One day carbs are okay; the next they’re the devil. Fat was the culprit of the obesity epidemic; now, loading up is touted as a quick-fix for weight loss. If you’re a health news consumer, you’re no stranger to nutrition advice that flip-flops so much it can make your head spin.
If you’ve been reading the headlines lately, you’ve probably come across another confusing example that’s sending shock waves through the nutrition field.
The conclusion of the meta-analysis, published in October 2019 in the Annals of Internal Medicine, challenges what many believed to be a truism in healthy eating: Limit red and processed meat, and you’ll help fend off chronic disease and live longer.
For the new analysis, researchers from Dalhousie University and McMaster University in Canada, and the Spanish (Iberoamerican) and Polish Cochrane Centers, performed four parallel systematic reviews involving randomized controlled trials and observational studies. They screened the published research on the effects of processed meat on cardiometabolic and cancer outcomes.
Researchers used a formula called GRADE, per an article published in December 2018 in BMC Medical Research Methodology, which prioritizes the presence of randomized controlled trials (RCTs) over observational studies. RCTs show a cause-and-effect relationship between two factors (like eating red meat and developing heart disease), while observational studies show an association between two factors.
Based on their reviews, a panel of 14 members from seven countries — part of an organization of self-appointed members called NutriRECS — voted on recommendations for red and processed meat consumption. Their verdict: A “weak” recommendation that most adults should continue to eat their current levels of red and processed meat intake — a finding that the authors acknowledge is contrary to almost all other guidelines that exist.
“Based on the research, we cannot say with any certainty that eating red meat or processed meat causes cancer, diabetes, or heart disease,” says lead author Bradley Johnston, PhD, associate professor in the department of community health and epidemiology at Dalhousie University Medicine in Halifax, Nova Scotia, Canada.
Indeed, the findings are at odds with recommendations from groups like the World Health Organization (WHO) and the U.S. Department of Agriculture (USDA) that advise limiting red meat, which is higher in saturated fat than plant protein sources, for optimal health. (Red meat includes beef, pork, lamb, goat, veal, mutton, and horse.)
Unsurprisingly, leaders in the nutrition community have tended to dismiss the conclusions. The American Cancer Society has affirmed its position on limiting the consumption of processed meat as well as red meat to save lives from cancer.
The Physicians Committee for Responsible Medicine, a nonprofit with 12,000 doctor members, promptly filed a petition with the Federal Trade Commission the day of the paper’s publication to “correct false statements regarding consumption of red and processed meat” released by the Annals of Internal Medicine, a publication of the American College of Physicians.
In a statement responding to the paper, researchers from the Harvard T.H. Chan School of Public Health called the new guidelines “unfortunate” and wrote that they could “harm individuals’ health, public health, and planetary health.”
“It may also harm the credibility of nutrition science and erode public trust in scientific research,” they added.
A Closer Look at How Researchers Evaluated Red Meat and Disease Risk
NutriRECS, the group that carried out the study, describes itself as an organization of international experts in nutrition and health research methodology whose aim is to improve the quality of nutritional guidelines. Citing the increasing call for more robust, evidence-based nutrition guidelines, the mission of NutriRECS is to “produce trustworthy nutritional guideline recommendations based on the values, attitudes, and preferences of patients and community members.”
Study authors wrote that they conducted the review to examine existing evidence to determine if people could reduce their risk for cancer, heart disease, diabetes, or early death by eating less red or processed meat, or reducing how often they ate it. The authors reported that on average, Americans eat red meat or processed meat about three to four times per week.
The authors based the recommendations on the following findings:
- Among 12 randomized control trials enrolling about 54,000 individuals, authors did not find a statistically significant or important association with the risk of heart disease, cancer, or diabetes for those that consumed less red and processed meat.
- In 55 cohort studies following more than four million participants, authors found a small reduction in risk among those who consumed three fewer servings of red or processed meat per week, with the certainty of evidence “low to very low.”
Differences in Opinion on What Makes for Quality Nutrition Research
A wealth of past research has suggested that limiting red and processed meat, and especially replacing intake of these animal protein with plant-based sources, has the potential to reduce the risk for chronic disease and early death.
For example, the famous PREDIMED study, published in April 2014 in The New England Journal of Medicine, found that a Mediterranean diet, which is low in red meat but high in fish and plants, can help reduce the risk of heart disease in people with a predisposition for the health condition.
In another previous study, researchers at the Harvard School of Public Health followed 121,342 people for up to 28 years to see if eating red meat increased the risk of dying early. They estimated that substituting red meat with fish, poultry, or a plant-based protein once a day was associated with a 7 to 19 percent lower mortality risk. Authors concluded that red meat consumption was associated with an increased risk of total, cardiovascular, and cancer mortality.
And a study published in August 2019 in the Journal of the American Heart Association found that plant-based diets are associated with a lower risk of heart disease and death from any cause. People with the highest adherence to a plant-based diet had up to a 25 percent lower risk of all-cause mortality.
How did the new analyses come to such a strikingly different conclusion?
It has to do with the way researchers conducted the new analysis, some critics argue. Their approach, GRADE — which stands for Grading of Recommendations Assessment, Development, and Evaluation — is not well-suited to nutrition research, according to Harvard, which notes that originally scientists developed the GRADE approach for drug trials.
For nutrition guidelines, many healthcare professionals and the official bodies that advise them — like the Centers for Disease Control and Prevention (CDC) and the USDA — rely on observational studies because randomized controlled trials involving people’s eating or lifestyle habits are difficult to conduct.
In randomized controlled trials, researchers randomly assign one group of people to an intervention and the other to a placebo (a control). The groups are comparable in their demographic, state of disease, and any potential medication they’re taking, noted an earlier article. This study model reduces the risk for researchers’ bias so that authors have the best chance of illustrating a cause-and-effect relationship between two factors, like eating red meat and developing heart disease.
Observational studies, on the other hand, simply analyze whether two factors are associated. These studies don’t necessarily prove a causal relationship, past research has shown.
Yet there are ways to evaluate the quality of observational studies. Hierarchies of Evidence Applied to Lifestyle Medicine (HEALM), per an article published in August 2019 in BMC Medical Research Methodology, and the 2015 Dietary Guidelines Advisory Committee Report from the National Institutes of Health are both examples of appropriate criteria that can be applied to observational studies about health habits and environmental factors.
Most existing lifestyle studies would never pass GRADE criteria, according to Harvard researchers. If these methods were used to evaluate other dietary studies (like soft drink consumption), lifestyle (such as physical inactivity and inadequate sleep), and environmental (such as passive smoking and air pollution) factors, none of the current recommendations on these factors would be supported by high- or even moderate-quality evidence, notes Frank Hu, MD, PhD, and fellow researchers at Harvard.
In the American Cancer Society’s statement, nutritional epidemiologist Marji McCullough, RD, explains: “While a long-term randomized trial of red and processed meat intake and cancer risk may provide support for the observational studies, it is unlikely that a trial of that nature would ever be conducted for practical and ethical reasons.”
One of those practical reasons is how long diet would need to be tracked to determine how it impacted health outcomes. “For example, atherosclerosis is a disease that develops over decades, so to apply an intervention and then ask several months later if that intervention had an impact on cardiovascular events is almost guaranteed to give a false-negative result,” says Donald McClain, MD, PhD, professor of endocrinology and metabolism and director of clinical and translation science institute at Wake Forest Baptist Health in Winston-Salem, North Carolina.
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Group’s Recommendation ‘Ill-Informed,’ Nutrition Experts Say
There are other noteworthy flaws of the current analysis, says David L. Katz, MD, MPH, founding director of Yale University’s Yale-Griffin Prevention Research Center in Derby, Connecticut, and the founder and president of the True Health Initiative.
First, in the systematic review of randomized trials, researchers found 12 eligible trials that met their criteria, and of those found only two that looked at main outcomes of cardiovascular, cancer, and diabetes mortality, incidence, and morbidity, Dr. Katz argues.
One of the trials had a meaningful difference in diet quality, which means that real conclusions might be drawn, he explains. Called the Lyon Diet Heart Study, the study looked at 423 people with a 46-month follow-up period and found subjects following the Mediterranean-style diet had a 50 to 70 percent lower risk of recurrent heart disease. The Mediterranean diet emphasizes eating fish and plant-based foods such as nuts, whole grains, and fruits and veggies, with olive oil and a little red wine. Red and processed meat are discouraged.
Katz argues that the authors excluded this study from their analysis not because there were any methodological problems — but because they concluded the treatment effects were “implausibly strong,” he says. “The one study where there was a major difference in diet design that produced a huge difference in outcomes was excluded because it worked ‘too well,’” he says. “To leave that study out when it’s one that actually had meaningful results and meaningful dietary differences in control and intervention is expressly misleading,” he adds.
After eliminating that trial, researchers were left only with the Women’s Health Initiative Dietary Modification Trial, a study that assigned postmenopausal women to either a regular American diet or a lowfat diet, says Katz. “It was not focused on meat per se,” he says. It’s also unusual that the authors ended up only including the data from this one study in what was originally intended to be a review of several studies, he adds.
The adherence to the assigned diet in both treatment arms in the Women’s Health trial was known for being very poor — the lowfat group consumed more fat than they were supposed to, and the regular diet arm less, says Katz. “If you have no difference between intervention and control, you’re not going to see much of a difference in outcomes,” says Katz. “But despite that, the results actually favored less meat intake,” he says.
There were also problems with the cohort studies selected, says Dr. McClain. The studies are a “hodgepodge” with variable follow-up times, several being only 6 to 12 months, argues McClain. “You would not expect to see differences in that short of a time period in these chronic diseases that take years to develop,” he adds. There were also highly variable populations in the included studies, he says. “Many of the studies had younger participants where the prevalence of chronic diseases is very low,” he says.
These reviews are not the basis for any kind of recommendation, McClain says. “We have studies where there is a significant shift from animal food to plant food that show massive improvements in health, and this research didn’t examine that body of literature at all,” he says.
Katz agrees. “I think this recommendation is ill-informed,” he says, adding that the review is also problematic for reasons beyond nutrition.
“This is a terribly ill-timed message, when we’re focused on climate change and there’s so much momentum there,” says Katz. “Reducing our intake of meat is a huge piece of this,” he says. “If these findings were right, it would still not be the best message, but the findings also just overtly wrong,” says Katz.
The panel acknowledges that it did not consider animal welfare or environmental concerns when making our recommendations, says Dr. Johnston. “We sought to clarify the evidence on health outcomes only, while noting that we are sympathetic to animal welfare and environmental concerns,” he says, adding that a number of the guideline panel members have eliminated or reduced their personal red and processed meat intake for animal welfare or environmental reasons.
So, Should You Binge on Red Meat or Keep Cutting Back?
This analysis doesn’t change the bottom line when it comes to the health risks of eating meat, and what we know from existing nutrition research, says Kelly Kennedy, RD, staff nutritionist at Everyday Health. “The fat in red and processed meats is primarily unhealthy saturated fat. This type of fat wreaks havoc in the body,” says Kennedy.
Making healthy substitutions to reduce meat intake can have health benefits, says Kennedy, but the key word here is “healthy.” If you swap saturated fat for healthy fats and fiber-rich carbohydrates, you can help lower your risk of heart disease and stroke. But if you replace saturated fat with refined carbohydrates, your risk for diseases like these may increase. Fiber-rich carbohydrates include fruits, veggies, and whole grains like whole-wheat bread and brown rice, and healthy fats include those from olive oil, avocado, and nuts.
The bottom line is that the fundamentals of good nutrition don’t change with every news cycle, says Katz. “That’s critically important to note,” he says. “This ‘playing ping-pong’ with science is incredibly injurious to public health.”