ROUND-UP: Heart disease and red meat (Nature Medicine*) – experts respond

Fri Apr 5, 2013

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EMBARGO LIFTED 03:00 AEST Monday 8 April, 2013

The nutrient L-carnitine, found in red meat and used as a dietary supplement, is associated with cardiovascular disease in people and causes vascular disease in mice, reports a new study by US researchers. The results point to L-carnitine, rather than saturated fat and cholesterol, as explaining the link between red meat consumption and cardiovascular disease. Also, the findings have relevance to the widespread use of L-carnitine as a dietary supplement, and the authors suggest that the safety of this practice should be further investigated.

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Professor Garry Jennings is Director of Baker IDI Heart and Diabetes Institute

“Red meat consumption remains controversial and therefore this study makes an important contribution to that discussion. However, the evidence is not sufficiently compelling to cause concern amongst the red meat industry and a balanced diet remains the best recommendation for Australians

While this paper makes some clever observations, the overall evidence that red meat is harmful is not consistent with a broader body of evidence. Some studies have shown a moderate adverse effect, others only with processed meats and others have shown no risk associated with red meat.

The proposal is that the bacterial flora of the gut is altered in meat eaters. These ‘new’ bacteria metabolise L-carnitine in meat, forming a chemical that in-turn is changed by the liver into TMAO. The paper provides evidence that TMAO can contribute to atherosclerosis, the cause of heart attacks, strokes and vascular diseases. The authors suggest that this explains reports of higher rates of cardiovascular disease with red meat consumption rather than the cholesterol and saturated fat in meat.

Carnitine is needed by the body to help oxidise fat. It is provided both in the diet from meat and dairy food, and is made by the body from lysine and vitamin B6. While Healthy people do not need extra carnitine, it’s a popular supplement with athletes such as footballers and body builders. However, there is very little evidence to support its efficacy.”  

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Professor John Funder is Executive Chairman of Obesity Australia. He is also a Senior Fellow at Prince Henry’s Institute and Professor in the Department of Medicine at Monash University

“The article in Nature Medicine by Robert Koeth and co-authors presents a fascinating series of studies, in mice and human subjects, pointing to a causative role for L-carnitine in atherosclerosis. The effect is not of L-carnitine itself, but of a product of its metabolism by resident bacteria in the gut called Trimethylamine-N-oxide, or TMAO for short. There are a number of things to note about the study, as follows. First, the authors include pork and duck into the list of red meat, whereas in fact they have very much lower L-carnitine levels. Secondly, relevant to Australian red meat eaters, kangaroo meat – long considered very healthy, given its very low fat content – has more L-carnitine per gram than any other red meat; on the basis of the authors’ findings, it may not be such a healthy option after all. Finally, as a scientist one unanswered question is that of the particular microbes in the gut of meat eaters that are absent from that of vegans; do they require something in red meat for their ability to colonize the gut, and what are they like in species that are primarily carnivorous (like cats) rather than omnivorous (like humans and mice).”

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Professor Mark Wahlqvist is Emeritus Professor of Medicine, Monash University and Past President of the International Union of Nutritional Sciences

“We are used to rather simple assumptions about how dietary components affect health. For heart disease this has been that dietary fat, including cholesterol and fat type (saturated, monounsaturated, omega-6 and omega-3 polyunsaturated and trans fatty acids) were the main diet story. That has been changing and increasingly it is clear that it is more reliable to consider food patterns and health outcomes and that an emphasis on variety and plant foods is the over-riding guideline for healthy eating.

The new study published in the journal Nature Medicine by Koeth and co-workers supports this, in that it finds yet another factor in meat which may increase the risk of heart disease and stroke, namely l-carnitine, which is important in fat metabolism. It seems that this is because carnitine can be converted, by certain microbial populations in the gut, to trimethylamine-N-oxide (TMAO), which damages arteries supplying the heart and brain. The same has been known for some time for choline which is found in lecithin (fatty substance used as food additive and dietary supplement). It is increasingly evident that neither carnitine nor choline may be safe as nutrient supplements and that the safest way to obtain them from the diet is from a varied plant-based diet.”

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Professor Peter Clifton is Professor of Nutrition at the Samson Institute at the University of South Australia and Affiliate Professor at the University of Adelaide, Flinders University and Baker IDI Heart and Diabetes Institute

It is a very interesting hypothesis but unprocessed red meat has not been found in meta-analyses to be associated with cardiovascular disease, while of course vegetarian sources of choline would also be a problem for the formation of TMAO (eg quinoa, soymilk, and broccoli). Other issues with the paper  include the assumption that fasting carnitine is an index of red meat intake and there is no data shown for this comparing vegetarians and omnivores. I am sure this was measured but the fact that is not shown suggests it wasn’t different apart from the one vegan and omnivore mentioned in supplementary figure 6. Fasting carnitine may derive from endogenous synthesis and not be related to diet.  Plasma TMAO was a bit different between vegetarians and omnivores (2 vs 2.5)  and the assumption is that this might be meat but it could be just a different microbiome as they have shown how important certain bacteria are in making TMAO and how bacteria are very different in vegetarians. More data is needed.

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Associate Professor Shawn Somerset is Associate Professor of Public Health at the Australian Catholic University 

“These results add further support to previous substantial research showing that excessive meat consumption is associated with increased risk of a range of chronic diseases. Recent national dietary surveys have consistently shown that average meat consumption in Australia significantly exceeds general population recommendations for meat consumption. Meat is an excellent dietary source of iron and a range of other minerals and there are some Australians with insufficient intakes of these nutrients. The public health challenge continues to centre around developing strategies to prevent deficiencies in these nutrients whilst avoiding the adverse consequences of excessive meat consumption.”

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[*Intestinal microbiota metabolism of l-carnitine, a nutrient in red meat, promotes atherosclerosis, Koeth et al., Nature Medicine, Advanced Online Publication, 8 April 2013]