New Health Culprit Carnitine Found in Red Meat
Doctors have long assumed that saturated fat and cholesterol in red meat
are what raise the risk of heart disease. But a study in the journal
Nature Medicine fingers another culprit: carnitine, a compound abundant
in red meat that also is sold as a dietary supplement and found in some
energy drinks.
Carnitine typically helps the body transport fatty acids into cells to be used as energy. But researchers at the Cleveland Clinic found that in both humans and mice, certain bacteria in the digestive tract convert carnitine to another metabolite, called TMAO, that promotes atherosclerosis, or a thickening of the arteries.
The researchers, led by Stanley Hazen, chief of cellular and molecular medicine at the Cleveland Clinic's Lerner Research Institute, tested the carnitine and TMAO levels of omnivores, vegans and vegetarians, and examined records of 2,595 patients undergoing cardiac evaluations. In patients with high TMAO levels, the more carnitine in their blood, the more likely they were to develop cardiovascular disease, heart attacks, stroke and death.
Many studies have linked consumption of red and processed meat to cardiovascular disease and some cancers. The Harvard School of Public Health reported last year that among 83,000 nurses and 37,000 male health professionals followed since the 1980s, those who consumed the highest levels of red meat had the highest risk of death during study, and that one additional serving a day of red meat raised the risk of death by 13%.
The new findings don't mean that red meat is more hazardous than previously thought, but may help explain the underlying risk, which some researchers have long thought was higher than the saturated fat and cholesterol content alone could explain.
Dr. Hazen speculated that carnitine could be compounding the danger. 'Cholesterol is still needed to clog the arteries, but TMAO changes how cholesterol is metabolized─like the dimmer on a light switch,' he said. 'It may explain why two people can have the same LDL level [a measure of one type of cholesterol], but one develops cardiovascular disease and the other doesn't.'
One surprising finding, Dr. Hazen said, was how a long-term diet that includes meat affected the amount of TMAO-producing bacteria in the gut and thus magnified the risk. In the study, when longtime meat-eaters consumed an eight-ounce steak and a carnitine supplement, their bacteria and TMAO levels rose considerably. But when a vegan ate the same combination, he showed no increase in TMAO or bacterial change.
'Vegans basically lose their ability to digest carnitine,' said Dr. Hazen.
The study, sponsored by the National Institutes of Health, didn't assess how little red meat people could consume and still have elevated TMAO. Nor did it look at how long someone had to abstain from red meat to end the process. 'We know it will be longer than one week, but shorter than one year,' Dr. Hazen said.
He and his colleagues have been exploring how altering gut bacteria might influence the risk of heart disease. 'In the future, maybe there will be a heart-healthy yogurt, or a drug to block the formation of TMAO,' he said.
Consumption of red meat─primarily beef, veal, lamb and pork─has been falling gradually since 1970.
Trade groups for meat producers have questioned the link between red meat and cardiovascular disease on the grounds that studies that ask people to recall what they ate over long periods are imprecise.
As a dietary supplement, carnitine is designated as 'generally regarded as safe' by the Food and Drug Administration, but few studies have looked at its long-term safety. A 2006 risk assessment found no adverse effects when subjects consumed 2,000 milligrams a day for six months. (An eight-ounce steak has roughly 200 mg of carnitine.)
Ads for supplements promote carnitine as helping boost energy levels, particularly in endurance sports, and assisting in recovery after intense exercise; some also claim that it helps shed pounds and improve brain function.
MELINDA BECK
Carnitine typically helps the body transport fatty acids into cells to be used as energy. But researchers at the Cleveland Clinic found that in both humans and mice, certain bacteria in the digestive tract convert carnitine to another metabolite, called TMAO, that promotes atherosclerosis, or a thickening of the arteries.
The researchers, led by Stanley Hazen, chief of cellular and molecular medicine at the Cleveland Clinic's Lerner Research Institute, tested the carnitine and TMAO levels of omnivores, vegans and vegetarians, and examined records of 2,595 patients undergoing cardiac evaluations. In patients with high TMAO levels, the more carnitine in their blood, the more likely they were to develop cardiovascular disease, heart attacks, stroke and death.
Many studies have linked consumption of red and processed meat to cardiovascular disease and some cancers. The Harvard School of Public Health reported last year that among 83,000 nurses and 37,000 male health professionals followed since the 1980s, those who consumed the highest levels of red meat had the highest risk of death during study, and that one additional serving a day of red meat raised the risk of death by 13%.
The new findings don't mean that red meat is more hazardous than previously thought, but may help explain the underlying risk, which some researchers have long thought was higher than the saturated fat and cholesterol content alone could explain.
Dr. Hazen speculated that carnitine could be compounding the danger. 'Cholesterol is still needed to clog the arteries, but TMAO changes how cholesterol is metabolized─like the dimmer on a light switch,' he said. 'It may explain why two people can have the same LDL level [a measure of one type of cholesterol], but one develops cardiovascular disease and the other doesn't.'
One surprising finding, Dr. Hazen said, was how a long-term diet that includes meat affected the amount of TMAO-producing bacteria in the gut and thus magnified the risk. In the study, when longtime meat-eaters consumed an eight-ounce steak and a carnitine supplement, their bacteria and TMAO levels rose considerably. But when a vegan ate the same combination, he showed no increase in TMAO or bacterial change.
'Vegans basically lose their ability to digest carnitine,' said Dr. Hazen.
The study, sponsored by the National Institutes of Health, didn't assess how little red meat people could consume and still have elevated TMAO. Nor did it look at how long someone had to abstain from red meat to end the process. 'We know it will be longer than one week, but shorter than one year,' Dr. Hazen said.
He and his colleagues have been exploring how altering gut bacteria might influence the risk of heart disease. 'In the future, maybe there will be a heart-healthy yogurt, or a drug to block the formation of TMAO,' he said.
Consumption of red meat─primarily beef, veal, lamb and pork─has been falling gradually since 1970.
Trade groups for meat producers have questioned the link between red meat and cardiovascular disease on the grounds that studies that ask people to recall what they ate over long periods are imprecise.
As a dietary supplement, carnitine is designated as 'generally regarded as safe' by the Food and Drug Administration, but few studies have looked at its long-term safety. A 2006 risk assessment found no adverse effects when subjects consumed 2,000 milligrams a day for six months. (An eight-ounce steak has roughly 200 mg of carnitine.)
Ads for supplements promote carnitine as helping boost energy levels, particularly in endurance sports, and assisting in recovery after intense exercise; some also claim that it helps shed pounds and improve brain function.
MELINDA BECK
紅肉中發現新的健康風險
Bloomberg News
牛肉中含有肉鹼。一項研究發現，肉鹼在人體消化道中被細菌轉化后產生的物質，可能增加人們罹患心臟病和中風等疾病的幾率。
長
期以來﹐醫生們一直認為紅肉中能夠增加心臟病風險的就是飽和脂肪和膽固醇。不過﹐《自然-醫學》(Nature Medicine)雜志的一項研究發現了另一個罪魁禍首：肉鹼。肉鹼在紅肉中含量豐富﹐同時也作為膳食補充劑出售﹐有些能量飲料中也含有肉鹼。肉鹼一般能夠幫助人體將脂肪酸運送到細胞內以產生能量。但是克里夫蘭醫學中心(Cleveland Clinic)的研究人員發現﹐在人體和小鼠體內﹐某些細菌在消化道可以將肉鹼轉化成另一種代謝物氧化三甲胺(TMAO)﹐而TMAO能夠誘發動脈粥樣硬化。
在 克里夫蘭醫學中心勒納研究所的細胞和分子醫學主任黑曾(Stanley Hazen)領導下﹐研究員人員們測試了雜食者、嚴格素食主義者和素食者體內肉鹼和TMAO的水平﹐研究了2,595名接受心臟評估的病人的記錄。在擁有 較高TMAO水平的病人中﹐血液中的肉鹼水平越高﹐他們越有可能罹患心血管疾病、心臟病、中風等疾病﹐也越有可能死亡。
許多研究證明﹐食 用紅肉和加工肉類會誘發心血管病和一些癌症。哈佛大學公共衛生學院(Harvard School of Public Health)去年發佈報告說﹐上世紀80年代開始追蹤的8.3萬名護士和3.7萬名男性健康專家中﹐食用紅肉水平最高的人死亡風險最高﹐每天多吃一份紅 肉﹐死亡的風險增加13%。
新的發現並不意味著紅肉比之前認為的更加危險﹐但可能有助於解釋紅肉的潛在風險。長期以來﹐一些研究人員認為﹐紅肉的風險難以僅用飽和脂肪和膽固醇來解釋。
黑曾認為﹐肉鹼可能加劇了紅肉的危害。他說﹐堵塞血管仍然需要膽固醇﹐但是TMAO改變了膽固醇的新陳代謝方式﹐就像光開關上的調光器﹐這可能解釋為什麼兩個人可以有同樣水平的低密度脂蛋白(LDL﹐一種膽固醇類型)﹐但是一個得了心血管疾病﹐另一個卻沒有。
黑 曾說﹐一個驚人的發現是﹐長期食肉的飲食習慣會影響消化道內產生TMAO的細菌的數量﹐並因此增大患病風險。在研究中﹐當長期的食肉者吃掉八盎司的牛排和 一種肉鹼補充劑時﹐他們體內的細菌和TMAO水平均大幅上升。但是當一個嚴格素食者吃掉同樣的東西之後﹐他的TMAO水平沒有上升﹐細菌水平也沒有發生變 化。
黑曾說﹐嚴格素食者失去了消化肉鹼的能力。
這項由美國國立衛生研究院(National Institutes of Health)資助的研究沒有評估增加TMAO水平至少需要食用多少紅肉。研究也沒有發現一些人戒掉紅肉多長時間後才能阻止TMAO的上升。黑曾說﹐我們知道這會長於一個星期﹐但是短於一年。
他和他的同事一直在探索改變腸道細菌如何影響人們患心臟病的風險。他說﹐未來也許將出現一種心臟健康酸奶﹐或一種藥物﹐來阻止TMAO的形成。
紅肉的消耗量自1970年以來一直在下降。紅肉主要包括牛肉、小牛肉、羊肉和豬肉。
肉類生產商的行業組織質疑了紅肉和心血管病有關的結論﹐他們的理由是﹐研究中人們對長期飲食的回憶並不精準。
作 為一種飲食補充劑﹐肉鹼被美國食品藥品管理局(Food and Drug Administration)認可為“一般公認安全物質”﹐但是幾乎沒有人研究過肉鹼的長期安全性。2006年的風險評估發現﹐受試者連續六個月每天攝 入2,000毫克肉鹼不會產生負面影響（八盎司的牛排大約含有200毫克肉鹼）。
食品補充劑的廣告中宣傳說﹐肉鹼能夠幫助補充能量﹐尤其是在耐力運動中增強體能﹐並有助於在大量運動之後恢復體力﹐有些廣告還說﹐肉鹼有助減肥並改善腦部功能。
MELINDA BECK
Culprit in Heart Disease Goes Beyond Meat’s Fat
April 08, 2013
科學家發現心臟病與腸道細菌有關
報道 2013年04月08日
正值早餐時間，參與一項對紅肉及其影響研究的人們吃了擺在面前冒着熱氣、滋滋作響的西冷牛排。研究員為了這自己買了一台喬治·福爾曼(George Foreman)牌烤架，協助他工作的護士負責烹調。
以科學研究的名義，這六名男、女每人吃了一份8盎司（約合227克）重的多汁牛排，一點都沒剩。然後，他們等待抽血。
It was breakfast time and the people participating in a study of red meat and its consequences had hot, sizzling sirloin steaks plopped down in front of them. The researcher himself bought a George Foreman grill for the occasion, and the nurse assisting him did the cooking.
For the sake of science, these six men and women ate every last juicy bite of the 8-ounce steaks. Then they waited to have their blood drawn.
主持這項研究的克利夫蘭醫學中心(Cleveland Clinic)的斯坦利·黑曾(Stanley Hazen)醫生和他的同事們已經積累了證據，為紅肉可能引發心臟病提出一個令人驚訝的新解釋。他們正以這次早餐實驗來驗證其結論。
Dr. Stanley Hazen of the Cleveland Clinic, who led the study, and his colleagues had accumulated evidence for a surprising new explanation of why red meat may contribute to heart disease. And they were testing it with this early morning experiment.
The researchers had come to believe that what damaged hearts was not just the thick edge of fat on steaks, or the delectable marbling of their tender interiors. In fact, these scientists suspected that saturated fat and cholesterol made only a minor contribution to the increased amount of heart disease seen in red-meat eaters. The real culprit, they proposed, was a little-studied chemical that is burped out by bacteria in the intestines after people eat red meat. It is quickly converted by the liver into yet another little-studied chemical called TMAO that gets into the blood and increases the risk of heart disease.
研究人員逐漸認識到，對心臟有害的不僅是牛排邊角上厚厚的肥肉，或中間幼嫩美味的大理石狀肉。事實上，這些科學家懷疑，飽和脂肪和膽固醇只是喜食紅 肉者心臟疾病多發的一小部分誘因。他們提出，真正的罪魁禍首卻是一種在人們吃了紅肉以後由腸道內細菌釋放出來的化學物質，此前甚少有人研究。這種物質會很 快被肝臟轉化成另一種少有研究的化學物質，稱為氧化三甲胺(TMAO)，進入血液後增加心臟病的風險。
至少理論上如此。所以，那天早上的問題就是：他們吃了牛排以後，血液中是否會爆發出大量的氧化三甲胺？至少一年沒吃肉的純素食者，吃了同樣的一頓飯後，是否會出現同樣的情況？
That, at least, was the theory. So the question that morning was: Would a burst of TMAO show up in people’s blood after they ate steak? And would the same thing happen to a vegan who had not eaten meat for at least a year and who consumed the same meal?
The answers were: yes, there was a TMAO burst in the five meat eaters; and no, the vegan did not have it. And TMAO levels turned out to predict heart attack risk in humans, the researchers found. The researchers also found that TMAO actually caused heart disease in mice. Additional studies with 23 vegetarians and vegans and 51 meat eaters showed that meat eaters normally had more TMAO in their blood and that they, unlike those who spurned meat, readily made TMAO after swallowing pills with carnitine.
“It’s really a beautiful combination of mouse studies and human studies to tell a story I find quite plausible,” said Dr. Daniel J. Rader, a heart disease researcher at the University of Pennsylvania School of Medicine, who was not involved in the research.
Researchers say the work could lead to new treatments for heart disease — perhaps even an antibiotic to specifically wipe out the bacterial culprit — and also to a new way to assess heart disease risk by looking for TMAO in the blood.
Of course, critical questions remain. Would people reduce their heart attack risk if they lowered their blood TMAO levels? An association between TMAO levels in the blood and heart disease risk does not necessarily mean that one causes the other. And which gut bacteria in particular are the culprits?
There also are questions about the safety of supplements, like energy drinks and those used in body building. Such supplements often contain carnitine, a substance found mostly in red meat.
But the investigators’ extensive experiments in both humans and animals, published Sunday in Nature Medicine, have persuaded scientists not connected with the study to seriously consider this new theory of why red meat eaten too often might be bad for people.
Dr. Hazen began his research five years ago with a scientific fishing expedition. He directs a study of patients who come to the Cleveland Clinic for evaluations. Over the years, there have been 10,000. All were at risk for heart disease and agreed to provide blood samples and to be followed so the researchers would know if any patient had a heart attack or died of heart disease in the three years after the first visit. Those samples enabled him to look for small molecules in the blood to see whether any were associated with heart attacks or deaths.
That study and a series of additional experiments led to the discovery that a red meat substance no one had suspected — carnitine — seemed to be a culprit. Carnitine is found in red meat and gets its name from the Latin word carnis, the root of carnivore, Dr. Hazen said. It is also found in other foods, he noted, including fish and chicken and even dairy products, but in smaller amounts. Red meat, he said, is the major source, and for many people who eat a lot of red meat, it may be a concern.
The researchers found that carnitine was not dangerous by itself. Instead, the problem arose when it was metabolized by bacteria in the intestines and ended up as TMAO in the blood.
That led to the steak-eating study. It turned out that within a couple of hours of a regular meat-eater having a steak, TMAO levels in the blood soared.
The researchers’ theory, based on their laboratory studies, is that TMAO enables cholesterol to get into artery walls and also prevents the body from excreting excess cholesterol.
But what is it about carnitine that bacteria like? The answer, Dr. Hazen said, is that bacteria use it as a fuel.
He said he worries about carnitine-containing energy drinks. Carnitine often is added to the drinks on the assumption that is will speed fat metabolism and increase a person’s energy level, Dr. Hazen said.
Dr. Robert H. Eckel, a professor of medicine at the University of Colorado and a past president of the American Heart Association, worried about how carnitine might be affecting body builders and athletes who often take it because they believe it builds muscle.
Those supplements, Dr. Hazen said, “are scary, especially for our kids.”
答案是：會，在五位食肉者身上的確出現了氧化三甲胺的激增；不會，純素食者沒有出現這樣的情況。經研究證明，氧化三甲胺的水平是能夠預測人類心肌梗 死風險的。研究人員還發現，氧化三甲胺的確會讓小鼠得心臟病。另外對23位素食者和純素食者以及51位肉食者的研究顯示，肉食者的血液中通常含有更多氧化 三甲胺，和拒絕吃肉的人不同，他們在服用了含有肉鹼的葯後就會立刻產生出氧化三甲胺。
賓西法尼亞大學醫學院(University of Pennsylvania School of Medicine)心臟病研究員丹尼爾·J·雷德(Daniel J. Rader)醫生稱，“通過小鼠和人體研究來共同闡述一個理論，這真的是一種完美結合，我覺得這一結論很可信。”雷德醫生沒有參與這項研究。
研究人員說，這項研究工作有可能為心臟疾病帶來新的治療方案，甚至可能找到一種抗生素，能有針對性地清除引發問題的細菌，也可能發現一種測定心臟病風險的新方法——查看血液中的氧化三甲胺。
當然，關鍵問題依然存在。如果人們能夠降低他們血液中氧化三甲胺的水平，是不是就能減少心肌梗死的風險？血液中氧化三甲胺的水平與心臟病風險的聯繫未必意味着，它們之間存在因果關係。並且，引發問題的到底是哪一種腸道細菌呢？
還有關於補劑的安全問題，如能量飲料和健美補劑。這類補劑通常含有肉鹼，一種主要存在於紅肉中的物質。
然而這項周日發表在《自然醫學》(Nature Medicine)雜誌上的研究所進行的大量人類和實驗，對和此次研究無關的科學家們來說已經足夠有說服力，在為何經常吃紅肉會對人有害的問題上，這項新理論引起了他們的重視。
黑曾醫生的研究始於五年前的一次科學調查。他主持了一項對克利夫蘭醫學中心患者的研究。幾年下來研究對象達到了1萬人。所有患者都有罹患心臟病的風 險，他們同意提供血液樣本並接受跟蹤調查，這樣研究人員就能夠了解，是否有患者在初次就診後三年內出現心肌梗死或死於心臟疾病。這些樣本使黑曾醫生能夠尋 找血液內的小分子，來看看其中是否有與心肌梗死或死亡相關的。
這項研究及一系列附加的實驗發現，紅肉中的一種從未成為懷疑對象的物質——肉鹼——似乎是罪魁禍首。黑曾醫生說“肉鹼”(carnitine)存在 於紅肉中，這個名稱來自於拉丁詞carnis，也就是食肉動物(carnivore)一詞的詞根。他指出，其他食物中也有肉鹼，包括魚肉和雞肉，甚至乳製 品，不過量都很小。他說紅肉是肉鹼的主要來源，對那些大量食用紅肉的人來說，它可能是一種隱患。
研究人員發現，肉鹼本身並不危險。然而，當肉鹼在腸道內被細菌分解，並最終成為血液中的氧化三甲胺時，問題就會出現。
這導致了那項吃牛排的研究。結果證明，一個經常食肉的人在食用牛排幾小時之內，血液內的氧化三甲胺水平會飆升。
研究者基於實驗室研究的理論是，氧化三甲胺令膽固醇進入動脈血管壁，並阻止身體排出多餘的膽固醇。
但是細菌看上了肉鹼什麼呢？黑曾醫生說，答案就是細菌把它當成一種能量來源。
他說他對那些含肉鹼的能量飲料感到擔憂。人們在這些飲料中加入肉鹼通常是認為它可以加速脂肪代謝，並提升一個人的能量水平。
令科羅拉多大學(University of Colorado)醫學教授、美國心臟病協會(American Heart Association)前會長羅伯特·H·埃克爾博士(Robert H. Eckel)擔心的是，肉鹼會對健美人士及運動員有影響，這些人認為肉鹼有增肌作用，因此經常飲用。
黑曾醫生說，這些補劑“很可怕，特別是對我們的孩子來說。”
以科學研究的名義，這六名男、女每人吃了一份8盎司（約合227克）重的多汁牛排，一點都沒剩。然後，他們等待抽血。
It was breakfast time and the people participating in a study of red meat and its consequences had hot, sizzling sirloin steaks plopped down in front of them. The researcher himself bought a George Foreman grill for the occasion, and the nurse assisting him did the cooking.
For the sake of science, these six men and women ate every last juicy bite of the 8-ounce steaks. Then they waited to have their blood drawn.
Dr. Stanley Hazen of the Cleveland Clinic, who led the study, and his colleagues had accumulated evidence for a surprising new explanation of why red meat may contribute to heart disease. And they were testing it with this early morning experiment.
The researchers had come to believe that what damaged hearts was not just the thick edge of fat on steaks, or the delectable marbling of their tender interiors. In fact, these scientists suspected that saturated fat and cholesterol made only a minor contribution to the increased amount of heart disease seen in red-meat eaters. The real culprit, they proposed, was a little-studied chemical that is burped out by bacteria in the intestines after people eat red meat. It is quickly converted by the liver into yet another little-studied chemical called TMAO that gets into the blood and increases the risk of heart disease.
研究人員逐漸認識到，對心臟有害的不僅是牛排邊角上厚厚的肥肉，或中間幼嫩美味的大理石狀肉。事實上，這些科學家懷疑，飽和脂肪和膽固醇只是喜食紅 肉者心臟疾病多發的一小部分誘因。他們提出，真正的罪魁禍首卻是一種在人們吃了紅肉以後由腸道內細菌釋放出來的化學物質，此前甚少有人研究。這種物質會很 快被肝臟轉化成另一種少有研究的化學物質，稱為氧化三甲胺(TMAO)，進入血液後增加心臟病的風險。
至少理論上如此。所以，那天早上的問題就是：他們吃了牛排以後，血液中是否會爆發出大量的氧化三甲胺？至少一年沒吃肉的純素食者，吃了同樣的一頓飯後，是否會出現同樣的情況？
That, at least, was the theory. So the question that morning was: Would a burst of TMAO show up in people’s blood after they ate steak? And would the same thing happen to a vegan who had not eaten meat for at least a year and who consumed the same meal?
The answers were: yes, there was a TMAO burst in the five meat eaters; and no, the vegan did not have it. And TMAO levels turned out to predict heart attack risk in humans, the researchers found. The researchers also found that TMAO actually caused heart disease in mice. Additional studies with 23 vegetarians and vegans and 51 meat eaters showed that meat eaters normally had more TMAO in their blood and that they, unlike those who spurned meat, readily made TMAO after swallowing pills with carnitine.
“It’s really a beautiful combination of mouse studies and human studies to tell a story I find quite plausible,” said Dr. Daniel J. Rader, a heart disease researcher at the University of Pennsylvania School of Medicine, who was not involved in the research.
Researchers say the work could lead to new treatments for heart disease — perhaps even an antibiotic to specifically wipe out the bacterial culprit — and also to a new way to assess heart disease risk by looking for TMAO in the blood.
Of course, critical questions remain. Would people reduce their heart attack risk if they lowered their blood TMAO levels? An association between TMAO levels in the blood and heart disease risk does not necessarily mean that one causes the other. And which gut bacteria in particular are the culprits?
There also are questions about the safety of supplements, like energy drinks and those used in body building. Such supplements often contain carnitine, a substance found mostly in red meat.
But the investigators’ extensive experiments in both humans and animals, published Sunday in Nature Medicine, have persuaded scientists not connected with the study to seriously consider this new theory of why red meat eaten too often might be bad for people.
Dr. Hazen began his research five years ago with a scientific fishing expedition. He directs a study of patients who come to the Cleveland Clinic for evaluations. Over the years, there have been 10,000. All were at risk for heart disease and agreed to provide blood samples and to be followed so the researchers would know if any patient had a heart attack or died of heart disease in the three years after the first visit. Those samples enabled him to look for small molecules in the blood to see whether any were associated with heart attacks or deaths.
That study and a series of additional experiments led to the discovery that a red meat substance no one had suspected — carnitine — seemed to be a culprit. Carnitine is found in red meat and gets its name from the Latin word carnis, the root of carnivore, Dr. Hazen said. It is also found in other foods, he noted, including fish and chicken and even dairy products, but in smaller amounts. Red meat, he said, is the major source, and for many people who eat a lot of red meat, it may be a concern.
The researchers found that carnitine was not dangerous by itself. Instead, the problem arose when it was metabolized by bacteria in the intestines and ended up as TMAO in the blood.
That led to the steak-eating study. It turned out that within a couple of hours of a regular meat-eater having a steak, TMAO levels in the blood soared.
The researchers’ theory, based on their laboratory studies, is that TMAO enables cholesterol to get into artery walls and also prevents the body from excreting excess cholesterol.
But what is it about carnitine that bacteria like? The answer, Dr. Hazen said, is that bacteria use it as a fuel.
He said he worries about carnitine-containing energy drinks. Carnitine often is added to the drinks on the assumption that is will speed fat metabolism and increase a person’s energy level, Dr. Hazen said.
Dr. Robert H. Eckel, a professor of medicine at the University of Colorado and a past president of the American Heart Association, worried about how carnitine might be affecting body builders and athletes who often take it because they believe it builds muscle.
Those supplements, Dr. Hazen said, “are scary, especially for our kids.”
答案是：會，在五位食肉者身上的確出現了氧化三甲胺的激增；不會，純素食者沒有出現這樣的情況。經研究證明，氧化三甲胺的水平是能夠預測人類心肌梗 死風險的。研究人員還發現，氧化三甲胺的確會讓小鼠得心臟病。另外對23位素食者和純素食者以及51位肉食者的研究顯示，肉食者的血液中通常含有更多氧化 三甲胺，和拒絕吃肉的人不同，他們在服用了含有肉鹼的葯後就會立刻產生出氧化三甲胺。
賓西法尼亞大學醫學院(University of Pennsylvania School of Medicine)心臟病研究員丹尼爾·J·雷德(Daniel J. Rader)醫生稱，“通過小鼠和人體研究來共同闡述一個理論，這真的是一種完美結合，我覺得這一結論很可信。”雷德醫生沒有參與這項研究。
研究人員說，這項研究工作有可能為心臟疾病帶來新的治療方案，甚至可能找到一種抗生素，能有針對性地清除引發問題的細菌，也可能發現一種測定心臟病風險的新方法——查看血液中的氧化三甲胺。
當然，關鍵問題依然存在。如果人們能夠降低他們血液中氧化三甲胺的水平，是不是就能減少心肌梗死的風險？血液中氧化三甲胺的水平與心臟病風險的聯繫未必意味着，它們之間存在因果關係。並且，引發問題的到底是哪一種腸道細菌呢？
還有關於補劑的安全問題，如能量飲料和健美補劑。這類補劑通常含有肉鹼，一種主要存在於紅肉中的物質。
然而這項周日發表在《自然醫學》(Nature Medicine)雜誌上的研究所進行的大量人類和實驗，對和此次研究無關的科學家們來說已經足夠有說服力，在為何經常吃紅肉會對人有害的問題上，這項新理論引起了他們的重視。
黑曾醫生的研究始於五年前的一次科學調查。他主持了一項對克利夫蘭醫學中心患者的研究。幾年下來研究對象達到了1萬人。所有患者都有罹患心臟病的風 險，他們同意提供血液樣本並接受跟蹤調查，這樣研究人員就能夠了解，是否有患者在初次就診後三年內出現心肌梗死或死於心臟疾病。這些樣本使黑曾醫生能夠尋 找血液內的小分子，來看看其中是否有與心肌梗死或死亡相關的。
這項研究及一系列附加的實驗發現，紅肉中的一種從未成為懷疑對象的物質——肉鹼——似乎是罪魁禍首。黑曾醫生說“肉鹼”(carnitine)存在 於紅肉中，這個名稱來自於拉丁詞carnis，也就是食肉動物(carnivore)一詞的詞根。他指出，其他食物中也有肉鹼，包括魚肉和雞肉，甚至乳製 品，不過量都很小。他說紅肉是肉鹼的主要來源，對那些大量食用紅肉的人來說，它可能是一種隱患。
研究人員發現，肉鹼本身並不危險。然而，當肉鹼在腸道內被細菌分解，並最終成為血液中的氧化三甲胺時，問題就會出現。
這導致了那項吃牛排的研究。結果證明，一個經常食肉的人在食用牛排幾小時之內，血液內的氧化三甲胺水平會飆升。
研究者基於實驗室研究的理論是，氧化三甲胺令膽固醇進入動脈血管壁，並阻止身體排出多餘的膽固醇。
但是細菌看上了肉鹼什麼呢？黑曾醫生說，答案就是細菌把它當成一種能量來源。
他說他對那些含肉鹼的能量飲料感到擔憂。人們在這些飲料中加入肉鹼通常是認為它可以加速脂肪代謝，並提升一個人的能量水平。
令科羅拉多大學(University of Colorado)醫學教授、美國心臟病協會(American Heart Association)前會長羅伯特·H·埃克爾博士(Robert H. Eckel)擔心的是，肉鹼會對健美人士及運動員有影響，這些人認為肉鹼有增肌作用，因此經常飲用。
黑曾醫生說，這些補劑“很可怕，特別是對我們的孩子來說。”
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