Australasian Science: Australia's authority on science since 1938

Red Herring for Red Meat Consumption

A nutrient used as a dietary supplement, not fat and cholesterol, is the link between red meat consumption and cardiovascular disease.

“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 colonise the gut, and what are they like in species that are primarily carnivorous (like cats) rather than omnivorous (like humans and mice).”

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.

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“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 overriding 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.”

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

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“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.”

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

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“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.”

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

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