We are frequently advised to keep our consumption of red meat to a bare minimum of perhaps once a week, or preferably avoid it all together. There is ample evidence demonstrating a link between red meat consumption and an increased incidence of cancer, particularly bowel cancer. We also know that red meat is associated with an increased risk of heart attacks and strokes, which has been attributed to elevated cholesterol levels in the blood derived from the high content of saturated fats found in red meat.
L-carnitine and choline (found in excess amounts in eggs, dairy products) is broken down by our gut microbes into a compound called TMA, which is then converted by liver enzymes into TMAO - associated with cardiovascular disease and atherosclerosis. TMAO is detected in the blood of omnivores but not vegans, following the intake of L-carnitine capsules for three days. It seems a species of bacteria belonging to the genus Prevotella is the main bacterial population that metabolises carnitine into TMAO.
And interesting point is that the presence of TMAO has no effect on the expression of the receptor for LDL (low density lipoproteins or bad cholesterol) or cholesterol production genes. So how can TMAO increase atherosclerosis? In the liver it prevents the breakdown of cholesterol into bile acids. It appears that TMAO also reduces a process called reverse cholesterol transport – the expulsion of excess cholesterol by gut cells, and macrophages in atherosclerotic plaques. TMAO does this by lowering the presence of cholesterol transporters in the gut. Thus if excess cholesterol cannot be eliminated from the circulation by gut cells, or the artery wall by macrophages, the development of atherosclerosis and the risk of cardiovascular disease increases.
So now we know how red meat on its own can increase cardiovascular disease. Over the years it has been documented how the food industry handles red meat to enhance its visual appeal and marketability such as the use of red food colouring and soaking meat in salt water to enrich its flavour. We cannot establish on what scale such tampering has affected the health of the general population. But one thing is for certain, don’t eat meat that looks red.
 Koeth, R. a, Wang, Z., Levison, B. S., Buffa, J. a, Org, E., Sheehy, B. T., … Hazen, S. L. (2013). Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis. Nature Medicine, 19, 576–85. doi:10.1038/nm.3145