托福听力备考如何开展精听训练

Think it's your inability to resist cheesecake that's making it tough to fit into your skinny jeans? Well, your bacteria may share some of the blame. Because a new study in mice shows that the response of intestinal microbes to a high-fat diet ends up triggering the release of a hormone that makes mammals feel hungry, causing them to eat even more. The finding is served up in the journal Nature.

Previous work has shown that the types of bacteria in the gut in diabetic or obese individuals are different from the bacteria in healthy people. But does this bacterial makeup contribute to these disorders? Or is it just a side effect?

To unravel this mystery, researchers put mice on a high-fat diet. The animals experienced a buildup of a chemical called acetate, particularly in the large intestine.

That location points to gut bacteria, which can produce acetate, as a possible culprit. So the researchers wiped out the microbes using antibiotics or a simple saline wash. And acetate levels plummeted.

Okay, so the gut bacteria in fat-fed mice make acetate. What does acetate do? Well, it gets the involuntary part of the nervous system, the parasympathetic nervous system, to put out the call to produce more insulin. Unfortunately, in this case, acetate also gets the parasympathetic nervous system to stimulate production of a hunger hormone called ghrelin. And the more fats an animal consumes, the more acetate it makes — which means the more ghrelin it produces and, of course, the more it eats. And bacteria make the whole sequence happen.

The researchers are now investigating whether the same biochemical events happen in humans. If they do, it's possible that obtaining a better assortments of gut bacteria could help us control our weight. Of course, the best way to get those good bacteria is from a fecal transplant — in which bacteria-rich feces come out of one person and into you. The very thought of which could help curb your appetite.

设想一下，你很难穿上紧身牛仔裤是因为你无法抗拒奶酪蛋糕吗?其实，你体内的细菌可能要承担部分责任。一项在小鼠身上进行的新研究表明，肠道细菌对高脂饮食的反应会最终引发荷尔蒙的释放，使哺乳动物感到饥饿、吃得更多。该项发现刊登在《自然》杂志上。

此前的研究表明，糖尿病患者或肥胖群体肠道中的细菌类型与健康人的不同。但是这种细菌组成会导致这些疾病吗?还是只是一种副作用?

为了解开这个谜团，研究人员让小鼠食用高脂饮食。因此，醋酸酯这种化学物质在这些动物的大肠内积累起来。

该位置指向肠道细菌，这种细菌能产生醋酸酯，是潜在的罪魁祸首。因此，研究人员用抗生素或简单的生理盐水来消灭微生物。这同时也使醋酸酯浓度下降。

食用高脂饮食的小鼠，其体内肠道细菌可以产生醋酸酯。醋酸酯的作用是什么?它可以使神经系统、副交感神经系统中的无意识部分发出信号，产生更多的胰岛素。不幸的是，在这种情况下，醋酸酯也会使副交感神经系统刺激饥饿激素的产生。同时，脂肪消耗的越多，体内产生的醋酸酯就越多，这意味着体内产生的饥饿激素越多，当然就吃的越多。而且细菌会使这一整个序列发生。

研究人员正在研究相同的生物化学反应是否会在人类身上发生。假如人类也会这样，那么对肠道细菌更好的分类也许可以帮助我们控制体重。当然，摄取这些有益细菌的最佳方式是通过粪便移植，即某人含有大量细菌的排泄物进入到你的体内。想到这里可能会有助于抑制你的食欲。

重点讲解：

1. end up 最终;结果;到头来;

例句：If you speak too slowly, you might end up sounding unnatural.

但如果说得太慢，就有可能导致发音不自然。

2. be different from 与……不同;

例句：My style of writing is very different from yours.

我写作的风格和你很不同。

3. contribute to 促成;促使;是导致…的原因之一;

例句：Exercise contributes to better health.

锻炼能促成更强健的体魄。

4. wipe out 摧毁;毁灭;使灭绝;

例句：The plague once could wipe out a village.

鼠疫曾一度可以夺走整个村庄村民的生命。

2020托福听力练习：城市灯光欺骗树木

The Earth's temperature is rising. And as it does, springtime phenomena—like the first bloom of flowers—are getting earlier and earlier. But rising temperatures aren't the only factor. Urban light pollution is also quickening the coming of spring. "So temperature and light are really contributing to a double whammy of making everything earlier." Richard ffrench-Constant, an entomologist at the University of Exeter.

He and his colleagues compiled 13 years of data from citizen scientists in the U.K., who tracked the first bud burst of four common trees. Turns out, light pollution—from streetlights in cities, and along roads—pushed bud burst a full week earlier. Way beyond what rising temperatures could achieve. This disruptive timing can ripple through the ecosystem.

"The caterpillars that feed on trees are trying to match the hatching of their eggs to the timing of bud burst. Because the caterpillars want to feed on the juiciest and least chemically protected leaves. And it's not just the caterpillars, of course, that are important. But the knock-on effect is on nesting birds, which are also trying to hatch their chicks at the same time that there's the maximum number of caterpillars." So earlier buds could ultimately affect the survival of birds, and beyond. The findings are in the Proceedings of the Royal Society B.

The world's becoming increasingly urbanized, and light pollution is growing—which ffrench-Constant says could trick trees into budding earlier and earlier. But smarter lighting—like LEDs that dial down certain wavelengths—could help. "Perhaps the exciting thing is, if we understand more about how light affects this bud burst, we might be able to devise smarter sort of street lighting that has less red components, and therefore less early bud burst." Thus keeping springtime an actual springtime phenomenon.

地球温度正在上升。因此，花儿首次盛开等春季现象的出现时间会越来越早。但是，不断升高的气温并不是唯一的影响因素。城市光污染同样加快了春天到来的脚步。“因此，在温度和光线的双重冲击下，一切都提早到来。”理查德·费伦奇·康斯坦特是英国埃克塞特大学的昆虫学家。

他和他的同事对英国公民科学家统计了13年的数据进行编制，这些科学家对四种常见树木进行追踪，观察它们第一次萌芽的时间。结果发现，城市及道路两边的路灯造成的光污染，会使树木的发芽时间提前整整一个星期。除此以外，不断上升的气温也会施加影响。这种破坏性的时机会对生态系统造成影响。

“以树木为食的毛毛虫正试图将自己的孵卵时间与树木的发芽时间相匹配。因为毛毛虫要以最多汁、最少受化学物质保护的树叶为食。不仅毛毛虫是这样，当然，这一点很重要。这种连锁反应体现在筑巢鸟类身上，它们试图在毛毛虫数量最多的时候孵化鸟蛋。”因此，提前发芽最终可能会影响鸟类的生存，甚至产生更加深远的影响。该项研究结果发表在《英国皇家学会学报B》上。

世界越来越城市化，光污染的范围也在逐渐扩大，费伦奇·康斯坦特表示这种情况会欺骗树木，使其发芽的时间越来越早。但是LED这样的智能灯光会调低特定的波长，这可能会有所帮助。“也许令人感到兴奋的是，如果我们对灯光如何影响发芽有更多了解，我们也许能设计出发出更少红光的智能路灯，这样就可以使发芽时间少提前一些。”进而让春天呈现出实际的春天现象。

重点讲解：

1. contribute to 促成;促使;是导致…的原因之一;

例句：Honesty and hard work contribute to success and happiness.

诚实加苦干有助于成功和幸福。

2. turn out 原来是;结果发现;

例句：I'm sure things will turn out all right in the end.

我确信最终一切都会好的。

3. at the same time 同时;

例句：The crash took place at the same time as he left.

就在他离开时，车祸发生了。

4. dial down 下调;降低;

例句：The retreating gasoline price helped to lift consumer spirits this month and dial down their inflation expectations.