(Psychomedia)
Une étude canadienne, publiée dans PLoS Biology, remet en question la théorie, qui prévaut depuis plus de 40 ans, selon laquelle les radicaux libres accélèrent le vieillissement en endommageant les cellules; dommages que préviennent les antioxydants en combattant ces radicaux libres.
Pour tester la théorie, Siegfried Hekimi et Wen Yang de l'Université McGill ont créé des vers mutants produisant plus de radicaux libres. Plutôt que de vivre moins longtemps, ces vers ont, au contraire, vécu plus longtemps. Et, plus est, l'administration d'antioxydants mettait fin à la longévité accrue.
Les chercheurs ont par la suite imité l'effet bénéfique apparent des radicaux libres en traitant des vers ordinaires au Paraquat, un herbicide qui entraîne l'augmentation de la production de radicaux libres. Les vers vivaient plus longtemps après cette exposition. Le Paraquat, toxique pour les humains et les animaux, est proscrit en Union européenne et son usage est limité dans plusieurs autres pays.
« Des expériences supplémentaires sont nécessaires pour découvrir exactement comment ces données pourraient modifier notre théorie sur le vieillissement », dit le professeur Hekimi. « Il est clair que les radicaux libres y contribuent, mais pas de la façon dont nous avions l'habitude d'y penser. »
Une étude britannique, publiée en 2008 dans la revue Genes and Development, mettait également en doute la théorie des radicaux libres du vieillissement en montrant que des vers modifiés pour présenter des pouvoirs antioxydants rehaussés ne vivaient pas plus longtemps.
Par ailleurs, une étude danoise analysant 67 recherches, publiée en 2008 dans la revue The Cochrane Library, montrait que les compléments alimentaires antioxydants ne réduisaient pas la mortalité et que certains (la vitamine A, la bêta-carotène et la vitamine E) pouvaient l'augmenter.
Affichage des articles dont le libellé est antioxydants. Afficher tous les articles
Affichage des articles dont le libellé est antioxydants. Afficher tous les articles
01 janvier 2011
17 novembre 2008
Vitamins do not reduce cancer risk, says study
James Randerson, science correspondent (The Guardian)
Taking vitamin A and E supplements does not lower your risk of cancer, according to the results of a large clinical trial involving nearly 15,000 men in the US.
Both vitamins are powerful antioxidants - substances that can tackle harmful byproducts of the body's metabolism which can cause DNA damage and hence trigger cancer. However, the study shows that taking the vitamins in supplement form has no effect at all on cancer.
"There have been a number of previous studies that have suggested that vitamin E and vitamin C might be important in the prevention of cancer," said Dr Howard Sesso, an epidemiologist at Brigham and Women's hospital in Boston, Massachusetts. These were mostly small lab studies or research on animals. But a 1998 study of men in Finland suggested that vitamin E supplements reduced prostate cancer cases by 32% and deaths by 41%.
"The lack of an effect that we observe for vitamin E or C on cancer does convince us that these particular doses that we tested really have no role for recommendation for cancer prevention," said Sesso.
His team recruited 14,641 male doctors and assigned them to four groups which took a different combination of the supplements or their placebos. The team looked at the number of deaths from cancer and found no statistical differences.
Sesso reported the results of the Physicians Health Study II trial at the American Association for Cancer Research's meeting in Prince George's county, Maryland.
Ed Yong, health information manager at Cancer Research UK, said there was growing evidence vitamin supplements did not prevent the risk of cancer. He said having a healthy diet was more important.
Taking vitamin A and E supplements does not lower your risk of cancer, according to the results of a large clinical trial involving nearly 15,000 men in the US.
Both vitamins are powerful antioxidants - substances that can tackle harmful byproducts of the body's metabolism which can cause DNA damage and hence trigger cancer. However, the study shows that taking the vitamins in supplement form has no effect at all on cancer.
"There have been a number of previous studies that have suggested that vitamin E and vitamin C might be important in the prevention of cancer," said Dr Howard Sesso, an epidemiologist at Brigham and Women's hospital in Boston, Massachusetts. These were mostly small lab studies or research on animals. But a 1998 study of men in Finland suggested that vitamin E supplements reduced prostate cancer cases by 32% and deaths by 41%.
"The lack of an effect that we observe for vitamin E or C on cancer does convince us that these particular doses that we tested really have no role for recommendation for cancer prevention," said Sesso.
His team recruited 14,641 male doctors and assigned them to four groups which took a different combination of the supplements or their placebos. The team looked at the number of deaths from cancer and found no statistical differences.
Sesso reported the results of the Physicians Health Study II trial at the American Association for Cancer Research's meeting in Prince George's county, Maryland.
Ed Yong, health information manager at Cancer Research UK, said there was growing evidence vitamin supplements did not prevent the risk of cancer. He said having a healthy diet was more important.
Voilà qui déplaira aux adeptes des théories de Linus Pauling, qui prétendait que des mégadoses de vitamine C permettait de lutter contre le cancer. Ces théories ont influencées des générations de parents éblouis par l'autorité du Prix Nobel de Chimie dans un domaine où il n'avait pas d'expertise. Il n'existe plus grand monde pour soutenir ces théories aujourd'hui, sauf les adeptes de la pseudo-médecine "orthomoléculaire", tels Matthias Rath et son lucratif business de cocktails vitaminés pour malades du tiers-monde.
30 avril 2007
Moderate Drinking Linked to Breast Cancer
WASHINGTON — Moderate alcohol consumption, or about two drinks a day, has often been touted as heart healthy in recent years, but a new study finds the same quantity causes cancer.
Mice given the human equivalent of two drinks daily developed breast tumors that were nearly double the weight of those in their “dry” relatives.
Nearly 179,000 U.S. women will develop breast cancer this year, according to the National Cancer Institute. Even so, scientists lack a strong grasp on why one woman develops the disease and another remains cancer free.
Presented here at the American Physiological Society (APS) annual meeting, the research not only shows the link between alcohol consumption and breast cancer, but it proposes how that glass of wine or bottle of beer works to stimulate tumor growth.
“Alcohol [consumption] is the most important avoidable risk factor for women getting breast cancer,” lead scientist Jian-Wei Gu of the University of Mississippi Medical Center told LiveScience. Genetic factors would be considered “unavoidable,” since people inherit DNA from their parents.
The APS meeting is being held as part of the larger, annual Experimental Biology meeting.
Drinkin’ mice
Gu and his colleagues, also from the University of Mississippi Medical Center, fed six-week-old female mice moderate levels of alcohol for four weeks which would be the equivalent of two drinks each day in humans. In the second week, they injected mouse breast-cancer cells into each mouse’s mammary glands.
They found that tumors in the alcohol-fed mice weighed on average about 1.4 grams, nearly two times the weight of tumors in the control mice. Images of the alcohol-fed mice show a visible lump in the upper portion of the animal’s chest.
The scientists suggest the difference in tumor weight is a result of jacked-up growth in blood vessels due to alcohol consumption. That’s because when a rodent, or a human, gulps down a drink, the cells in their bodies go into overdrive to get rid of the “toxins.”
The stressed-out cells send out a hormone dubbed VEGF that stimulates the growth of blood vessels—a tumor’s means of getting oxygen and nutrients to survive. Supporting that idea, the alcohol-fed mice showed a significant increase in VEGF as well as more blood vessels than the other mice.
Animal models
This is the first study to use an animal model that accurately mimics human breast cancer, Gu said.
Previous studies injected human breast-cancer cells into “nude” mice, or those lacking an immune system. Without a line of defense, the mice’s bodies would let the foreign cells grow and scientists could run experiments.
“In regular mice, if you implant a human tumor, it won’t grow because it’s foreign [so the immune system would stage an attack],” said study team member Emily Young. That also meant any results would apply only to an organism lacking an immune system.
Instead, “we used regular mice and we implanted the mouse breast-cancer form into it,” Young said.
As well, many studies have used toxic levels of alcohol, leading to results that were less applicable to humans who typically don’t consume that much booze.
The researchers say their results, unlike the others, can be squarely extrapolated to humans and have implications to both prevent and help treat breast cancer.
“Normal people produce cancer cells every day,” Gu said, “but at the beginning the cancer doesn’t have blood vessels.” So it’s easier for the body’s immune system to fend them off. Once the cancerous cells acquire a blood-vessel lifeline, which this study suggests is fostered by alcohol, the tumor growth takes off.
Mice given the human equivalent of two drinks daily developed breast tumors that were nearly double the weight of those in their “dry” relatives.
Nearly 179,000 U.S. women will develop breast cancer this year, according to the National Cancer Institute. Even so, scientists lack a strong grasp on why one woman develops the disease and another remains cancer free.
Presented here at the American Physiological Society (APS) annual meeting, the research not only shows the link between alcohol consumption and breast cancer, but it proposes how that glass of wine or bottle of beer works to stimulate tumor growth.
“Alcohol [consumption] is the most important avoidable risk factor for women getting breast cancer,” lead scientist Jian-Wei Gu of the University of Mississippi Medical Center told LiveScience. Genetic factors would be considered “unavoidable,” since people inherit DNA from their parents.
The APS meeting is being held as part of the larger, annual Experimental Biology meeting.
Drinkin’ mice
Gu and his colleagues, also from the University of Mississippi Medical Center, fed six-week-old female mice moderate levels of alcohol for four weeks which would be the equivalent of two drinks each day in humans. In the second week, they injected mouse breast-cancer cells into each mouse’s mammary glands.
They found that tumors in the alcohol-fed mice weighed on average about 1.4 grams, nearly two times the weight of tumors in the control mice. Images of the alcohol-fed mice show a visible lump in the upper portion of the animal’s chest.
The scientists suggest the difference in tumor weight is a result of jacked-up growth in blood vessels due to alcohol consumption. That’s because when a rodent, or a human, gulps down a drink, the cells in their bodies go into overdrive to get rid of the “toxins.”
The stressed-out cells send out a hormone dubbed VEGF that stimulates the growth of blood vessels—a tumor’s means of getting oxygen and nutrients to survive. Supporting that idea, the alcohol-fed mice showed a significant increase in VEGF as well as more blood vessels than the other mice.
Animal models
This is the first study to use an animal model that accurately mimics human breast cancer, Gu said.
Previous studies injected human breast-cancer cells into “nude” mice, or those lacking an immune system. Without a line of defense, the mice’s bodies would let the foreign cells grow and scientists could run experiments.
“In regular mice, if you implant a human tumor, it won’t grow because it’s foreign [so the immune system would stage an attack],” said study team member Emily Young. That also meant any results would apply only to an organism lacking an immune system.
Instead, “we used regular mice and we implanted the mouse breast-cancer form into it,” Young said.
As well, many studies have used toxic levels of alcohol, leading to results that were less applicable to humans who typically don’t consume that much booze.
The researchers say their results, unlike the others, can be squarely extrapolated to humans and have implications to both prevent and help treat breast cancer.
“Normal people produce cancer cells every day,” Gu said, “but at the beginning the cancer doesn’t have blood vessels.” So it’s easier for the body’s immune system to fend them off. Once the cancerous cells acquire a blood-vessel lifeline, which this study suggests is fostered by alcohol, the tumor growth takes off.
27 février 2007
Antioxidant Supplements Up Death Risk
Study Shows No Benefit, Slightly Higher Death Risk for Antioxidant Supplements
By Daniel DeNoon
WebMD Medical News
Reviewed By Louise Chang, MD
Feb. 27, 2007 – Use of the popular antioxidant supplements beta-carotene, vitamin E, or vitamin A slightly increases a person's risk of death, an overview of human studies shows.
The study also shows no benefit -- and no harm -- for vitamin C supplements. Selenium supplements tended to very slightly reduce risk of death.
Oxidative stress -- caused by highly reactive "free radical" compounds circulating in the blood -- is a factor in most diseases.
Antioxidants sweep up these free radicals. It seems to be a no-brainer that taking antioxidant supplements would protect your health. But it may not be that simple.
A new, detailed analysis of human studies of beta-carotene, vitamin A, and vitamin E shows that people who take these antioxidant supplements don't live any longer than those who don't take them. In fact, those who take the supplements have an increased risk of death.
The finding, reported in The Journal of the American Medical Association, comes from Goran Bjelakovic, MD, DrMedSci, of the University of Nis in Serbia; Christian Gluud, MD, DrMedSci, of Copenhagen University Hospital in Denmark; and colleagues.
"Our findings have already changed the way I counsel my patients about antioxidant supplements," Bjelakovic tells WebMD in an email interview. "According to our findings, beta-carotene, vitamin A, and vitamin E cannot be recommended. I am telling them that they should stop using these supplements."
"There is no reason to take anything that hasn't been proven beneficial. And these antioxidant supplements do not seem beneficial at all," Gluud tells WebMD.
Not everyone agrees. Nutritionist Andrew Shao, PhD, is vice president for scientific and regulatory affairs at the Council for Responsible Nutrition, a supplement-industry trade group.
"Consumers can feel confident in relying on their antioxidant supplements as they always have," Shao tells WebMD. "They can continue to take them knowing they will provide the same benefits -- and this article does not change that."
Antioxidant Supplements and Death Risk
Bjelakovic, Gluud, and colleagues analyzed data from 68 randomized clinical trials of antioxidant supplements that included 232,606 people. When they looked at all the trials together, they found that the supplements offered no benefit but did no harm.
However, some of the trials were more exactly controlled than others. There were 21 trials that had a "high bias risk." These trials had one or more problems with randomizing study participants to the supplement or placebo groups, with blinding both the participants and the investigators to whether participants received supplements or placebos, and/or with following up on all participants until the end of the study.
So the researchers looked only at the 47 "low-bias-risk" studies -- which included nearly 181,000 participants and which did not include people taking selenium. They found that:
* Taking vitamin A supplements increased the risk of death by 16%.
* Taking beta-carotene supplements increased the risk of death by 7%.
* Taking vitamin E supplements increased the risk of death by 4%.
* Taking vitamin C supplements did not have any effect on risk of death.
Shao says it just isn't fair to study antioxidants in this way.
"What these authors have done is combine studies that are incredibly dissimilar in all sorts of ways," he says. "These studies looked at different nutrients at different doses at different durations with different lengths of follow-up -- and in different populations, ranging from folks who were incredibly healthy to people with cancercancer and other diseases."
Moreover, Shao says, the researchers looked only at studies in which people died. That left out 405 clinical trials, which he says skews the results in favor of death risk. And he points out that the researchers original 68 studies did not show any harm from supplements.
"These questions cause one to step back and wonder if the findings are relevant to the healthy population that uses these supplements to maintain health and avoid chronic disease," Shao says. "That is a point they don't make: that antioxidants are not used to treat cancer or heart diseaseheart disease. They are used for disease prevention."
Edgar R. Miller III, MD, PhD, associate professor of medicine at Johns Hopkins University, in 2004 analyzed clinical trials of vitamin E. He found that high doses of vitamin E did more harm than good. Miller has high praise for the Bjelakovic/Gluud study.
"This is a great study. It is the highest form of scientific evidence," Miller tells WebMD. "I don't think that [Shao's] criticism is legitimate. I argue this is the best technique to analyze all this information."
Gluud and Bjelakovic strongly disagree that they "cherry picked" only studies that fit some preconceived conclusion. They point out that all of their methods are "transparent" and open to public view.
"Anyone is welcome to criticize our research," Gluud says. "But my question is, what is your evidence? I think the parties that want to sell or use these antioxidant supplements in the dosages used in these trials, they want [to see only] positive evidence that it works beneficially."
Advice to Consumers
Kathleen Zelman, MPH, RD, LD, is director of nutrition for WebMD. She reviewed the Bjelakovic/Gluud study for this article.
"This is a very comprehensive, to-be-respected analysis. This isn't just another study coming out," Zelman says. "The bottom line is that antioxidant supplements are not a magic bullet for disease prevention. We hoped maybe they were, but they are not."
If you are interested in protecting your health, Zelman says, pills aren't the answer.
"There is no single food or nutrient that is going to be the answer. The secret really is lifestyle," she says. "And the most important things about lifestyle are being at a healthy weight, being physically active, and eating a healthy diet."
Shao says he's not persuaded to stop taking antioxidant supplements.
"I take antioxidant supplements every day," he says. "I know more about these nutrients than most people do, including the authors of this study, who are not nutritionists. This does not change a thing for me. You can take that to the bank."
Zelman has this advice: If you plan to continue taking antioxidant supplements, don't exceed the recommended daily doses.
"For nutritional insurance, my suggestion would be a once-daily multivitamin," she says. "But for those people who take multiple supplements, and are going to continue to do so, heed the warning and be sure to respect the safe upper dosage limits."
"If you are in doubt, take the time and go to your doctor and talk with her or him," Gluud advises.
SOURCES: Bjelakovic, G. The Journal of the American Medical Association, Feb. 28, 2007; vol 297: pp 842-857. Goran Bjelakovic, MD, DrMedSci, University of Nis, Serbia (email interview). Christian Gluud, MD, DrMedSci, Copenhagen University Hospital, Denmark. Andrew Shao, PhD, vice president for scientific and regulatory affairs, Council for Responsible Nutrition, Washington, D.C. Edgar R. Miller III, MD, PhD, associate professor of medicine, Johns Hopkins University, Baltimore. Kathleen Zelman, MPH, RD, LD, director of nutrition, WebMD, Atlanta.
By Daniel DeNoon
WebMD Medical News
Reviewed By Louise Chang, MD
Feb. 27, 2007 – Use of the popular antioxidant supplements beta-carotene, vitamin E, or vitamin A slightly increases a person's risk of death, an overview of human studies shows.
The study also shows no benefit -- and no harm -- for vitamin C supplements. Selenium supplements tended to very slightly reduce risk of death.
Oxidative stress -- caused by highly reactive "free radical" compounds circulating in the blood -- is a factor in most diseases.
Antioxidants sweep up these free radicals. It seems to be a no-brainer that taking antioxidant supplements would protect your health. But it may not be that simple.
A new, detailed analysis of human studies of beta-carotene, vitamin A, and vitamin E shows that people who take these antioxidant supplements don't live any longer than those who don't take them. In fact, those who take the supplements have an increased risk of death.
The finding, reported in The Journal of the American Medical Association, comes from Goran Bjelakovic, MD, DrMedSci, of the University of Nis in Serbia; Christian Gluud, MD, DrMedSci, of Copenhagen University Hospital in Denmark; and colleagues.
"Our findings have already changed the way I counsel my patients about antioxidant supplements," Bjelakovic tells WebMD in an email interview. "According to our findings, beta-carotene, vitamin A, and vitamin E cannot be recommended. I am telling them that they should stop using these supplements."
"There is no reason to take anything that hasn't been proven beneficial. And these antioxidant supplements do not seem beneficial at all," Gluud tells WebMD.
Not everyone agrees. Nutritionist Andrew Shao, PhD, is vice president for scientific and regulatory affairs at the Council for Responsible Nutrition, a supplement-industry trade group.
"Consumers can feel confident in relying on their antioxidant supplements as they always have," Shao tells WebMD. "They can continue to take them knowing they will provide the same benefits -- and this article does not change that."
Antioxidant Supplements and Death Risk
Bjelakovic, Gluud, and colleagues analyzed data from 68 randomized clinical trials of antioxidant supplements that included 232,606 people. When they looked at all the trials together, they found that the supplements offered no benefit but did no harm.
However, some of the trials were more exactly controlled than others. There were 21 trials that had a "high bias risk." These trials had one or more problems with randomizing study participants to the supplement or placebo groups, with blinding both the participants and the investigators to whether participants received supplements or placebos, and/or with following up on all participants until the end of the study.
So the researchers looked only at the 47 "low-bias-risk" studies -- which included nearly 181,000 participants and which did not include people taking selenium. They found that:
* Taking vitamin A supplements increased the risk of death by 16%.
* Taking beta-carotene supplements increased the risk of death by 7%.
* Taking vitamin E supplements increased the risk of death by 4%.
* Taking vitamin C supplements did not have any effect on risk of death.
Shao says it just isn't fair to study antioxidants in this way.
"What these authors have done is combine studies that are incredibly dissimilar in all sorts of ways," he says. "These studies looked at different nutrients at different doses at different durations with different lengths of follow-up -- and in different populations, ranging from folks who were incredibly healthy to people with cancercancer and other diseases."
Moreover, Shao says, the researchers looked only at studies in which people died. That left out 405 clinical trials, which he says skews the results in favor of death risk. And he points out that the researchers original 68 studies did not show any harm from supplements.
"These questions cause one to step back and wonder if the findings are relevant to the healthy population that uses these supplements to maintain health and avoid chronic disease," Shao says. "That is a point they don't make: that antioxidants are not used to treat cancer or heart diseaseheart disease. They are used for disease prevention."
Edgar R. Miller III, MD, PhD, associate professor of medicine at Johns Hopkins University, in 2004 analyzed clinical trials of vitamin E. He found that high doses of vitamin E did more harm than good. Miller has high praise for the Bjelakovic/Gluud study.
"This is a great study. It is the highest form of scientific evidence," Miller tells WebMD. "I don't think that [Shao's] criticism is legitimate. I argue this is the best technique to analyze all this information."
Gluud and Bjelakovic strongly disagree that they "cherry picked" only studies that fit some preconceived conclusion. They point out that all of their methods are "transparent" and open to public view.
"Anyone is welcome to criticize our research," Gluud says. "But my question is, what is your evidence? I think the parties that want to sell or use these antioxidant supplements in the dosages used in these trials, they want [to see only] positive evidence that it works beneficially."
Advice to Consumers
Kathleen Zelman, MPH, RD, LD, is director of nutrition for WebMD. She reviewed the Bjelakovic/Gluud study for this article.
"This is a very comprehensive, to-be-respected analysis. This isn't just another study coming out," Zelman says. "The bottom line is that antioxidant supplements are not a magic bullet for disease prevention. We hoped maybe they were, but they are not."
If you are interested in protecting your health, Zelman says, pills aren't the answer.
"There is no single food or nutrient that is going to be the answer. The secret really is lifestyle," she says. "And the most important things about lifestyle are being at a healthy weight, being physically active, and eating a healthy diet."
Shao says he's not persuaded to stop taking antioxidant supplements.
"I take antioxidant supplements every day," he says. "I know more about these nutrients than most people do, including the authors of this study, who are not nutritionists. This does not change a thing for me. You can take that to the bank."
Zelman has this advice: If you plan to continue taking antioxidant supplements, don't exceed the recommended daily doses.
"For nutritional insurance, my suggestion would be a once-daily multivitamin," she says. "But for those people who take multiple supplements, and are going to continue to do so, heed the warning and be sure to respect the safe upper dosage limits."
"If you are in doubt, take the time and go to your doctor and talk with her or him," Gluud advises.
SOURCES: Bjelakovic, G. The Journal of the American Medical Association, Feb. 28, 2007; vol 297: pp 842-857. Goran Bjelakovic, MD, DrMedSci, University of Nis, Serbia (email interview). Christian Gluud, MD, DrMedSci, Copenhagen University Hospital, Denmark. Andrew Shao, PhD, vice president for scientific and regulatory affairs, Council for Responsible Nutrition, Washington, D.C. Edgar R. Miller III, MD, PhD, associate professor of medicine, Johns Hopkins University, Baltimore. Kathleen Zelman, MPH, RD, LD, director of nutrition, WebMD, Atlanta.
C'est le même cas que pour les études montrant les effets positifs des pseudo-médecines (homéopathie, acupuncture,...). Plus les études sont scientifiques, plus les résultats sont proches de l'effet placebo. Le conflit d'intérêt des labos producteurs aidant, seuls les résultats positifs font l'objet de publicité. Ce qui laisse un énorme doute sur les prétentions de ces labos.
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