problem for the country’s waterways. The nation’s wastewater is laden with the leftovers from protein digestion： nitrogen compounds that can feed toxic algal blooms1 and pollute the air and drinking water. This source of nitrogen pollution even rivals that from fertilizers washed off of fields growing food crops， new research suggests.

美国人蛋白质吃得太多，超过身体所需。摄入过量蛋白质对人体健康也许无害，却成为影响该国水道的问题。美国的废水中充斥着蛋白质消化后的残留物：含氮化合物——它们会为有害藻华提供营养，还会污染空气和饮用水。新的研究表明，这种氮污染堪比灌溉庄稼地时肥料流失造成的污染。

When we overconsume protein—whether it comes from lentils， supplements2 or steak—our body breaks the excess down into urea， a nitrogen-containing compound that exits the body via urine and ultimately ends up in sewage. Maya Almaraz， a biogeochemist at the University of California， Davis， and her colleagues wanted to see how much of this nitrogen is being flushed into the U.S. sewage system because of a protein-heavy diet. The researchers combined population data and previous work on how much excess protein the average American eats and found that the majority of nitrogen pollution present in wastewater—some 67 to 100 percent—is a by-product of what people consume. “We think a lot about sewage nitrogen. We know that’s an issue，” Almaraz says. “But I didn’t know how much of that is actually affected by the choices we’re making way upstream—when we go the grocery store， when we cook a meal and what we end up putting in our bodies.”

当我们过度摄入蛋白质——无论来自小扁豆、膳食补充剂还是牛排——我们的身体就会把多余的蛋白质分解成尿素，这种含氮化合物会通过尿液排出体外并最终进入污水系统。加州大学戴维斯分校的生物地球化学家马娅·阿尔马拉斯和她的同事想了解，有多少因高蛋白饮食习惯而产生的氮，正不断地被冲入美国的污水系统。研究人员将人口数据和先前对美国人平均摄入多少过量蛋白质的研究成果结合在一起进行分析，发现目前废水中的大部分氮污染——约67%至100%——是人们饮食的副产物。“我们仔细考量了污水中的氮，认为这的确是个问题。”阿尔马拉斯说，“但我不知道，我们在上游所做的那些选择——去杂货店购物时、做饭时，以及最终吃进肚里的东西——究竟在多大程度上影响了这个问题。”

Once it enters the environment， the nitrogen in urea can trigger a spectrum of ecological impacts known as the “nitrogen cascade3.” Under certain chemical conditions， and in the presence of particular microbes， urea can break down to form gases of oxidized nitrogen. These gases reach the atmosphere， where nitrous oxide （N2O） can contribute to warming via the greenhouse effect and nitrogen oxides （NOx） can cause acid rain. Other times， algae and cyanobacteria4， photosynthetic bacteria5 also called blue-green algae， feed on urea directly. The nitrogen helps them grow much faster than they would normally， clogging vital water supplies with blooms that can produce toxins that are harmful to humans， other animals and plants. And when the algae eventually die， the problem is not over. Microorganisms that feast on dead algae use up oxygen in the water， leading to “dead zones，” where many aquatic species simply cannot survive， in rivers， lakes and oceans. Blooms from Puget Sound to Tampa， Fla.， have caused large fish die-offs.

尿素中的氮一旦进入环境，就会引发称作“氮级联效应”的一系列生态影响。在一定化学条件下，且存在特定微生物时，尿素会分解形成氮氧化物气体。这些气体进入大气后，其中的一氧化二氮（N2O）会通过温室效应加剧全球变暖，而氮氧化物（NOx）则会造成酸雨。此外，藻类和蓝细菌（即被称为蓝绿藻的光合作用菌）直接以尿素为食。氮可以促进藻类超常生长，生出藻华阻塞重要水源，而藻华还会产生危害人类及其他动植物的毒素。并且，麻烦不会因藻类最终死亡而结束。以死藻为食的微生物会耗尽水中的氧，从而在河流、湖泊和海洋中形成“死亡区域”，许多水生物种根本无法在这些区域生存。从华盛顿西北部的皮吉特湾到佛罗里达西岸的坦帕湾，藻华已导致鱼类大量死亡，数量锐减。

One option for preventing nitrogen from getting into the environment is improving wastewater treatment plants. The technology exists to remove 90 percent of nitrogen from wastewater， but only 1 percent of all U.S. sewage is currently treated this way， partly because the technology is so expensive. Equipping plants in China to remove nitrogen from three quarters of the country’s urban sewage cost more than $20 billion. Almaraz and her team suggest， however， that curbing nitrogen pollution could be approached more quickly with a change in eating habits that could save billions of dollars in the long term.

防止氮进入环境的一个办法是改良污水处理厂。现有技术可从废水中去除90%的氮，但现在全美只有1%的污水采用这种技术处理，原因之一是这种技术过于昂贵。在中国，处理厂采用的技术设备可从四分之三的城市污水中去除氮，耗资超过200亿美元。不过，阿尔马拉斯及其团队认为，改变饮食习惯可能更快地遏制氮污染，从长远看还可以节省数十亿美元。

Their new study， published in Frontiers in Ecology and the Environment， broke down protein requirements by age （adults 50 to 70 years old need the most） for the current U.S. population and projected future populations out to 2055. By midcentury， the country’s population is expected to be larger overall and to have a greater percentage of older people. The researchers calculated the amount of nitrogen that would enter the environment if people ate today’s average American diet and if they instead reduced their protein intake to only what is nutritionally needed. This shift in diet alone could reduce the amount of nitrogen reaching aquatic ecosystems by 12 percent today and by nearly 30 percent in the future， according to the study’s results. Such a change could also help reduce damaging nitrogen pollution while wastewater infrastructure catches up.

他们的新研究成果发表在期刊《生态与环境前沿》上，研究根据美国当前及预测到2055年的人口情况，按年龄细化分析了美国人的蛋白质需求（50至70岁的成人对蛋白质需求最高）。到本世纪中叶，预计美国总人口会更庞大，而老龄人口占比将更高。研究人员计算了两种情况下进入环境的氮的数量：一种是保持美国人当下的普遍饮食习惯；另一种是将蛋白质摄入量减少到仅够保证营养。研究结果表明，单单饮食习惯的改变就可以将目前进入水生态系统中的总氮量减少12%，未来则可减少近30%。随着废水处理基础设施迎头赶上，这样的变化可能还会助力减少破坏性氮污染。

（译者是“《英语世界》杯”翻译大赛获奖者）

1 algal bloom藻华（也称藻花或水华），水体中藻类大量增殖使水体变色的生态现象。通常是水体富营养化的标志，给生态系统和人类生产生活带来负面影响。  2 = dietary supplements膳食补充剂，一种作为饮食辅助的口服补剂，用以补充人体所需的氨基酸、维生素、矿物质和微量元素等。

3 nitrogen cascade氮级联效应。级联效应（cascade effect）指由于影响系统的一个行为/过程而导致一系列不可避免和意料之外事件发生的效应。  4 cyanobacteria蓝细菌，亦称蓝藻或蓝绿藻。一些种类在受氮、磷等元素污染后会引起水体的富营养化。  5 photosynthetic bacteria光合细菌。能将光能转化为生物代谢活动能量的原核微生物。亦称光能营养细菌，包括蓝细菌、紫细菌、绿细菌和盐细菌。