L'ail ne fait pas baisser le "mauvais" cholestérol

JEAN-MICHEL BADER
LeFigaro.fr

DEPUIS 3 500 ANS, l'ail (allium sativum) et l'ail sauvage (allium ursinum) sont utilisés pour lutter contre les maladies cardiovascu­laires. La première description date du Codex Ebers de l'ancienne Égypte. Depuis deux cents ans, la médecine occidentale s'est emparée des propriétés médicinales de l'ail et plus de 100 études cliniques ont été publiées sur ses effets préventifs dans les maladies cardiaques et vasculaires. Les firmes commerciales font une promotion sans freins de cet effet supposé protecteur contre le cholestérol et ce n'est pas par hasard qu'aux États-Unis, l'ail sous forme de suppléments soit le n° 1 des ventes des épices. L'ail écrasé libère de l'alliine et son enzyme l'alliinase, pour produire l'allicine, principe actif.

Or, une étude publiée aujourd'hui dans les Archives of Internal Medicine, par Christopher Gardner (université de Stanford), met à mal cette croyance. En effet, pendant six mois, près de 200 sujets volontaires âgés de 30 à 65 ans, et ayant une concentration de LDL cholestérol (le « mauvais » cholestérol) « modérément élevée » (130 à 190 mg par dl), ont participé à un essai clinique de l'ail. Six jours sur sept, les participants ont reçu soit de l'ail cru réduit en poudre, soit l'une de deux préparations commerciales d'ail, la Garciline ou le Kyolic 100, soit un placebo.

Première étude d'envergure

La quantité d'ail quotidienne correspondait environ à une gousse de taille moyenne. Tous les mois, la composition des suppléments à l'ail et la concentration sanguine de LDL cholestérol des cobayes humains étaient mesurées. Entre novembre 2002 et juin 2005, 169 sujets de l'étude ont poursuivi le traitement. Ni l'ail cru, ni les deux préparations commerciales ne sont capables de diminuer significativement le LDL cholestérol des sujets de cette étude, la première de cette envergure pu­bliée dans une revue savante. La diminution maximale observée de la concentration sanguine de LDL cholestérol ne dépasse pas les 10 mg par décilitre de sang.

Le « bon » cholestérol (HDL), les triglycérides et le rapport des deux cholestérols, marqueurs de la santé lipidique de l'individu, n'ont pas du tout été modifiés.

Seul effet secondaire notable : une haleine et une odeur d'ail ­prononcée ont été observées par 28 participants du groupe ayant consommé de l'ail cru.

Pour autant, ce n'est pas la démonstration définitive que l'ail n'a aucune vertu préventive. Pour évaluer l'effet protecteur de tout agent sur le coeur et les vaisseaux, dans de tels essais cliniques, explique un éditorial de la revue américaine, signé du Dr Mary Charlson de l'université Cornell (New York), il faut définir l'objectif primaire ad hoc. « Le LDL est la cible princi­pale mais pas la seule », a-t-elle expliqué au Figaro.

Si les anomalies de la concentration des lipides (comme le cholestérol par exemple) sont des facteurs contributifs importants, ce ne serait pas uniquement le LDL cholestérol qui contribuerait à la plaque d'athérome. « Il faut compter avec l'inflammation, l'hyper­tension artérielle, l'agrégation des plaquettes san­guines, le diabète, le tabagisme et la génétique », complète Mary Charlson.

Un certain nombre de 'croyances de grand-mères' subsistent bien des années après qu'on les ait invalidées par des études scientifiques. A un moment, il faudra bien arrêter de se les répéter.

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.

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.