Increased heart disease risk from red meat may stem from gut microbe response to digestion
American Heart Association
In
the United States and around the world, cardiovascular disease is the leading
cause of death. While the risk of developing cardiovascular disease, including
heart attack and stroke, increases with age, other risk factors are influenced
by lifestyle. Lifestyle and behaviors that are known to improve cardiovascular
health include eating healthy foods, especially fruits and vegetables; regular
physical activity; obtaining sufficient sleep; maintaining a healthy body
weight; stopping smoking; and controlling high blood pressure, high cholesterol
and high blood sugar.
"Most of the focus on red meat intake and health has been around dietary saturated fat and blood cholesterol levels," said co-lead author of the study Meng Wang, Ph.D., a postdoctoral fellow at the Friedman School of Nutrition Science and Policy at Tufts University in Boston. "Based on our findings, novel interventions may be helpful to target the interactions between red meat and the gut microbiome to help us find ways to reduce cardiovascular risk."
Previous
research has found that certain metabolites -- chemical byproducts of food
digestion -- are associated with a greater risk of cardiovascular disease . One
of these metabolites is TMAO, or trimethylamine N-oxide, which is produced by
gut bacteria to digest red meat that contains high amounts of the chemical
L-carnitine .
High
blood levels of TMAO in humans may be associated with higher risks of CVD,
chronic kidney disease and Type 2 diabetes. However, whether TMAO and related
metabolites derived from L-carnitine may help explain the effects of red meat
intake on cardiovascular risk, and to what extent they may contribute to
cardiovascular risk associated with meat consumption, are still unknown.
To
understand these questions, the researchers conducting this study measured
levels of the metabolites in blood samples. They also examined whether blood
sugar, inflammation, blood pressure and blood cholesterol may account for the
elevated cardiovascular risk associated with red meat consumption.
Study participants included nearly 4,000 of the 5,888 adults initially recruited from 1989 to 1990 for the Cardiovascular Health Study (CHS). The participants selected for the current study were free of clinical cardiovascular disease at time of enrollment in the CHS, an observational study of risk factors for cardiovascular disease in adults aged 65 or older.
The CHS follows 5,888
participants recruited from four communities: Sacramento, California;
Hagerstown, Maryland; Winston-Salem, North Carolina; and Pittsburgh,
Pennsylvania. The average age of participants at enrollment was 73, nearly
two-thirds of participants were female and 88% of participants self-identified
as white. The median follow-up time for participants was 12.5 years, and up to
26 years in some cases. At follow-up appointment, participants' medical
history, lifestyle, health conditions and sociodemographic characteristics --
such as household income, education and age -- were assessed.
Several
blood biomarkers were measured at the start of the study and again in
1996-1997. The fasting blood samples stored frozen at -80 degrees Celsius were
tested for levels of several gut-microbiome linked to red meat consumption
including TMAO, gamma-butyrobetaine and crotonobetaine.
Additionally, all study participants answered two validated food-frequency questionnaires about their usual dietary habits, including intake of red meat, processed meat, fish, poultry and eggs, at the start of the study and again from 1995 to 1996.
For the first questionnaire, participants indicated how often, on average in
the previous 12 months, they had eaten given amounts of various foods, ranging
from "never" to "almost every day or at least five times per
week," based on medium portion sizes, which varied based on the food
source. The second questionnaire used a ten-category frequency of average
intake over the past 12 months, ranging from "never or less than once per
month" to "six+ servings per day," with defined standard portion
sizes.
For
the current analyses, the researchers compared the risk of cardiovascular
disease among participants who ate different amounts of animal source foods
(i.e., red meat, processed meat, fish, chicken and eggs). They found that
eating more meat, especially red meat and processed meat, was linked to a
higher risk of atherosclerotic cardiovascular disease -- a 22% higher risk for
about every 1.1 serving per day .
According
to the authors, the increase in TMAO and related metabolites found in the blood
explained roughly one-tenth of this elevated risk. They also noted that blood
sugar and general inflammation pathways may help explain the links between red
meat intake and cardiovascular disease. Blood sugar and inflammation also
appear to be more important in linking red meat intake and cardiovascular
disease than pathways related to blood cholesterol or blood pressure. Intake of
fish, poultry and eggs were not significantly linked to higher risk of
cardiovascular disease.
"Research
efforts are needed to better understand the potential health effects of
L-carnitine and other substances in red meat such as heme iron, which has been
associated with Type 2 diabetes, rather than just focusing on saturated
fat," Wang said.
The
study had several limitations that may have affected its results. The study was
observational, meaning it could not control for all risk factors for
cardiovascular disease, and may not prove cause and effect between meat
consumption and cardiovascular disease or its mediation by gut
microbe-generated chemicals. Additionally, food consumption was self-reported,
so errors in reporting were possible. And, as most of the study participants
were older, white men and women in the United States, the findings may not
apply to populations that are younger or more racially diverse.
The
co-lead author is Zeneng Wang, Ph.D. Co-authors are Yujin Lee, Ph.D.; Heidi
T.M. Lai, Ph.D.; Marcia C. de Oliveira Otto, Ph.D.; Rozenn N. Lemaitre, Ph.D.,
M.P.H.; Amanda Fretts, Ph.D., M.P.H.; Nona Sotoodehnia, M.D., M.P.H.; Matthew
Budoff, M.D.; Joseph A. DiDonato, Ph.D.; Barbara McKnight, Ph.D.; W. H. Wilson
Tang, M.D.; Bruce M. Psaty, M.D., Ph.D.; David S. Siscovick, M.D., M.P.H.;
Stanley L. Hazen, M.D., Ph.D.; and Dariush Mozaffarian, M.D., Dr.Ph. Authors'
disclosures are listed in the manuscript.
The study was funded by the National Heart, Lung, and Blood Institute, with additional support from the National Institutes of Health, the National Institute of Neurological Disorders and Stroke, and the National Institute on Aging.
