Diets high in red
meat are known to be associated with greater risk of heart disease. The association between red meat and heart
health has been thought to be mostly due to the unhealthy fats found in red
meat that raise blood cholesterol levels and contribute to atherosclerosis,
clogging of the arteries. Now, new
research has uncovered an additional way in which red meat consumption promotes
atherosclerosis. Researchers have found that
when carnitine, a compound plentiful in meat, is broken down by bacteria in the
digestive tract, a substance that promotes arterial inflammation and clogging
of the arteries, trimethylamine-N-oxide (TMAO), is produced.
The research, led by Dr. Stanley Hazen, tested the carnitine and TMAO levels of meat-eaters, vegans and
vegetarians, and examined the carnitine blood levels of 2,595 patients
undergoing elective cardiac evaluations. Among those undergoing cardiac evaluations,
they found that high carnitine levels were consistently associated with a
raised risk of heart disease, heart attack, stroke and heart-related death. Also, in an experimental trial they fed steaks
to persons who regularly ate red meat and measured levels of TMAO before and
after the meal. They found sharp increases
in TMAO blood levels following the meal and also found similar results when
feeding these persons carnitine supplements.
Next, participants were given a course of antibiotics to wipe out gut
bacteria, and then again fed steaks, and, on another occasion, carnitine
supplements. After the antibiotic
treatment, neither the steak nor the supplements increased TMAO levels,
indicating that indeed it was gut bacterial producing the TMAO.
The type of bacteria
living in one's digestive tract is strongly influenced by one's long-term
dietary patterns. Researchers found that a diet high in carnitine shifts the gut
microbe composition to the types that digest carnitine. Thus, TMAO production is increased in meat-eaters.
In contrast, researchers found that levels of TMAO and the bacteria which
produce TMAO were very low in persons who were following a vegetarian (no meat,
fish, poultry) or vegan (no animal products including no eggs and dairy) diet.
It is believed that
TMAO contributes to atherosclerosis because it enables blood cholesterol to get
into artery walls and cause damage. It
is also believed to interfere with the body's ability to eliminate excess blood
cholesterol. Thus, a diet high in meat
not only increases blood cholesterol but increases the harm caused by the
higher levels of cholesterol.
Dr. Hazen, reports that he has taken his findings
to heart. He had been a lifelong meat lover eating large portions several times
a week, but, convinced by his findings, he now limits his intake to 4-6 oz. once
every two weeks.
What to do: Besides meat, carnitine also occurs naturally
in chicken, fish, and dairy but levels are much higher in red meat. Carnitine is also added to energy drinks and
supplements because it has been thought to help build muscle. While meat is a great source of protein,
iron, and B vitamins, because of the risk it poses to heart health, try
limiting its consumption to 2 times per week or less. Opt instead for lean protein sources such as
fish, skinless poultry, lowfat/nonfat dairy, eggwhites, and beans. Make sure to avoid supplements or energy
drinks containing added carnitine.
Adapted from articles available at:
Source:
Koeth RA, Wang Z, Levison
BS, et al. Intestinal microbiota
metabolism of l-carnitine, a nutrient in red meat, promotes atherosclerosis. Nature Medicine; 2013. Doi:10.1038/nm.3145. Available at: http://www.nature.com/nm/journal/vaop/ncurrent/full/nm.3145.html
