Vegetarian or Vegan diets represent a minority of the world’s dietary patterns with strong disparity of frequencies between countries (30% of vegetarians in India as against 2 to 5% in France according to verified sources). Interest in these diets is growing, as they bring consumers to think about ethical and environmental matters. In this article BIOFORTIS highlights some of the knowledge publically available to date as well as different study perspectives with regards to the impact of a vegetarian diet on health and the microbiome (Glick-Bauer and Yeh 2014).
Microbiome and dietary pattern
Researches have been conducted in order to investigate if dietary patterns can be associated with enterotypes. An enterotype dominated by Bacteroides is associated to diets high in protein and animal fat, while the Prevotella enterotype is associated with carbohydrate metabolism and found on vegetarian subjects. Wu et al. observed on ten subjects, who switched from a high-fat/low-fiber to low-fat/high-fiber diet displayed detectable changes in their microbiome composition within 24 hours, although their enterotype identity remained stable for the duration of the 10-day trial (Wu et al. 2011).
In another trial conducted by Zimmer et al, the fecal microbiota profile of diets from vegans (n = 105), vegetarians (n = 144), and an equal number of controls on an omnivorous diet were investigated. Samples from vegan subjects had significantly lower microbial counts than their omnivore counterparts for four bacterial taxa: Bacteroides, Bifidobacterium, E. coli and Enterobacteriaceae. The Vegan diet was the most distinct from omnivores, but not necessarily significantly different from that of vegetarians (Zimmer et al. 2012).
Contradictory findings are discussed in the review by Glick-Bauer and highlight the methodologies lacking (impact of long term effects, comparable subjects in term of age, gender, BMI, etc. and the impact of microbiota analyses methods for example) (Glick-Bauer and Yeh 2014).
In a general way, the impact of fiber intake appears to play a key role in shaping the microbiome as vegetarian diets comprise large consumption of fruits and vegetables in quantity and diversity.
Another aspect to consider with regard to foods consumed in vegetarian and vegan diets is the bioavailability differences between micro and macronutrients. For example, the fact that essential amino acids, and especially the branched-chain amino acids (BCAAs), are more concentrated in animal-based protein compared to plant protein, is a recurrent argument. In order to investigate the digestibility of proteins, the Food and Agriculture Organization created in 2011 an index called the Digestible Indispensable Amino Acid Score (DIAAS). Typically, animal protein has higher DIAAS scores compared to plant proteins, reflecting therefore higher digestibility. However, to date, it is recognized that consuming a balanced vegetarian or vegan diet that includes a variety of plant protein sources has consistently been shown to be nutritionally adequate in terms of providing sufficient amounts of essential amino acids (Lynch, Johnston, and Wharton 2018).
The major concern does not regard proteins but vitamins and minerals. The risk of B12 deficiency is critical for vegan people, as it is found exclusively in meat and fish and, to a lesser extent, in milk and eggs. In consequence, food supplements are strictly recommended.
Iodine deficiency can also be an issue, as it is contained in iodine salt, fish and seafood. Some minerals found in plant-based foods (calcium, zinc, iron) are less bioavailable than those found in animal-based foods. In case of iron, its weak bioavailability in plants can be partially compensated by the fact that vegetarians and vegans consume a lot of fruit, and in consequence have high vitamin C intake, which furthers iron absorption.
Results of studies have to be taken with caution regarding the fact that even diets which allow moderate amounts of animal products may be protective against disease. A pool of analysis of five prospective cohort studies, involving 76,000 subjects found that both vegetarians and those who consumed small amounts of red meat benefited from a reduced risk of coronary heart disease and type 2 diabetes (McEvoy, Temple, and Woodside 2012). That’s why the exact food consumption (in quality and quantity) has to be really representative.
Another matter to take into consideration is the characterization of the subjects to be compared. Compared to the general population, vegans tend to declare less smoking, lower alcohol consumption and more physical activity. In addition it was also observed that vegans follow dietary recommendations, such as regular intake of fruit, whole grains, seafood and plant-based protein, and sodium . (Parker and Vadiveloo 2019)
So, it could be interesting to investigate the effect of vegetarian and vegan diets versus “balanced omnivores diets”, to avoid subject’s inter-variability bias.
While observational studies remain the most common way to study this kind of diet, interventional studies, randomized and controlled could be an interesting way to investigate short term effects and possibly, long term effect of diets
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Zimmer, J., B. Lange, J.-S. Frick, H. Sauer, K. Zimmermann, A. Schwiertz, K. Rusch, S. Klosterhalfen, and P. Enck. 2012. ‘A Vegan or Vegetarian Diet Substantially Alters the Human Colonic Faecal Microbiota’. European Journal of Clinical Nutrition 66 (1): 53–60. https://doi.org/10.1038/ejcn.2011.141.
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