Dr. Neville S. Wilson –July, 2013.
Does the Carnitine in red meat increase your chances of developing heart disease ?
INTRODUCTION :
Fearsome pronouncements about the health dangers of animal meat have once again hit world media headlines.
A recently published medical article, linking Carnitine from animal red meat, to cardio-vascular disease, has attracted the attention of the popular media, and widely publicised as a warning for consumers to limit their intake of red meat, and to also avoid taking Carnitine in supplemental form.
The recent widespread media outbursts, containing health warnings about red meat consumption, are based on the findings of a study paper released by Cleveland Clinic researchers.
The speculation that animal meat is causally linked to heart disease is entrenched in the official dietary dogma of the American Heart Association, and interpreted in its dietary recommendations to limit the intake of saturated fat and cholesterol containing foods, as part of a “healthy diet”, aimed at reducing the risk of heart disease.
The purpose of this paper is to expose the limitations and weakness of the study and it’s recommendations, and to correct the false perception that either red meat or Carnitine supplementation is injurious to health.
THE STUDY:
A group of researchers at Cleveland Clinic hypothesized a link between carnitine, in red meat, and cardiovascular disease, and designed a study to test their hypothesis. (1)
To their credit they acknowledged that there is no conclusive evidence for the long-held view of a positive association between animal meat (saturated fat ) and cardiovascular disease (2 ) (3) (4).
With this recognition they undertook to explore an alternative mechanism for a “cause” of cardiovascular disease in humans, this time looking at the possible role that intestinal microbiota might play in the development of atherosclerotic disease, following the consumption of a diet rich in red meat carnitine.
Their recently published study, in Nature Paper, proposed that L-Carnitine, found in red meat, could increase levels of a metabolite called trimethylamine-N-Oxide (TMAO), and thereby accelerate plaque progression in the walls of arteries, thereby increasing the risk for heart disease in humans.
The recommendations, following their study conclusions, are to limit the dietary intake of red meat, and to avoid including supplemental L-carnitine in a dietary regimen, in order to minimise the risk of developing heart disease.
This advice is not markedly different from previous public health exhortations to limit the intake of animal meat, a dietary policy which is entrenched in the guideline statements of the American Heart Association and its allied health bodies, such as the National Institutes of Health, and currently remains the official dietary policy in many western countries, including Ireland.
What is new, however, is the focus on the popular health supplement, L-Carnitine, the safety and benefit of which is now called into question, based on the findings of this study.
STUDY RATIONALE:
The underlying intent of this study was to demonstrate a correlation between dietary L-carnitine (in red meat, and also in supplemental form) and cardiovascular disease, via the formation of trimethylamine (TMA) and trimethylamine oxide (TMAO), which are natural metabolites of L-carnitine, under the action of human gut microbes.
Acknowledging that there is not sufficient credible evidence for the long-standing conventional view, that animal fat and dietary cholesterol are causally linked to cardiovascular disease, the researchers embarked on exploration into an alternative theory for the possible causes of cardiovascular disease, this time searching for an “alternative disease-promoting mechanism”, which they postulated could be environmental.
(It is noteworthy that this study has the appearances of another attempt to causally implicate cholesterol in the pathogenesis of cardiovascular disease, albeit through a different route !)
Their “environmental” quest leads them to a consideration of human intestinal microbiota, and their impact on dietary L-carnitine, resulting in TMA and TMAO formation, and subsequent heart disease, through an indirect effect of cholesterol infiltration into the arterial wall.
Their proposal suggests that TMA/TMAO production is potentially harmful through its action of inhibiting the process of reversecholesterol uptake, and thereby causing deposition of cholesterol into the arterial wall, with the formation of plaque, and increased risk for stroke or heart attack.
(The proposed mechanism, according to the researchers, for the initiating cause in cardiovascular disease, is not markedly different from the conventional view of cholesterol deposition in the arterial wall in the form of LDL-cholesterol.
I have previously shown that reverse cholesterol uptake, one of several functions of HDL-C, is not the sole mechanism whereby HDL-C effects it’s protective role in cardiac health.
The inhibition of reverse cholesterol uptake, therefore, can hardly be explained as the sole mechanism for causing heart disease). (5).
What is novel in this study is the idea that intestinal microbes impact on L-carnitine, a component of red meat, producing metabolites that are indirectly causative in cholesterol deposition in the arterial wall).
The Study, conducted by researchers at Cleveland Clinic in Ohio, USA, was funded by the National Institutes of Health (NIH), and reported in NATURE MEDICINE, May 2013, and widely promoted through media outlets as a warning about the potential harm to cardiovascular health of dietary red meat and carnitine.
(The NIH is renowned for it’s anti-fat and anti-cholesterol position, and for it’s close ties to the American Heart Association, which consistently warns against the dangers of eating animal meats, and the dietary intake of food containing saturated fats and cholesterol) (6)
CARNITINE – THE FACTS:
L-carnitine is present in a variety of foods, in addition to meat,
(carne = meat), and is utilised by human cells for the production of cellular energy, through transport of energy rich fatty acids into cellular mitochondria.
It is present in beef, chicken breast, cod fish, and in smaller amounts in cheese and bread.
Dietary carnitine is not “free carnitine”, but occurs as acetyl- L-carnitine esters that facilitate efficient absorption in the small intestine.
Excess carnitine, not absorbed, is broken down in the colon by commensal bacteria, and converted to trimethylamine (TMA), which in turn, is transported to the liver, where it is broken down and detoxified to form trimethylamine oxide (TMAO), and finally excreted in the urine and faeces as TMAO.
TMA production in the human body is a naturally occurring consequence of eating certain foods that contain precursors to TMA, such as meat, fish and cruciferous vegetables, in which choline is present. (7) and (8)
The authors of this paper have failed to explain why TMAO excretion, following the ingestion of a healthy meal, containing fish and cruciferous vegetables, is not a risk factor for developing heart disease.
(It is difficult to conceptualise why a natural occurring urine excretion, like TMA or TMAO, can be a cause of cardiovascular disease, as implied in this study.)
L-Carnitine is also a popular nutritional supplement that is widely used by health conscious people for heart protection, or fatigue, and by athletes for enhanced energy.
Many studies have conclusively demonstrated the health benefits of supplemental carnitine. ( 9 )
THE STUDY:
The title of the study paper implicates L-carnitine as a possible causal factor in cardiovascular disease, despite the acknowledgement by the researchers, that such a relationship has not been conclusively demonstrated in scientific studies.
In the 1st leg of the study, an experimental group of 5 meat-eating volunteers (4 males and 1 female) was tested against a control group of 5 vegans or vegetarians, after being given an 8 oz steak plus 250 mg of Carnitine, as a labeled isotope, for easy detection, and their urine tested for concentrations of TMAO, an end product of L-Carnitine, following passage through the gut, and detoxification in the liver.
The 5 omnivores produced more TMA and TMAO in their blood and urine than the 5 vegans.
In the 2nd leg of the study the 5 omnivores were then given antibiotics to suppress the effect of intestinal microbes on carnitine, and they produced less TMAO. (ie. Intestinal microbiota convert carnitine to TMA/TMAO)
The 5 vegans were not given antibiotics, and did not produce increased levels of TMA/TMAO.
( ie Human intestinal microbiota convert Carnitine > TMA/TMAO )
In a 3rd leg of the experiment a group of 5 omnivores was tested against a group of 5 vegetarians, who had no steak to eat, but given Carnitine supplements instead.
These omnivores produced more TMAO than the vegetarians, replicating the findings from the first experiment.
(We don’t know if these were the same omnivores, or a different group, as this distinction may influence TMAO blood levels produced in the first group of omnivores).
In the 4th leg of the experiment 30 omnivores and 23 vegans were again compared for baseline TMA and levels of TMAO following an addition of certain strains of bacteria. Those given the bacteria developed an increase output of TMAO, again suggesting a link between intestinal microbiota and increased excretion of TMAO.
In a 5th leg of the experiment, the levels of L-Carnitine were also checked in a large group (N=2595) of subjects who were undergoing a regular cardiovascular assessment.
More males than females produced increased levels of TMAO. (a gender influence ?) and for some there was a positive correlation between plasma carnitine and coronary artery disease (CAD), peripheral artery disease (PAD) and cardiovascular disease (CVD).
Note: ( correlation does not mean cause )
Since most of these patients were already taking various forms of medication, such as ace-inhibitors and beta-blockers for hypertension (high blood pressure), and statins (for high cholesterol, presumably), and asprin as a blood thinner) we cannot dismiss the possible effects of these various drugs on the end outcome of the study.
Furthermore, supplementary data from the study showed a correlation between increasing carnitine levels and increasing age, higher smoking rates, and poor lifestyle.
All of these drugs, as well as the increased age and lifestyle factors, are determinates which can confound the outcome. They are thus termed “confounders”, but they appear to have been ignored by the researchers !
In a 6th leg of the experiment, 2 groups of mice were also investigated, following a L-Carnitine ingestion by one group, and a normal diet by the other group, and the comparative build-up of arterial plaque measured in each group.
The mice that were given their normal chow, plus supplemental Carnitine, developed more atherosclerosis than those eating their normal chow.
(Does carnitine cause atherosclerosis in mice, and if so, does carnitine, by extrapolation, cause atherosclerosis in humans ?)
Notably, these were not ordinary mice.
For experimental purposes they were specifically engineered to lack a specific gene, called ApoE, which confers protection against developing atherosclerosis.
The apoE deficient mice, therefore, had no protection against the development of atherosclerosis, and having been given a diet for which they were not evolved to metabolise efficiently, they became more vulnerable to atherosclerotic changes.
STUDY OBSERVATIONS : (A Summary)
- The omnivore group who ate the 8 oz steak containing carnitine plus 250 mg of free L-carnitine produced more urinary TMAO than the non-meat eating vegan group.
- The one vegan male produced no TMAO following his steak plus supplemental Carnitine, whereas the female omnivore did produce TMAO.
- The use of bacteria to suppress microbial activity, caused an increase in TMAO production, suggesting a role for microbiota in the production of TMA/TMAO.
- In a group of persons having a cardiac evalualuation, a correlation was found between L-carnitine intake, and increased risk for heart disease in those subjects who had high levels of TMAO, but not in those having low levels of TMAO, suggesting a possible link between high levels of TMAO and increased risk for heart disease.
- Plaque build up in the arteries of experimental mice correlated with the intake of dietary L-carnitine, suggesting a link between TMA and cardiovascular disease.
STUDY CONCLUSIONS BY THE AUTHORS :
- In the normal gut, intestinal flora (in the absence of gut antibiotics) may be causative agents in the conversion of L-carnitine to TMA and finally to TMAO.
- High levels of TMAO may contribute to arterial plaque build up, by inhibiting the process that prevents the absorption of cholesterol into the arterial wall, with subsequent increased risks for myocardial infarction, stroke and death.
- Supplemental L-Carnitine, or consumed red animal meat may increase the risk for cardiovascular disease via conversion in the normal gut to TMA and TMAO.
THE STUDY SHORTCOMINGS:
This study paper has several shortcomings which raise important questions about the validity of it’s conclusions, and he merits of it’s dietary recommendations.
The uncritical acceptance, by the media, of this paper as an authoritative statement about cardiovascular risk, and it’s widespread warnings about carnitine ingestion, without consideration of the many confounding factors, do not serve the best health interests of the public.
Accordingly, the shortcoming and limitations need to be addressed and the dietary recommendations viewed with caution.
PROBLEM 1 :
The authors of this paper have conceded that there is no supportive evidence for the popular notion that animal meat is CAUSALLY linked to cardiovascular disease (CVD). (2)(3)(4)
They make reference to the NURSES’ HEALTH STUDY (NHS), in which red meat was “associated” with high rates of cardiovascular disease. (CHD) and for which there is no evidence of cause. (3)
The NHS is confounded by the fact that increased meat consumption, although associated with high rates of CHD, was also associated with increased rate of smoking, increased dietary trans-fats, history of angina, diabetes, high blood pressure, and decreased physical activity.
These “associations” do not imply “cause” since any one, or more, of the factors measured, could have been a causative agent.
They have also made reference to 2 other studies in which the association of animal meat (saturated fat) with CVD was evaluated, and found to be absent. ( 2 ) ( 4 ).
The study would have gained greater credibility and relevance had it been a comparison of 2 groups of randomised persons, with one control group, and the other an intervention group being given dietary carnitine.
PROBLEM 2 :
The omnivore diets consisted of a meaty 8oz steak plus supplemental L-Carnitine and produced TMAO.
But, we do not know whether the steak, without the supplemental Carnitine would have produced the TMAO. Assuming that it would, the steak meal on its own would not present a risk for heart disease, according to the acknowledgement of the authors in their introductory remarks.
The one vegan male and one female omnivore can hardly be expected to represent the wider population of vegans and omnivores, since body sizes, rates of absorption and gender differences in the wider population would account for higher or lower levels of TMA / TMAO produced.
Neither do we know what the dietary habits were of the subjects in the study, or what the other components of their diet was, including carbohydrate ingestion, prior to ingesting the study diet.
PROBLEM 3 :
Supplemental L-Carnitine is not identical to natural acetyl-L-carnitine present in red meat, and the experiment does not represent the digestive and absorption process of natural acetyl-L- carnitine in the human body, and the degree to which acetyl-L-carnitine may or may not produce TMA/TMAO.
PROBLEM 4 :
In the group of 30 omnivores studied increased levels of TMAO was detected, but different strains of bacteria were also detected, raising the question of whether only certain strains of microbiota are implicated in the production of TMA/TMAO.
PROBLEM 5 :
Amongst the group of 2595 participants who were having a cardiac evaluation there were individuals who did demonstrate a positive association between carnitine ingestion and coronary artery disease, peripheral arterial disease and cardiovascular disease.
However, a review of the supplementary data (not quoted in the press !) revealed that as carnitine consumption increased, so did the age of the participants, their smoking habits, and unhealthy lifestyle.
These “confounding factors” are not unlike those found in the Nurses’ Health Study, which are “associations” and do NOT explain cause.
PROBLEM 6 :
The researchers selected only one food/nutrient item (carnitine) which they sought to implicate as a producer of TMA/TMAO and cardiovascular disease.
In doing so they ignored a large range of foods, which are recognized as being healthy, including “heart healthy fish” which also raise TMA/TMAO levels, but do not contribute to cardiovascular disease or to increased mortality. (10)
( Furthermore, choline, a common ingredient in a range of popular healthy foods, like eggs, liver, nuts, seafood and cruciferous vegetables also has the potential for forming TMA. )
While fish consumption does raise plasma levels of TMA they do not explain why TMA in these cases does not “inhibit reverse cholesterol uptake” and cause cholesterol deposition in the arterial wall.
The researchers should have been familiar with a study in 1999 in which 6 human volunteers were given 46 different foods to eat, following which an increased excretion of TMA/TMAO was caused only by fish and seafood. ( 11 )
PROBLEM 7:
In the 1st branch of the experiment 5 omnivores were given an 8 oz steak (=180 mg L-carnitine) and 250 mg of an isotope labeled L-Carnitine, and had their carnitine and TMAO levels measured.
No information is provided regarding the source of the meat given, an important factor in considering the presence or absence of bacteria in the animal meat, which could have played a role in the reduced levels of TMA produced by the vegans who were given a steak to eat.
The method of preparing the steaks was described as being that of a George Forman Cooker. This particular method of cooking may create high temperatures with the production of harmful nitrosamines, which in turn may be precursors of TMAO, and at the same time a potential cause for cancer development.
This fact has not been accounted for by the research team.
The source of animal meat is also an important determinant of health, since organic grass fed animals produce a superior quality of meat compared to animals in Concentrated Animal Feeding Operations (CAFO).
CAFO animals have been injected with hormones and antibiotics, and their feed contaminated with mycotoxins, and no reference is made in the study to the presence or absence of mycotoxins and their potential for cardiovascular disease.
PROBLEM 8 :
The extrapolation of dietary effects in mice to outcomes for humans cannot be done without recognition of the significant differences (size, diet and metabolic preference ) between these 2 groups of experimental subjects.
An added complication is that specifically engineered mice , who have no protection against developing atherosclerosis, and in addition to their standard experimental chow, were given a meal that is foreign to that for which they have evolved to metabolise.
The strain of mice used in the experiment had their protective gene, ApoE, removed, making them vulnerable to diseases of the arteries and heart.
As such, they were not ideal subjects for a comparative study of dietary intake or for end point measurement such as cardiovascular disease or death.
Historically, animal feeding experiments fail to account for the vast differences between carnivores and herbivores in handling food for which they have been specifically evolved to metabolise.
The engineered mice, already vulnerable to developing atherosclerosis, were given supplemental Carnitine with their chow, and predictably, developed atherosclerosis.
The researchers acknowledged this difference in their examination of gut flora in the mice and in the human subjects.
Furthermore, the levels of TMAO produced by the mice varied according to the species of gut bacteria in the mice, indicating another variable that needs to be accounted for.
PROBLEM 9 :
Not only were the experimental mice vulnerable due to the removal of their apoE protection, the chow given to them was highly suspect also, and may have been a significant factor in their development of atherosclerosis.
Their experimental chow was “TEKLAD 2018”, a standard Rodent chow containing low fat, with 18% calories from protein and 58% calories from refined carbohydrates.
High dietary intakes of refined carbohydrates stimulate insulin release, which in excess is injurious to the inner lining (endothelium) of arteries, and may initiate the process of atherosclerosis.
Was the causal agent in these demonstrations of cardiovascular disease perhaps the 58% of calories from dietary refined carbohydrate, and not the carnitine ?
PROBLEM 10 :
Since Carnitine studies are not new, questions may be raised as to why this experiment was conducted.
Previous Carnitine studies have demonstrated a protective role for carnitine in cardiovascular disease.
The role of L-Carnitine and it’s 2 metabolites, acetyl-L- Carnitine and propionyl-L-Carnitine, has been demonstrated in the treatment of acute ischaemic events. (12)
Researchers in Italy demonstrated a decrease in arterial plaque progression and aortic atherosclerotic lesions with carnitine.
(13 )
In patients with peripheral artery disease (PAD) improved walking performance was measured in patients using carnitine. (14)
Carnitine improved sexual dysfunction in a study evaluating mortality risks. (15 )
Carnitine decreased mortality risk in heart attack victims. (15)
Carnitine is essential for healthy cellular energy production and a deficiency of carnitine could compromise cardiac health, as observed in a condition of carnitine insufficiency called Primary Carnitine Deficiency. (16)
THE MAYO CLINIC REVIEW 2013 :
Following a Systematic Review of 13 controlled studies, between 1989 and 2007, researchers at the Mayo Clinic came to a different conclusion than that of the Cleveland researchers. (17)
They showed that L-Carnitine, in comparison with a placebo, produced a reduction in death from all causes, and a highly significant reduction in ventricular arrhythmias and angina attacks, following a heart attack.
In their review, which involved 3629 patients, the Mayo researchers recorded 250 deaths, 22 cases of new Heart Failure and 38 recurrent heart attacks, and reported:
A 27% reduction in all cause mortality (NNT 38)
A 65% reduction in ventricular arrhythmias (NNT 4)
A 40% reduction in the devilment of angina (NNT 3)
Reduction in infarct size in association with the dietary intake of L-Carnitine.
Their findings all had high statistical significance, an important consideration in validating their conclusions that L-Carnitine had a strong protective role in the prevention and management of cardiovascular disease.
These positive outcomes for L-Carnitine therapy establishes it as a valid therapeutic tool, in addition to other current therapies, for the management of patients with acute myocardial infarction.
The Mayo authors state, “ L-CARNITINE therapy can already be considered in selected patients with high risk or persistent angina after acute myocardial infarction, who cannot tolerate treatment with ACE inhibitors or beta-blockers, considering it’s low cost and excellent safety profile
CONCLUSION:
The relevance of animal studies (mice) in evaluating the causes of human disease is of questionable value, and the use of engineered mice in this study weakens the argument of the researchers.
The scant regard, by the researchers, for multiple confounding factors in the development of the study hypothesis invalidates their conclusions.
The concluding proposition, by the researchers, that Carnitine may cause cardiovascular disease is not supported by reputable studies.
The recently published findings of the Mayo researchers (2013) contradict the suggestions of a correlation between L-Carnitine and heart disease, as proposed in the NATURE PAPER.
THE MOTIVE ?
The underlying motive for the NATURE PAPER is speculative, given the strong associations the authors have with several pharmaceutical companies.
An added concern is the revelation that members of the Cleveland Clinic, who were involved in the design and execution of the study in question, have applied for a patent to register TMA testing as a clinical risk assessment for cardiovascular disease.
The study hardly provides a credible basis for using TMA/TMAO measurements for this purpose.
The names of the applicants seeking this patent are the Cleveland Clinical Foundation, with listed inventors, named as Stanley Hazen, Bruce Levison and Zeneng Wang, each who have “the right to receive royalty payments for inventions and discoveries related to cardiovascular diagnostics from Liposciences” (18) See Competing Financial Interests in Study Paper.
The Cleveland paper fails to provide conclusive evidence for a link between dietary or supplemental Carnitine and cardiovascular disease, and it’s dietary recommendations are to be considered with scepticism and caution.
Dr. Neville S. Wilson.
July 2013.
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- Nature Medicine 2013; April 7, Koeth R A et al.
- Am J Clin Nutr 2010; 91:535-546, Siri-Tarine, P W et al.
- Circulation 2010, Aug 31; 122(9): 876-88, The Nurses’ Health Study.
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- Higher is Better – www.drnevillewilson.com
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- www.nal.usda.gov/fnic/foodcomp/data/choline.pdf
- Journal of Food Composition and Analysis, vol 15,issue June 2002, 277-282. Mitchell, S C.
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- Public Health Nutrition 2012, April 15(14): 725-737.
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- Atherosclerosis 2013 March 15; Delaney C.
- Drugs Under Experimental and Clinical Research 1992, 18: 355-365.
- Ghr.nim.nih.gov/condition/primary-carnitine-deficiency.
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- www.ipaustralia.com.au/applicant/the-cleveland-clinic-foundation/patents au2010753965.
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