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Imagine the typical office birthday party.
It's after lunch, so everybody is full. Then, in comes a luscious chocolate confection. The sight, the smell -- even the sound of the word 'cake!' -- stimulate the reward-and-pleasure circuits of the brain, activating memory centers and salivary glands as well.
Those reactions quickly drown out the subtle signals from the stomach that are saying, in effect, 'Still digesting down here. Don't send more!' Social cues add pressure and permission to indulge. Soon, everybody is having a slice -- or two.
Scholars have understood the different motives for eating as far back as Socrates, who counseled, 'Thou shouldst eat to live, not live to eat.' But nowadays, scientists are using sophisticated brain-imaging technology to understand how the lure of delicious food can overwhelm the body's built-in mechanism to regulate hunger and fullness, what's called 'hedonic' versus 'homeostatic' eating.
One thing is clear: Obese people react much more hedonistically to sweet, fat-laden food in the pleasure and reward circuits of the brain than healthy-weight people do. Simply seeing pictures of tempting food can light up the pleasure-seeking areas of obese peoples' brains.
Two conferences this week on obesity are each examining aspects of how appetite works in the brain and why some people ignore their built-in fullness signals. Scientists hope that breakthroughs will lead to ways to retrain people's thinking about food or weight-loss drugs that can target certain brain areas.
In a study presented this week at the International Conference on Obesity in Stockholm, researchers from Columbia University in New York showed pictures of cake, pies, french fries and other high-calorie foods to 10 obese women and 10 non-obese women and monitored their brain reactions on fMRI scans. In the obese women, the images triggered a strong response in the ventral tegmental area (VTA), a tiny spot in the midbrain where dopamine, the 'desire chemical,' is released. The images also activated the ventral pallidum, a part of the brain involved in planning to do something rewarding.
'When obese people see high-calorie foods, a widespread network of brain areas involved in reward, attention, emotion, memory and motor planning is activated, and all the areas talk to each other, making it hard for them to resist,' says lead investigator Susan Carnell, a research psychiatrist at the New York Obesity Research Center at Columbia University.
Similar brain reactions occurred in the obese subjects even when researchers merely said the words 'chocolate brownie' -- but not when they saw or heard about lower calorie foods such as cabbage and zucchini. Reactions were far less pronounced in the non-obese subjects.
More such studies are being presented in Pittsburgh this week at the annual meeting of the Society for the Study of Ingestive Behavior. In one, neuroscientists from Yale University's John B. Pierce Laboratory had 13 overweight and 13 normal-weight subjects smell and taste chocolate or strawberry milkshakes and observed their brains with fMRI scans.
The overweight subjects had strong reactions to the food in the amygdala -- the emotional center of the brain -- whether they were hungry or not. The healthy-weight subjects showed an amygdala response only when they were hungry.
'If you are of normal weight, your homeostatic mechanisms are functioning and controlling this region of the brain,' says lead investigator Dana Small. 'But in the overweight group, there is some sort of dysfunction in the homeostatic signal so that even though they weren't hungry, they were vulnerable to these external eating cues.'
Studies have found that a diet of sweet, high-fat foods can indeed blunt the body's built-in fullness signals. Most of them emanate from the digestive tract, which releases chemical messengers including cholecystokinin, glucagon-like peptide and peptide YY when the stomach and intestines are full. Those signals travel up to the brain stem and then the hypothalamus, telling the body to stop eating.
Obesity also throws off the action of leptin, a hormone secreted by fat tissue that tells the hypothalamus how much energy the body has stored. Leptin should act as a brake against overeating, and it does in normal-weight people. But most obese people have an overabundance of leptin, and somehow their brains are ignoring the signal.
All these findings beg the question, which came first? Does obesity disrupt the action of leptin, or does a malfunction in leptin signaling make people obese?
Similarly, are some people obese because their brains overreact to tempting food, or do their brains react that way because something else is driving them to overeat? Researchers at Yale and elsewhere are turning to such questions next. 'It's possible that these changes reflect how the brain has adapted to eating patterns in obese people, and that could create a vicious circle, putting them at risk for even more disordered eating.' says Dr. Small.
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讓我們來(lái)想象一場(chǎng)典型的同事們給某人過(guò)生日的派對(duì)場(chǎng)面。
中飯吃完了，大家都吃得很飽。然后，服務(wù)員送進(jìn)來(lái)一個(gè)美味的巧克力蛋糕。蛋糕的形狀和香味──甚至是“蛋糕”這個(gè)詞本身──都激起人們腦部的快感滿足回路(pleasure and reward circuit)，對(duì)蛋糕美味的記憶隨之開(kāi)啟，唾腺開(kāi)始分泌。
此時(shí)，人們的胃部正發(fā)出微弱的信號(hào)：“這里還在消化，別再吃了。”然而，對(duì)蛋糕的自然反應(yīng)很快占據(jù)壓倒性優(yōu)勢(shì)。集體的心理暗示讓每個(gè)人都覺(jué)得吃蛋糕是一種理所當(dāng)然的快樂(lè)之事。很快，大家就開(kāi)始分而食之，一塊，或是兩塊。
學(xué)者們?cè)缇椭�，人�?lèi)的進(jìn)食存在各種各樣的動(dòng)機(jī)，希臘哲學(xué)家蘇格拉底(Socrates)有一句勸誡的名言：“你應(yīng)該為了活而吃，而不是為了吃而活。”('Thou shouldst eat to live, not live to eat.')但現(xiàn)在，科學(xué)家正借助先進(jìn)的腦部成像技術(shù)來(lái)探查，為何美食的誘惑能夠戰(zhàn)勝體內(nèi)控制饑餓和飽腹感的生理機(jī)制，也就是“為了快樂(lè)而吃”和“為了需求而吃”到底孰強(qiáng)孰弱。
有一點(diǎn)很清楚：肥胖人群在面對(duì)高糖分高脂肪的食品時(shí)，腦部快感體驗(yàn)回路的反應(yīng)要比體重正常的人群積極得多。只要看到美食的圖片，肥胖人群腦部尋求快感的區(qū)域就馬上活躍起來(lái)。
2010年7月中旬舉辦的兩場(chǎng)肥胖問(wèn)題研討會(huì)都在研究人體腦部的食欲反應(yīng)機(jī)制，以及為什么有些人會(huì)忽視體內(nèi)的飽腹感信號(hào)�？茖W(xué)家希望在該領(lǐng)域的突破將有助于改變?nèi)藗儗?duì)食物的渴求程度，或研究出能影響腦部特定區(qū)域的減肥藥。
其中一個(gè)會(huì)議是在斯德哥爾摩舉辦的肥胖問(wèn)題國(guó)際研討會(huì)(International Conference on Obesity)。在會(huì)上，來(lái)自紐約州哥倫比亞大學(xué)(Columbia University)的研究人員向10名超重女性和10名非超重女性展示了蛋糕、餡餅、薯?xiàng)l等高熱量食品的圖片，并通過(guò)功能磁共振成像(fMRI)掃描儀觀察她們的腦部反應(yīng)。超重女性的中腦腹側(cè)被蓋區(qū)(ventral tegmental, VTA)迅速產(chǎn)生強(qiáng)烈的反應(yīng)，導(dǎo)致中腦一個(gè)微小的區(qū)域分泌出被稱為“欲望化學(xué)物質(zhì)”的多巴胺(dopamine)。這些圖片也激活了她們的腹側(cè)蒼白球(ventral pallidum, VP)，一個(gè)跟快感體驗(yàn)行為相關(guān)的腦部區(qū)域。
“肥胖人群看到高熱量食品時(shí)，涉及快感體驗(yàn)、關(guān)注、情緒、記憶和動(dòng)作計(jì)劃的一系列腦部區(qū)域開(kāi)始啟動(dòng)，各個(gè)腦部區(qū)域相互溝通，導(dǎo)致他們很難抗拒進(jìn)食的誘惑。”哥倫比亞大學(xué)紐約肥胖研究中心(New York Obesity Research Center)的首席研究員蘇珊.卡奈爾(Susan Carnell)說(shuō)道。
即使研究人員只是開(kāi)口說(shuō)“黑森林蛋糕”，接受觀察的肥胖者腦部也會(huì)產(chǎn)生同樣的反應(yīng)，但當(dāng)他們看到或聽(tīng)到卷心菜和西葫蘆等低熱量食品時(shí)，腦部對(duì)此無(wú)動(dòng)于衷。對(duì)于非肥胖人群來(lái)說(shuō)，這種反差則要小得多。
攝食行為研究學(xué)會(huì)(Society for the Study of Ingestive Behavior)的年會(huì)本周在匹茲堡舉行，有更多的研究成果在會(huì)上公布出來(lái)。舉例而言，耶魯大學(xué)(Yale University)約翰•皮爾斯實(shí)驗(yàn)室(John B. Pierce Laboratory)神經(jīng)學(xué)家們讓13名超重者和13名正常體重者用鼻子聞并品嘗巧克力奶昔或者草莓奶昔，通過(guò)功能磁共振成像掃描儀觀察他們的腦部活動(dòng)。
超重者的腦部杏仁核區(qū)域（大腦的情緒中心）對(duì)食物產(chǎn)生強(qiáng)烈反應(yīng)──無(wú)論他們是否感到饑餓。體重正常者則只有在饑餓的時(shí)候杏仁核區(qū)才會(huì)有類(lèi)似反應(yīng)。
“如果你體重正常，那說(shuō)明你體內(nèi)的自平衡機(jī)制在發(fā)揮作用，正控制著腦部的這塊區(qū)域。” 約翰•皮爾斯實(shí)驗(yàn)室首席研究員斯莫爾(Dana Small)說(shuō)，“但對(duì)超重者來(lái)說(shuō)，他們的自平衡機(jī)制有點(diǎn)問(wèn)題，因此即使不餓，也抵抗不了外界的美食誘惑。”
研究還發(fā)現(xiàn)，高糖分高脂肪的食品確實(shí)會(huì)讓人體內(nèi)的飽腹信號(hào)變得遲鈍起來(lái)。絕大多數(shù)這類(lèi)信號(hào)來(lái)自于消化道，當(dāng)胃部和腸道裝滿食物時(shí)，會(huì)釋放出一些化學(xué)信使來(lái)傳遞飽腹信號(hào)，包括膽囊收縮素、胰高血糖素樣肽和肽YY。這些信號(hào)依次傳輸?shù)侥X干和丘腦下部，指示身體停止進(jìn)食。
肥胖人群受瘦素(leptin)的影響也不明顯。瘦素是一種由脂肪組織分泌的激素，能告訴丘腦下部體內(nèi)已經(jīng)儲(chǔ)存了多少能量，還能制止人體過(guò)多進(jìn)食。正常體重者體內(nèi)的瘦素在發(fā)揮作用，但大多數(shù)肥胖者瘦素分泌過(guò)剩，導(dǎo)致其腦部忽略了瘦素傳遞出的信息。
所有這些研究都指向一個(gè)問(wèn)題：誰(shuí)才是罪魁禍?zhǔn)�？是肥胖�(jǐn)_亂了瘦素的作用，還是瘦素信號(hào)失常導(dǎo)致人們變得肥胖？
還有與此類(lèi)似的一個(gè)問(wèn)題，是一些人的腦部對(duì)美食做出過(guò)激反應(yīng)造成其身體變胖，還是有別的因素驅(qū)使他們過(guò)多進(jìn)食從而導(dǎo)致腦部作出過(guò)激反應(yīng)？耶魯大學(xué)和其他機(jī)構(gòu)的研究人員下一步就要研究這些問(wèn)題。“是否存在這種可能性，即肥胖人群對(duì)美食的過(guò)激反應(yīng)表明其腦部已經(jīng)適應(yīng)了不良的飲食習(xí)慣，從而形成惡性循環(huán)，導(dǎo)致肥胖者出現(xiàn)更嚴(yán)重的進(jìn)食紊亂。”斯莫爾說(shuō)道。
