American College of Physicians: Internal Medicine — Doctors for Adults ®

Wednesday, June 12, 2013

A new beef with meat and eggs? My gut reaction

As you have likely heard, assuming you weren't grazing on another planet over the past couple of weeks, there may be a new reason to eat less meat. While we were still digesting the study about meat, we were served the news that there may be a new reason to eat fewer eggs, as well. But before we wind up overcooking this potential new beef with meat and eggs, we'd best carve up the particulars and chew carefully.

The new studies address the generation of a compound called trimethylamine-N-oxide, or TMAO. TMAO has been shown to induce atherosclerosis in mice, and levels of TMAO in the blood of humans correlate with rates of cardiovascular disease, although that does not prove causality.

TMAO is potentially generated in the body following intake of l-carnitine, a protein found in meat, and following ingestion of choline found in lecithin, also known as phosphatidylcholine, a fat-like compound present in eggs among other foods. But this part of the story is anything but straightforward.

The first complicating consideration is that the generation of TMAO from foods containing either l-carnitine or lecithin depends on the presence of specific bacteria in the GI tract. It is the actions of intestinal bacteria that metabolize nutrient precursors into TMAO. But we also know that dietary pattern influences those same bacterial populations. So food intake influences what happens to our intestinal microbes, and our intestinal microbes influence what happens to our food. Stated differently, we may not yet know whether chicken and eggs are chicken, or egg, in this scenario.

The study linking meat intake to TMAO production demonstrated that more carnitine was converted to TMAO by omnivores than by vegans. This suggests that eating meat routinely affects the balance of intestinal bacteria, which in turn affects what happens when meat is eaten. Vegans, it seems, have gut microbes less prone to make TMAO even when they do encounter carnitine, although they do make some.

The second consideration is that both l-carnitine and choline are found in diverse foods, including plants. Carnitine is most concentrated in meat, but it is found in wheat and asparagus, along with dairy and some fish. Choline is most concentrated in eggs, and to a lesser extent, meat, but it is also found in fish, grains, vegetables, and fruits. And so, whatever the new research means, it does not mean we should aim to avoid either carnitine or choline completely, as no healthful, balanced diet would allow for that. Nor can we avoid making some TMAO. So this is a story about gradations, not either/or choices.

The third consideration is that the study of meat compared the metabolic responses of vegans to those of omnivores. But there are many ways to be an omnivore. In our culture, the most common kind of omnivore is a devotee of the typical American diet. The typical American diet isn't just a mix of plant and animal foods, however; it's a mix of real food and junk. A third to half the calories in the prevailing version of modern omnivorousness comes from junk food.

In contrast, vegans tend to be among the most conscious and careful eaters in modern society. Being vegan generally indicates a commitment to some blend of health and ethics, and both require thoughtful choices. A balanced, prudent vegan diet isn't just totally plant-based, it is substantially junk-free. Vegans eating junk exist, of course, but they are few and far between, and they are doing veganism wrong.

We Homo sapiens are constitutional omnivores. We have in fact been omnivorous far longer than we have been sapiens. But in modern context, an omnivore is generally not someone who eats mostly plants along with some wild salmon, organic eggs, and a bit of bison. In our culture, an omnivore is someone who eats mac n' cheese, salami, pepperoni pizza, and Big Macs. In contrast, vegans are, generally, not just people who only eat plants. The differences here are not subtle, and go well beyond the inclusion of some meat or eggs in the diet. A comparison between a group of omnivores and a group of vegans is likely to be a comparison between one group eating badly, and another eating well. That such differences would influence metabolism, intestinal flora, and health outcomes is far from shocking.

Yet another consideration is that our Stone Age ancestors, while reliably omnivores, didn't just eat mammoth rather than meatloaf; they did so in a pristinely germy world, free of antibiotics, food chemicals, or processing other than applications of heat and cold. We know that antibiotics can wreak havoc with our intestinal microbes, as well as propagate antimicrobial resistance -- and that antibiotics in feed animals may be a more important source for most of us than antibiotics we take on purpose. Perhaps some of the TMAO story is not all about carnitine or choline, but also the company they keep in the flesh of modern feed animals.

A fifth consideration is the tendency to conflate new research about potential health risks, with new potential health risks. When new studies are published, we may tend to think we face new threats. But meat and eggs are not suddenly more dangerous than they were yesterday; we just have new insight into the mechanisms of any such danger.

And yesterday, meat and eggs per se may not have been dangerous at all. We have evidence from large cohort studies that processed meat is bad for health, but pure meats are less so, and in moderation may not be harmful. Health benefits associated with the Paleo diet suggest that game intake might even confer health benefit. Health benefit has consistently been associated with fish intake, even despite the worry about contaminants. The weight of evidence suggests that egg intake is largely or entirely free of harmful effects for most people, and might be associated with some benefit depending on how foods are used to replace one another. And studies of healthful dietary patterns that include eggs and certain meats, the Mediterranean diet noteworthy among them, show health benefits at least as decisive as any linked to veganism. New insights about TMAO do not undo any of the epidemiologic science that came before.

That said, there are excellent arguments for veganism. Eating only plants, done well, figures among contenders for the most healthful diet, although it's not the clear winner. In addition, veganism is much about ethics, and avoids the abuse of animals practiced on behalf of often unknowing omnivores. It makes more efficient use of food energy as well, and is a kinder, gentler approach to the planet. In a world of 7 billion Homo sapiens, the case for veganism beyond individual health is strong.

Getting back to TMAO, a sixth and final consideration is salient. This new research highlights the dependence of our health on the bacterial populations within. This is a fascinating, profoundly important area we are only beginning to understand, though some relevant insights have been germinating for decades. I commend to all interested in this burgeoning field the book An Epidemic of Absence. We and our bacterial tenants are in this together, and are together influenced by genes, diet, and environmental exposures.

Which leads to my conclusions. We had compelling reasons to eat mostly plants and close to nature before, and TMAO may be a new entry on that list. We knew before that junk food, processed meat, and random antibiotic exposures were doing us no favors. We had no such clear indictments, however, of fish, lean meats, and eggs. And for those inclined to include these in their diets, the TMAO studies do not as yet provide a clear argument to do otherwise. We should all eat mostly plants, but arguments for eating only plants derive from considerations other than individual health.

The new research about TMAO is provocative, and clearly important. We have more to learn about how our food choices influence the microbes that populate our guts, and how those microbes in turn influence our gut reactions. For now, my gut reaction to the news about TMAO is that context matters. It's not really just about meat or eggs in the diet, but about the overall dietary pattern, the kinds of meat, the feed and treatment of the egg-laying hens, and our exposures to chemicals and antibiotics. Whenever we have invested too much passion, anxiety, or hope in one food, nutrient, or chronic disease risk marker, we have been proved wrong in the fullness of time.

What we eat influences our health, and it turns out we are a community inside our own skins. So "our" health is not just ours, but shared with the other residents in our personal ecosystem. What we eat influences that whole community.

So gut reactions and the germs on which they depend are far more germane to this tale than most. There is a village within, on which the digestion of an egg, or edamame for that matter, and the basic care and feeding of Homo sapiens all depend. Figuring out how each of us alone, and all of us together, may best tend that ecosystem is vitally important food for thought, and we are really just starting to chew on it.

David L. Katz, MD, FACP, MPH, FACPM, is an internationally renowned authority on nutrition, weight management, and the prevention of chronic disease, and an internationally recognized leader in integrative medicine and patient-centered care. He is a board certified specialist in both Internal Medicine, and Preventive Medicine/Public Health, and Associate Professor (adjunct) in Public Health Practice at the Yale University School of Medicine. He is the Director and founder (1998) of Yale University's Prevention Research Center; Director and founder of the Integrative Medicine Center at Griffin Hospital (2000) in Derby, Conn.; founder and president of the non-profit Turn the Tide Foundation; and formerly the Director of Medical Studies in Public Health at the Yale School of Medicine for eight years. This post originally appeared on his blog at The Huffington Post.

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Members of the American College of Physicians contribute posts from their own sites to ACP Internistand ACP Hospitalist. Contributors include:

Albert Fuchs, MD
Albert Fuchs, MD, FACP, graduated from the University of California, Los Angeles School of Medicine, where he also did his internal medicine training. Certified by the American Board of Internal Medicine, Dr. Fuchs spent three years as a full-time faculty member at UCLA School of Medicine before opening his private practice in Beverly Hills in 2000.

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Ira S. Nash, MD, FACP, is the senior vice president and executive director of the North Shore-LIJ Medical Group, and a professor of Cardiology and Population Health at Hofstra North Shore-LIJ School of Medicine. He is Board Certified in Internal Medicine and Cardiovascular Diseases and was in the private practice of cardiology before joining the full-time faculty of Massachusetts General Hospital.

Zackary Berger
Zackary Berger, MD, ACP Member, is a primary care doctor and general internist in the Division of General Internal Medicine at Johns Hopkins. His research interests include doctor-patient communication, bioethics, and systematic reviews.

Controversies in Hospital Infection Prevention
Run by three ACP Fellows, this blog ponders vexing issues in infection prevention and control, inside and outside the hospital. Daniel J Diekema, MD, FACP, practices infectious diseases, clinical microbiology, and hospital epidemiology in Iowa City, Iowa, splitting time between seeing patients with infectious diseases, diagnosing infections in the microbiology laboratory, and trying to prevent infections in the hospital. Michael B. Edmond, MD, FACP, is a hospital epidemiologist in Iowa City, IA, with a focus on understanding why infections occur in the hospital and ways to prevent these infections, and sees patients in the inpatient and outpatient settings. Eli N. Perencevich, MD, ACP Member, is an infectious disease physician and epidemiologist in Iowa City, Iowa, who studies methods to halt the spread of resistant bacteria in our hospitals (including novel ways to get everyone to wash their hands).

db's Medical Rants
Robert M. Centor, MD, FACP, contributes short essays contemplating medicine and the health care system.

Suneel Dhand, MD, ACP Member
Suneel Dhand, MD, ACP Member, is a practicing physician in Massachusetts. He has published numerous articles in clinical medicine, covering a wide range of specialty areas including; pulmonology, cardiology, endocrinology, hematology, and infectious disease. He has also authored chapters in the prestigious "5-Minute Clinical Consult" medical textbook. His other clinical interests include quality improvement, hospital safety, hospital utilization, and the use of technology in health care.

Juliet K. Mavromatis, MD, FACP, provides a conversation about health topics for patients and health professionals.

Dr. Mintz' Blog
Matthew Mintz, MD, FACP, has practiced internal medicine for more than a decade and is an Associate Professor of Medicine at an academic medical center on the East Coast. His time is split between teaching medical students and residents, and caring for patients.

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Toni Brayer, MD, FACP, blogs about the rapid changes in science, medicine, health and healing in the 21st century.

Vineet Arora, MD, FACP, is Associate Program Director for the Internal Medicine Residency and Assistant Dean of Scholarship & Discovery at the Pritzker School of Medicine for the University of Chicago. Her education and research focus is on resident duty hours, patient handoffs, medical professionalism, and quality of hospital care. She is also an academic hospitalist.

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John H. Schumann, MD, FACP, provides transparency on the workings of medical practice and the complexities of hospital care, illuminates the emotional and cognitive aspects of caregiving and decision-making from the perspective of an active primary care physician, and offers behind-the-scenes portraits of hospital sanctums and the people who inhabit them.

Gut Check
Ryan Madanick, MD, ACP Member, is a gastroenterologist at the University of North Carolina School of Medicine, and the Program Director for the GI & Hepatology Fellowship Program. He specializes in diseases of the esophagus, with a strong interest in the diagnosis and treatment of patients who have difficult-to-manage esophageal problems such as refractory GERD, heartburn, and chest pain.

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Mike Aref, MD, PhD, FACP, is an academic hospitalist with an interest in basic and clinical science and education, with interests in noninvasive monitoring and diagnostic testing using novel bedside imaging modalities, diagnostic reasoning, medical informatics, new medical education modalities, pre-code/code management, palliative care, patient-physician communication, quality improvement, and quantitative biomedical imaging.

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William Hersh, MD, FACP, Professor and Chair, Department of Medical Informatics & Clinical Epidemiology, Oregon Health & Science University, posts his thoughts on various topics related to biomedical and health informatics.

David Katz, MD
David L. Katz, MD, MPH, FACP, is an internationally renowned authority on nutrition, weight management, and the prevention of chronic disease, and an internationally recognized leader in integrative medicine and patient-centered care.

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Kevin Pho, MD, ACP Member, offers one of the Web's definitive sites for influential health commentary.

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Elaine Schattner, MD, FACP, shares her ideas on education, ethics in medicine, health care news and culture. Her views on medicine are informed by her past experiences in caring for patients, as a researcher in cancer immunology, and as a patient who's had breast cancer.

Mired in MedEd
Alexander M. Djuricich, MD, FACP, is the Associate Dean for Continuing Medical Education (CME), and a Program Director in Medicine-Pediatrics at the Indiana University School of Medicine in Indianapolis, where he blogs about medical education.

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The Blog of Paul Sufka
Paul Sufka, MD, ACP Member, is a board certified rheumatologist in St. Paul, Minn. He was a chief resident in internal medicine with the University of Minnesota and then completed his fellowship training in rheumatology in June 2011 at the University of Minnesota Department of Rheumatology. His interests include the use of technology in medicine.

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Neil Mehta, MBBS, MS, FACP, is interested in use of technology in education, social media and networking, practice management and evidence-based medicine tools, personal information and knowledge management.

Peter A. Lipson, MD
Peter A. Lipson, MD, ACP Member, is a practicing internist and teaching physician in Southeast Michigan. The blog, which has been around in various forms since 2007, offers musings on the intersection of science, medicine, and culture.

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Other blogs of note:

American Journal of Medicine
Also known as the Green Journal, the American Journal of Medicine publishes original clinical articles of interest to physicians in internal medicine and its subspecialities, both in academia and community-based practice.

Clinical Correlations
A collaborative medical blog started by Neil Shapiro, MD, ACP Member, associate program director at New York University Medical Center's internal medicine residency program. Faculty, residents and students contribute case studies, mystery quizzes, news, commentary and more.

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PLoS Blog
The Public Library of Science's open access materials include a blog.

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One of the most popular anonymous blogs written by an emergency room physician.

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