«Nutrients 2014, 6, 4822-4838; doi:10.3390/nu6114822 OPEN ACCESS nutrients ISSN 2072-6643 Review The Health Advantage ...»
The authors had previously suggested a novel pathway by which gut microbiota metabolize choline/phosphatidyl choline to produce the intermediate compound trimethylamine (TMA). TMA is oxidized to TMAO which has been directly linked to atherosclerotic heart lesions , and has been show in rodent models to alter cholesterol and sterol metabolism, resulting in an increase in atherosclerosis . Koeth et al.  sought to determine whether gut microbes are also responsible for TMAO production from dietary L-carnitine by testing vegans, vegetarians and omnivores with a “carnitine challenge”.
The study found that TMAO production from dietary L-carnitine is dependent upon intestinal microbiota and the capacity to produce this TMAO was negligible in vegans. The preliminary carnitine challenge was conducted on one (n = 1) long-term (5 years) vegan who displayed virtually no capacity to generate TMAO. When 23 additional vegans and vegetarians were examined and compared to 51 omnivorous subjects, fasting baseline TMAO levels were significantly lower in both the vegan and vegetarian subjects compared to omnivores. The long-term (1 year) vegans and vegetarians who underwent the oral carnitine challenge displayed a reduced capacity to produce TMAO from dietary carnitine. Moreover, analysis of fecal samples from vegans/vegetarians (n = 23) and omnivores (n = 30) revealed that several bacterial genera were significantly associated with both plasma TMAO levels and dietary category (vegan/vegetarian vs. omnivore), suggesting that established dietary habits directly impact the ability to synthesize TMAO. Thus a direct link was established between diet type, intestinal bacterial taxa, plasma TMAO levels and associated risk for atherosclerosis.
It should be noted that in this study both vegans and vegetarians were found to be distinct from omnivores, yet not necessarily from each other. Vegans and vegetarians overall have been shown to have lower plasma carnitine concentrations than omnivores, and more efficient renal reabsorption of carnitine, particularly when dietary intake is limited . Thus there appears to be a distinct vegetarian response to low dietary carnitine ingestion, characterized by both a greater efficiency at retaining carnitine, necessary for fuel metabolism in skeletal muscle, as well as a lack of the intestinal bacteria responsible for metabolizing dietary carnitine to the proatherosclerotic TMAO. This suggests that a vegan diet may be the best option to reduce proatherosclerotic TMAO and thus reduce CVD risk, but that similar benefits may be attained by a vegetarian diet. These findings support the idea of a continuum in which the vegan gut microbiota is most different from that of omnivores, yet not necessarily distinct from that of other vegetarians.
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4. Vegan Gut Microbiota May Be Protective against Inflammatory Diseases
Studies have noted a link between vegan diets and protection against autoimmune diseases . For example, an analysis of the Adventist cohort found that a vegan diet, but not a vegetarian diet, was associated with a lower risk of hypothyroid disease . Ling and Hanninen  tested subjects on both a conventional Western diet and a raw vegan diet for one month and found that four fecal hydrolytic enzymes, associated with toxic and inflammatory products, diminished during the vegan diet. However, these changes in fecal urease, choloylglycine hydrolase, β-glucuronidase and β-glucosidase disappeared within two weeks of resuming a conventional diet. The authors attribute these reductions in fecal enzymes not only to the activity of bacteria during the dietary shift, but also to the high fiber content of the vegan diet which can affect fecal weight, transit time and bacterial metabolism.
More in-depth research has focused on vegan diets and the “extreme” raw vegan diet (the Living Food movement) as a promising treatment for rheumatoid arthritis (RA). This possibility that a vegan diet can induce a rapid change in gut profile was supported by studies of rheumatoid arthritis patients in which a one-month switch to a vegan diet was sufficient to significantly alter the fecal microflora, as determined by stool sample gas-liquid chromatography profiles of bacterial cellular fatty acids [7,62].
Peltonen et al.  conducted a study of 53 RA patients and found a significant change in intestinal flora after a one-year shift from a conventional diet to a vegan and then a lactovegetarian diet. They also noted a significant difference between the fecal flora of test subjects in the high improvement group and the low improvement group, suggesting a direct connection between gut profiles and levels of disease activity.
To further test the role of fecal flora in diet-induced levels of rheumatoid arthritis activity, 43 RA patients were randomly assigned to either a raw vegan diet rich in lactobacilli or an omnivorous diet.
After one month, there was a significant change in the fecal flora of the 18 subjects in the vegan diet group who completed the study; no such change was found in the omnivore control group.
Importantly, the vegan diet also induced a decrease in disease activity in some of the RA patients, leading the authors to conclude that changes in the fecal flora are associated with diet-induced changes in disease activity .
Kjeldsen-Kragh  followed upon their work by putting rheumatoid arthritis patients on a fast followed by 3.5 months of a vegan diet, followed by a 9-month lactovegetarian diet. Subjects in the vegan/vegetarian diet group improved significantly over those maintained on an omnivorous diet.
Similar to other studies, the authors found that subjects’ fecal flora during times of clinical improvement differed significantly from times of no or minor improvements. Others have found that a raw vegan diet rich in lactobacilli and fiber decreased symptoms of rheumatoid arthritis, suggesting that the probiotic lactobacilli, among other components of a raw vegan diet, may be helpful to RA patients [64–66]. However, while these studies effectively linked diet, microbiota profile and RA symptom severity, they either focused on vegans to the exclusion of vegetarians, or combined the diet types, thus allowing no way to discern if the improvement in RA symptoms in these studies can be attributed to a vegan diet per se, or whether comparably beneficial results could be attained by a less-restrictive and potentially easier-to-follow vegetarian diet.
Caution is warranted in interpreting the studies on vegan diets and RA. Although diet-induced modification in intestinal flora and a reduction in inflammation severity may be a contributing factor to Nutrients 2014, 6 4832 the improvements seen in RA patients, it is important to note that other features of a vegan diet have been credited with alleviating RA symptoms among vegan diet adherents. These include an increase in fruit, vegetable and fiber intake, a reduction in saturated fat and caloric intake, altered antioxidant levels, weight loss, and a reduction in food allergies and intolerances [67,68].
To the authors’ knowledge, this review is the first to assess the association between vegan diets, gut microbiota and human health outcomes. Nonetheless, several limitations should be noted. A substantial limitation of comparing studies of gut microbiota lies in the different methodologies utilized, as molecular biology has advanced rapidly in recent years, beyond conventional culturing techniques, to allow substantially greater detection of the number and diversity of human gut microbiota [32,50,69].
Moreover, comparisons of studies on vegan gut microbiota may be problematic in that fecal microbial profiles vary not only by diet, but by the subjects’ age, gender, and body mass . Dietary fiber in particular may be a confounding variable when comparing vegan and omnivorous diets, given the interrelationship between fiber, microbial SCFA production, and the host’s inflammatory response .
A note of caution must be sounded in comparing health indices and disease risk of long-term vegans with those who follow a more typical Western diet, as lifestyle factors are significant confounding variables in these analyses. Among 100 vegans surveyed in the U.S., 47% cited health as the motive for their diet choice. This subset of vegans was associated with regular exercise, minimal alcohol and smoking practices, and frequent consumption of vegetables, nuts and grains [70,71]. The German Vegan study similarly found that, compared to the general population, vegans tend to refrain from smoking, consume limited alcohol, and engage in higher levels of physical activity . Both studies found that vegans tend to have a low or normal BMI [70,71], which may be a factor in the lower rates of metabolic disease and cancer in this population. Thus more trials are warranted to determine the strength of the association between diet, microbiota profile, degree of inflammation and autoimmune response, and ultimately the manifestations of disease symptoms.
The relationship between diet and the intestinal microbial profile appears to follow a continuum, with vegans displaying a gut microbiota most distinct from that of omnivores, but not always significantly different from that of vegetarians [34,42]. The vegan gut profile appears to be unique in several characteristics, including a reduced abundance of pathobionts, including Enterobacteriacea [33,42], and a greater abundance of protective species such as F. prausnitzii [34,51–53]. Vegans also appear to lack the intestinal microbiota for converting dietary L-carnitine into the proatherosclerotic TMAO .
Reduced levels of inflammation may be the key feature linking the vegan gut microbiota with protective health benefits. The role of dietary fiber in promoting lower levels of inflammation in subjects following a vegan diet warrants further exploration. The influence of plant versus animal protein sources on microbial profiles, metabolic syndrome and inflammation may be an avenue for future research as well.
New research holds great promise in revealing the mechanisms linking dietary patterns with gut microbiota profiles, obesity, inflammation, and disease states. For example, recent studies have Nutrients 2014, 6 4833 revealed the link between inflammasomes, a group of protein complexes that recognize inflammation-inducing stimuli, and obesity, metabolic syndrome, insulin signaling and atherosclerosis [55,72–74]. Inflammasomes play a role in regulating gut microbiota and gut homeostasis, which in turn affects the immune homeostasis of the entire host organism . The role of inflammasomes in regulating gut flora and the subsequent impact on metabolic syndrome is an exciting new field of study which may help elucidate the mechanism by which diet impacts gut microflora, inflammation and health.
An area in need of future research is the distinction between long-term diet adherence and short-term diet therapy. Wu et al.  postulate that long-term diets are responsible for the distinct enterotypes seen between those who follow an omnivorous diet and those who eat little or no animal products. Yet some studies have found that a dietary change can induce a partial shift in gut microbiota in a matter of weeks or even days [7,43]. It seems likely that sudden changes in diet, such as adopting a vegan diet to improve disease outcome, may alter the relative abundance of different taxa in the gut, without shifting the host into an entirely new enterotype . As vegan diets are gaining in interest as medical nutrition therapy, it is important to discern the long-term advantage of short-term diet change.
If patients need to maintain a strict vegan diet for long periods of time to reap continued health benefits, compliance may become an issue.
Diet compliance by patients is a critical factor in the feasibility of a vegan diet as adjuvant therapy in disease management. Barnard et al.  found that patients following a low-fat vegan diet for management of type 2 diabetes had diet compliance rates comparable to that of patients following a conventional diabetes diet after 74 weeks. Study participants adhering to the low-fat vegan diet demonstrated increased carbohydrate and fiber intake and decreased fat and cholesterol intake, suggesting that the diet has not only acceptability but applications for medical nutrition therapy.
However, high drop-out rates are a concern in studies in which subjects are compelled to adopt a vegan or even a vegetarian diet . While vegan diets have been shown to improve metabolic conditions in type 2 diabetes patients, similar improvements have been achieved with other diets including the Mediterranean diet, a low-carbohydrate/high-protein diet, and a vegetarian diet. Thus a patient’s personal taste and cultural traditions may need to dictate whether a vegan diet is the ideal choice for medical nutrition therapy .
Acknowledgements The authors acknowledge the reviewers for their insightful comments and recommendations.
Author Contributions All authors equally contributed to the preparation of the manuscript and have approved the final version.
Conflicts of Interest
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