[趣味] 為何熬夜會讓人想吃高熱量的食物？ 20191020 Science

Why skimping on sleep makes your brain crave sweets

By Eva Frederick

在減肥戒有一個老生常談：熬夜會使人更想吃高脂肪、高熱量的食物。現在，研究人員覺得他們大概知道為什麼了：缺少睡眠與大麻，對於嗅覺神經傳導的影響，非常相似。“這是個非常特別的研究！”，克莉絲丁 班尼迪克，一位與此研究無關，位在瑞典Uppsala大學，研究睡眠缺乏對於代謝影響的神經科學家說。

It’s a truism of the diet industry that getting too little sleep can make fatty, sweet foods more tempting. Now, researchers think they know why: Sleep loss influences the same smell-processing neural pathway that smoking marijuana does. “This is an exceptional study,” says Christian Benedict, a neuroscientist at Uppsala University in Sweden who has worked on the effects of sleep loss on metabolism but was not involved with the new research.

長久以來，睡眠缺乏一直都被認為會造成人們想要攝取更多高熱量的食物。為了研究這個機制如何作用，在伊利諾州，芝加哥西北大學芬堡醫學學校的神經科學家，涉爾斯坦 肯特從一個研究人類缺乏睡眠時會增加內生性大麻素系統內某些分子而得到靈感。內生性大麻素系統是個複雜神經傳導物質與受器的網絡，會受到許多物質的調控，其中包含了大麻。在大鼠中的研究顯示，這個系統會影響大腦如何感受味道。 而味道是對於引發食慾的一個重要驅力，就像是任何一個加油站的肉桂捲，總是如此吸引人。 然而，在這之前，從來沒有人將睡眠缺乏、內生性大麻素系統、味覺、以及人的食慾連結再一起。 肯特說：我們就提出這個想法然後說，好，讓我們對人來測試這件事，讓我們來連結這些機制。

Sleep deprivation has long been known to make people crave higher calorie foods. To find out how that process works, Thorsten Kahnt, a neurologist at Northwestern University’s Feinberg School of Medicine in Chicago, Illinois, took inspiration from studies linking sleep deprivation in humans to an increase in certain molecules in the endocannabinoid system, a complex network of neurotransmitters and receptors that, among other things, is affected by marijuana. Studies in mice have shown this system influences how the brain processes smells. And smell is a powerful driver of appetite—as illustrated by any gas station cinnamon roll shop.

Previously, though, no one had established clear links between sleep, the endocannabinoid system, smell, and appetite in humans. “We came in and said, ‘OK, let’s test this in humans. Let’s put these things together,’” Kahnt says.

為了要達到這個目標，肯特與他的團隊邀請25位自願受測的人員，分別每晚睡四小時或八小時。四個星期以後，這些人在重複相同的測試，但是每個人睡眠的時間都變更為另一個時間（原來四小時，改睡八小時）。在接下來的夜晚，他們取得這些受測人員的血液樣本。如預期的，睡眠缺乏的人，血液中的2-油酸甘油酯（已知可能會驅動內生性大麻素受器的分子）的量，明顯地較告。這些睡眠缺乏的人，並沒有顯示他們有較高的飢餓感，與同樣睡眠充足的人，在吃到飽的餐廳吃飯時，他們攝取食物的卡路里量，也沒有比較多。但是，睡眠缺乏的人，持續的選擇每克含有較高卡路里的食物，例如說淋上糖霜的甜甜圈，而非藍莓馬芬。

To do so, he and his team asked 25 healthy volunteers to sleep for either 4 hours or 8 hours per night. Four weeks later, the volunteers repeated the experiment, but those who slept 4 hours during the first round slept 8 hours, and vice versa. The following evening, the volunteers provided blood samples. Sleep-deprived volunteers, as expected, had higher levels of 2-oleoylglycerol, a molecule that likely acts on endocannabinoid receptors. The sleep-deprived group didn’t report feeling hungrier than their well-rested fellows, and when they were given a buffet of food, both groups consumed the same average amount of calories. However, people in the sleep-deprived group consistently chose foods that packed more energy per gram—for example, glazed doughnuts instead of blueberry muffins.

為了測試睡眠是否影響了腦對於嗅覺的反應，研究人員也在受試者想用吃到飽的晚餐前，進行了核磁共振掃描。在儀器中，受試者會聞到許多味道，一些跟食物有關，一些則無關聯，其中包含了烤牛肉、肉桂捲、大蒜和冷杉。研究人員測試了在大腦周圍梨狀皮質，一個負責處理味覺的梨形區域的反應。在大鼠中，有許多內生性大麻素的受器散布在這個區域中。如果說睡眠缺乏所產生的內生性大麻素系統物質，會改變人們對於嗅覺的反應，進一步影響人的食慾的話，研究人員預測，受試人員在這個大腦區域，因為所聞到味道的改變，應該會與他們選擇食物的傾象的改變有一致性。

To test whether sleep was affecting the odor-processing parts of the brain, the researchers also took prebuffet MRI scans. While in the scanner, study participants smelled a variety of food and nonfood odors, including pot roast, cinnamon rolls, garlic, and fir trees. The researchers examined scans of the piriform cortex, a pear-shaped region responsible for interpreting smells in the brain. In mice, it is peppered with endocannabinoid receptors. If increased endocannabinoid system molecules changed how the brain interpreted smell—and therefore a person’s appetite—the researchers reasoned that the piriform cortex should show variations in smell-processing activity that lined up with volunteers’ changes in food preferences.

但是，事情並非這麼簡單。研究人員雖然發現了，睡眠不足的人在聞到食物相關的味道時，腦部的梨形區域的活動有明顯的上升，但是與食慾傾向的變化，似乎沒有直接的關連。例如：兩位受試怎也許在這部位的活動有上升，但是在吃到飽的餐廳中，卻選擇了不同卡路里與脂肪量的食物。因此，研究團隊的人採取了另一個方針。他們猜想，也許，睡眠缺乏對於人傾向於高熱量食物食慾的影響是發生在腦的另外一個區域。當他們觀察大腦島葉（位處於腦的深處，自責人們對於食物攝取與梨形區域的溝通）的資訊流動時，他們發現，有較高的2-油酸甘油酯2的受試者，明顯的在這兩區域間的溝通，明顯地較少。

It wasn’t that simple, though. The researchers found that sleep-deprived participants’ piriform cortices showed increased activity in response to food related smells, but not in a way that directly correlated with their changes in appetite. For example, two volunteers with the same increase in odor encoding might have chosen foods with different amounts of fat and calories at the buffet.

So, the team members took a different tack. Maybe, they reasoned, the sleep-related changes that led to high-calorie cravings took place somewhere else in the brain. When they looked at information flow between the insula, a region deep inside the brain that helps regulate food intake, and the piriform cortex, they found that volunteers with higher levels of 2-oleoylglycerol showed consistently less “chatter” between the two regions.

這些改變，再加上食慾的變化，暗示了睡眠缺乏影響嗅覺系統與食物攝取上的可能路徑。（發表在這星期的eLife期刊）。肯特強調，對於這兩個腦部區域間的影響原因與其效果，仍然不是很清楚。“我們不清楚是誰在發出訊息，又是誰在接收”，他說。但是這個研究強化了睡眠缺乏對於感知系統的影響。“我們也發現，嗅覺對於味道的感測，對於食物選擇上的影響，真的被低估了”，他這麼說。 肯特也表示，如果我們知道這些外在因子如何改變人們的味覺與食慾，我們也許可以發展出對抗肥胖的新發法。

Those changes, along with the changes in appetite, show a possible pathway for how lack of sleep affects the olfactory system and food intake, the researchers report this month in eLife. Kahnt stresses that cause and effect in the two brain regions are unclear. “We don’t know who is speaking and who is listening,” he says. But the work solidifies the connection between sleep deprivation and sensory processes. “It also really underscores the role that the sense of smell has in guiding food choices,” he says. Knowing more about how external factors can affect smell processing and appetite could lead to new approaches to treating obesity or eating disorders, he says.

班尼迪克說，這個設計良好的計畫，對於未來的研究提供了許多不同的路徑。然而，他也注意到了許多除了睡眠之外，可能會影響這個研究結果的變因。 例如，八小時睡眠的人，是從晚上11點睡到早上7點，而四小時睡眠的人會在午夜1點上床，睡到早上5點。“我們知道睡醒的時間也會對於生理時鐘有一定程度的影響”，他表示。在太陽升起前起床，也許會會讓你的生理時鐘與日出後起床的人相比，相對的不同。

Benedict says the well-designed project provides plenty of avenues for future research. However, he notes that variables aside from sleep duration could have affected the results. For example, the people who slept 8 hours went to bed at 11 p.m. and woke up at 7 a.m., whereas the 4-hour group slept from 1 a.m. to 5 a.m. “We know that [time of] awakening has some effect on the circadian rhythm,” he says. Waking before dawn might throw your biological clock off compared with someone who woke up to sunshine.

肯特與他的團隊，希望接下來測試人們在一天中，對於味覺變化的微小改變，也想了解這樣的改變如何影響食物的攝取。另一個計畫將會測試人的身理時鐘如何被定時攝食所改變。肯特很好奇，這樣的改變是否存在，如果有的話，這也會引發其他正在研究嗅覺的研究人員的”食慾“。

Kahnt and his team hope to next look at how a person’s sense of smell changes throughout the day, and how that might contribute to food intake. Another project will examine how the body’s circadian rhythm is affected by extended fasting, in which a person limits their food intake to a short window of time each day. Kahnt wonders whether such changes exist, and if so, how they will whet the appetites of other researchers studying smell processing.

原文出處： https://www.sciencemag.org/news/2019/10/here-s-how-skimping-sleep-can-change-your-appetite?utm_campaign=SciMag&utm_source=JHubbard&utm_medium=Facebook