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Tuesday, July 2, 2013

Little evidence that low-salt diet prevents heart disease

We know that people with high cholesterol have a higher risk for strokes and heart attacks than people with low cholesterol. So if a medicine lowers cholesterol it should also lower the frequency of strokes and heart attacks too. Right? Not necessarily. Estrogen lowers cholesterol and doesn't lower stroke or heart attack risk. We also know that people with high blood pressure have a higher risk for strokes and heart attacks. Does that mean that a food that elevates blood pressure increases stroke and heart attack risk? Again, not necessarily.

The confusion here is a misunderstanding of the difference between clinical outcomes and intermediate outcomes. A clinical outcome is something that a patient notices herself and that impacts her life directly, like a stroke, a heart attack, or a bone fracture. An intermediate outcome is something that is measured by the doctor and that doesn't cause symptoms directly, for example, elevated blood pressure, elevated cholesterol, or low bone density. Intermediate outcomes can be risk factors for clinical outcomes but shouldn't be confused with them.

What does this have to do with salt?

Lots of evidence shows that eating more salt raises blood pressure, so doctors have always made the assumption that eating more salt also increases the risk of strokes and heart attacks. But as we've seen with estrogen and many other examples, guessing the effects on clinical outcomes from intermediate outcomes is frequently incorrect.

In 2005, the U.S. Department of Agriculture and Department of Health and Human Services wanted to revise their dietary recommendations for salt intake. Given the very little scientific evidence they had, what they did was both simple and presumptuous. They knew that 1,500 mg of sodium intake daily was the minimum needed for adequate nutrition. They also knew that at daily intake levels above 2,300 mg of sodium (which is about a teaspoon of salt) blood pressure begins to increase. So the U.S. recommendations since 2005 have been that everyone should eat no more than 2,300 mg of sodium daily, and that people at very high risk of stroke and heart attack should ingest no more than 1,500 mg.

How are we doing? Well, on average Americans ingest 3,400 mg of sodium daily, well above the recommendations. A host of policy initiatives has been spawned by the recommendations in an effort to educate consumers, clarify food labels and coerce restaurants to lower sodium.

But did anyone test the effects on the clinical endpoints?

The institute of Medicine (IOM) was commissioned to review all the studies relating to the health effects of sodium intake. Their report (which is over 150 pages) was released last week. A major conclusion of the IOM paper is that the quality of the current evidence linking salt to health outcomes is very poor. There are virtually no randomized studies and the rest of the studies suffer from important methodological flaws (like imprecisely measuring salt intake or using self-reported food diaries to estimate salt intake). The surprising and worrisome finding was that some of the randomized trials actually found worse outcomes with very low salt intakes. This isn't as preposterous as it may sound. We have no solid understanding on salt's effect on the body beyond that on blood pressure, so there could be many mechanisms that could explain worse cardiovascular outcomes with a very low salt diet.

The IOM endorsed the current belief that there is very likely a quantity of daily salt intake above which the risk of cardiovascular disease increases. The current evidence is simply insufficient to figure out what that limit is.

I'm always impressed when science comes up with the answer, "We have no idea," because that's very likely to be honest. Those who are more committed to enacting policy than to figuring out the truth are less likely to confess ignorance and to wait for better studies before making up their minds. The American Heart Association issued a press report criticizing the IOM paper and arguing essentially "But salt increases blood pressure!" which no one disputes.

So for now add me to the list of salt agnostics. I frequently ask patients to cut down on salt in the short term to avoid fluid retention, for example when traveling. But we should have the honesty to admit that in terms of long term outcomes we don't know how much salt is too much.

And if you're not going to eat that pickle, can I have it?

Learn more:
No Benefit Seen in Sharp Limits on Salt in Diet (New York Times)
Low-Salt Benefits Questioned (Wall Street Journal)
Is Eating Too Little Salt Risky? New Report Raises Questions (NPR)
Sodium Intake in Populations: Assessment of Evidence (Institute of Medicine)
Shaking the Salt Habit (American Heart Association)
New IOM report an incomplete review of sodium's impact, says American Heart Association (American Heart Association Media Alert)
Merck Knows More about Zetia than They're Telling Us (my post in 2007 explaining the difference between clinical outcomes and intermediate outcomes)

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. Holding privileges at Cedars-Sinai Medical Center, he is also an assistant clinical professor at UCLA's Department of Medicine. This post originally appeared at his blog.

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1 Comments:

Anonymous Stephen in Cleveland said...

Thanks for sharing this post. Too me this research shows just how much we still have left to learn when it comes to conditions like high blood pressure and high cholesterol. In fact, this post reminded me of another article that I read just last year on HDL or good cholesterol. Apparently, researchers had found that higher levels of HDL did not appear to help reduce the risk of heart attacks like what had previously been claimed.

August 19, 2013 at 12:01 AM  

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Blog log

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.

And Thus, It Begins
Amanda Xi, ACP Medical Student Member, is a first-year medical student at the OUWB School of Medicine, charter class of 2015, in Rochester, Mich., from which she which chronicles her journey through medical training from day 1 of medical school.

Auscultation
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.

DrDialogue
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.

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

FutureDocs
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.

Glass Hospital
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.

I'm dok
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.

Informatics Professor
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.

Just Oncology
Richard Just, MD, ACP Member, has 36 years in clinical practice of hematology and medical oncology. His blog is a joint publication with Gregg Masters, MPH.

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

MD Whistleblower
Michael Kirsch, MD, FACP, addresses the joys and challenges of medical practice, including controversies in the doctor-patient relationship, medical ethics and measuring medical quality. When he's not writing, he's performing colonoscopies.

Medical Lessons
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.

More Musings
Rob Lamberts, MD, ACP Member, a med-peds and general practice internist, returns with "volume 2" of his personal musings about medicine, life, armadillos and Sasquatch at More Musings (of a Distractible Kind).

Prescriptions
David M. Sack, MD, FACP, practices general gastroenterology at a small community hospital in Connecticut. His blog is a series of musings on medicine, medical care, the health care system and medical ethics, in no particular order.

Reflections of a Grady Doctor
Kimberly Manning, MD, FACP, reflects on the personal side of being a doctor in a community hospital in Atlanta.

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.

Technology in (Medical) Education
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.

Why is American Health Care So Expensive?
Janice Boughton, MD, FACP, practiced internal medicine for 20 years before adopting a career in hospital and primary care medicine as a locum tenens physician. She lives in Idaho when not traveling.

World's Best Site
Daniel Ginsberg, MD, FACP, is an internal medicine physician who has avidly applied computers to medicine since 1986, when he first wrote medically oriented computer programs. He is in practice in Tacoma, Washington.

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.

Interact MD
Michael Benjamin, MD, ACP member, doesn't accept industry money so he can create an independent, clinician-reviewed space on the Internet for physicians to report and comment on the medical news of the day.

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

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

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