April 15, 2013
Need more inspiration to put down the burger and pile on the veggies this Meatless Monday? A new paper from the Cleveland Clinic (released online last week in Nature) may further explain— beyond saturated fat and cholesterol—the connection between meat consumption and heart disease. This new mechanism involves gut bacteria and a nutrient known as L-carnitine, which is found in red meat, many dietary supplements, and energy drinks.
This groundbreaking and comprehensive study has shown for the first time that L-carnitine can be metabolized by intestinal bacteria for fuel to produce trimethylamine-N-oxide (TMAO). This compound is associated with increased cholesterol deposits in the bloodstream, contributing to the formation of artery-clogging plaques that lead to heart disease.
Through a series of studies highlighted in the paper, the researchers tested the effects of L-carnitine on mice and humans. In mouse studies, they found that chronic L-carnitine supplementation actually caused heart disease. However, the carnitine by itself wasn’t dangerous; it was the TMAO produced as a result of its digestion that caused the harm. In humans, TMAO levels were found to be predictive of risk for heart attacks and other major cardiac events.
In perhaps the most fascinating part of the paper, researchers conducted a study investigating the differing production of TMAO in omnivores vs. vegans/vegetarians. They theorized that as a result of not having consumed meat in over a year, the vegans/vegetarians had a different intestinal microflora composition with a reduced number of the bacteria that synthesize TMAO from L-carnitine. To test this out, they sampled blood TMAO levels of both omnivores and vegans/vegetarians before and after consuming an 8-oz steak and found that both baseline and post-meal TMAO levels in vegans/vegetarians were significantly lower and even non-existent in some cases than the omnivores.
As a student in microbiology myself this semester, this study adds a new perspective on the importance of intestinal microflora in both advancing and harming human health. Interesting discussions, and further studies, are sure to come out of this paper. For one, it will be interesting to see whether L-carnitine-containing supplements and energy drinks currently on the market may be re-assessed for safety. Second, testing blood TMAO levels may become a new (and perhaps cheaper?) method for identifying those with a higher risk for heart disease. Finally, how much will this study add to the current trajectory of a reduction in meat consumption in America, and will it have any effect on some of the increasing appetites abroad?