One fundamental and unanswered question in obesity research is what kinds of food contribute most to the condition. Experts variously blame， for example， fatty or sugary fare or food that lacks protein， which may prompt us， unconsciously， to overeat. Plenty of anecdotal evidence can be marshaled against any of the culprits， but there has been little long-term， large-scale experimental research on peoples comparative eating habits. It is neither ethical nor practical to have healthy subjects gorge themselves on one diet for years until they are obese.

肥胖研究中一個很根本卻一直懸而未解的問題是：什么樣的食物最容易導致肥胖？對此問題專家們有各種看法，例如，高脂肪、高糖分或缺乏蛋白質的食物可能會無意識地促使我們暴飲暴食。雖然大量的經(jīng)驗證據(jù)可以用來否定那些我們認為會讓人發(fā)胖的食物，但是關于人們的飲食習慣，卻少有長期的、大規(guī)模的對比試驗研究。因為讓健康的人多年暴食一種食物，直到肥胖，這既不道德，也不現(xiàn)實。

It is possible， though， to conduct this sort of experiment on mice. For a diet study published in Cell Metabolism， researchers randomly assigned one of 29 different diets to hundreds of adult male mice. （The scientists hope to include female mice in later experiments.） Some diets supplied up to 80 percent of their calories in the form of saturated and unsaturated fats， with few carbohydrates; others included little fat and consisted largely of refined carbohydrates， mostly from grains and corn syrup. Yet other diets were characterized by extremely high or low percentages of protein. The mice stayed on the same diet for three months—estimated to be the equivalent of roughly nine human years—while being allowed to eat and move about their cages at will. The mice were then measured by weight and body composition， and their brain tissue was examined for evidence of altered gene activity.

但是，對老鼠進行這種實驗卻是可能的。在《細胞代謝》雜志發(fā)表的一項飲食研究中，研究人員將29種不同的飲食隨機分配給數(shù)百只成年雄性老鼠。（科學家希望在之后的實驗中納入雌性老鼠）有些食物以飽和及不飽和脂肪的形式提供高達80%的卡路里，碳水化合物則很少;另一些則包含少量脂肪，主要由來自谷物和玉米糖漿的精制碳水化合物組成。其他飲食種類的特點是蛋白質含量極高或極低。這些老鼠保持相同的飲食三個月——估計相當于人類飲食的九年——同時允許它們隨意進食和在籠子里走動。然后研究人員測量老鼠的體重和身體成分，并對其腦組織進行檢查，以尋找基因活性改變的證據(jù)。

Only some of the mice became obese—almost every one of which had been on a high-fat diet. These mice showed signs of changes in the activity of certain genes too， in areas of the brain related to processing rewards; fatty kibble made them happy， apparently. None of the other diets， including those rich in sugar， led to significant weight gain or changed gene expression in the same way. Male mice on relatively high-fat diets became obese. The others did not.

只有一些老鼠變得肥胖，這些老鼠幾乎都吃的高脂肪食物。在這些老鼠處理獎勵信息的大腦區(qū)域中，它們的基因活性有改變的跡象，顯然，吃高脂肪的粗磨食物會讓它們開心。吃其他食物（包括高糖分食物）的老鼠并沒有表現(xiàn)出明顯的增重或相同的基因表達過程。吃高脂肪食物的雄性老鼠變肥胖了，而其他老鼠并沒有。