The connection between red meat and atherosclerotic cardiovascular disease has been well established, but newly reported findings indicate that metabolites in the gut microbiome may explain that relationship more than cholesterol and blood pressure.
“Eating more meat, especially red meat and processed meats, is associated with a higher risk of cardiovascular disease, even later in life,” co–lead study author Meng Wang, PhD, said in an interview.
The, from a large community-based cohort of older people, included 3,931 U.S. participants aged 65 and older in the Cardiovascular Health Study (CHS). It found that gut microbiota–generated metabolites of dietary L-carnitine, including trimethylamine N-oxide (TMAO), have a role in the association between unprocessed red meat intake and incident ASCVD.
“TMAO-related metabolites produced by our gut microbes as well as blood-glucose and insulin homeostasis and systematic inflammation appeared to explain much of the association, more so than blood cholesterol or blood pressure,” added Dr. Wang, of the Friedman School of Nutrition Science and Policy at Tufts University, Boston.
Dr. Wang said this study was unique because it focused specifically on older adults; the average participant age was 72.9 years. “Older adults are at the highest risk of CVD, and for them adequate intake of protein may help to offset aging-related loss of muscle mass and strength,” she said. However, the study population was largely white (88%), so, she said, the results may not be generalizable to populations that are younger or of different nationalities and races.
The researchers performed a multivariable analysis that showed that participants who had higher intakes of unprocessed red meat, total meat, and total animal source foods (ASF) had higher hazard ratios of ASCVD risk. The study had a median follow-up of 12.5 years. It divided the study population into five quintiles based on how much unprocessed red met they consumed at baseline and analyzed dietary exposure in the differences between the midpoints of the first and fifth quintiles.
Earlier studies of meat intake and CVD risk focused mostly on saturated fat and blood cholesterol, Dr. Wang added. “But our findings suggest that other components in red meat, such as L-carnitine and heme iron, might play a more important role than saturated fat,” she said.
Higher intake of unprocessed red meat was linked to a 15% higher incidence of ASCVD per interquintile range (hazard ratio, 1.15; 95% confidence interval, 1.01-1.30; P = .031). Total meat intake, defined as unprocessed plus processed red meat, was tied to a 22% higher incidence of ASCVD (HR, 1.22; CI, 1.07-1.39; P = .004).
The study found no significant association between fish, poultry, or egg intake and incident ASCVD, but found total ASF intake had an 18% higher risk (HR, 1.18; CI, 1.03–1.34; P = .016).
Explaining the red meat–CVD connection
“The more novel part of our study is about the mediation analysis,” Dr. Wang said. “It helps explain why meat intake was associated with a higher risk of CVD. We identified several biological pathways, including the novel one through TMAO-related metabolites produced by the gut microbiome.”
Three gut microbiota–generated metabolites of L-carnitine – TMAO, gamma-butyrobetaine, and crotonobetaine – seem to partly explain the association between unprocessed red meat intake and incident ASCVD, the study reported.
The study found 3.92 excess ASCVD events per 1,000 person years associated with each interquintile range of higher unprocessed red meat intake; 10.6% of them were attributed to plasma levels of the three L-carnitine metabolites (95% CI, 1.0-114.5).
In this study, neither blood cholesterol nor blood pressure levels seemed to explain the elevated risk of ASCVD associated with meat intake, but blood glucose and insulin did, with mediation proportions of 26.1% and 11.8%, respectively.
Study strengths are its size and its general population cohort with well-measured CVD risk factors, Dr. Wang pointed out. All participants were free of clinically diagnosed CVD at enrollment, which minimized selection bias and reverse causation, she said. However, she acknowledged that the use of self-reported diet intake data, along with the largely white population, constitute limitations.
“Our study findings need to be confirmed in different populations and more research efforts are needed to better understand the health effects of some of the components in red meat, such as L-carnitine and heme iron,” Dr. Wang said.
“This study is interesting in that it doesn’t just ask the question, ‘Is eating red meat associated with coronary disease and atherosclerotic disease?’ but it tells what the mechanism is,” Robert Vogel, MD, professor at University of Colorado at Denver, Aurora, said in an interview.
The association between red meat and ASCVD is “an established science,” he said. “Where this study adds to the literature is that it suggests that elevated LDL cholesterol or blood pressure, things – especially the former – that are thought to be associated with coronary disease, may or may not be the mechanism.” He cautioned, however, “this is all associative data.”
The study “produces incremental knowledge for the association between eating red met and atherosclerosis, but it does not establish causality,” Dr. Vogel added.
Dr. Wang has no relevant disclosures. Dr. Vogel is a consultant to the Pritikin Longevity Center in Miami.