The battle between vegetarians and carnivores was already present in the time of dinosaurs. In the modern age, the benefits of vegetarianism are highly praised and are still getting more and more attention. Not only the common health improvements but also the effects on physical and athletic performance in sports are examined and verified by evidence-based researches. The focus on fruits and vegetables concerning athletic performance has already been object of numerous studies and researches. For example, David C. Nieman examined the relation between physical fitness and vegetarian diets by comparing endurance capability of vegetarians and non-vegetarians. (Nieman, 1999) Moreover, there are several top-athletes who decided to use a vegetarian dietary pattern, such as tennis professionals Venus and Serena Williams or the German basketball player Dirk Nowitzki. Another successful example is the boxer Timothy Bradley who went for a three-month vegan training diet and became WBO welterweight champion in 2012.
There are different types of vegetarian diets, which contain and exclude different nutriments. During a vegan diet for instance, one completely abstains from animal products such as meat, milk, eggs or honey, whereas lacto-ovo vegetarians, the probably most common variation only go without meat, poultry, and fish (Figure 1). In this literature review the effects of vegetarianism on athletic performance will be investigated and evaluated. Therefore, the macronutrients carbohydrates and protein, the micronutrients Vitamin B12 and Iron, and the special case creatine will be structured thematically with the help of current literature.
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Figure 1: Classification of Dietary Patterns
Craddock, Probst and Peoples (2016) Vegetarian and Omnivorous Nutrition - Comparing Physical Performance S.213, International journal of sport nutrition and exercise metabolism
The most important macronutrient especially for endurance athletes are carbohydrates. In order to reload glycogen stores in muscles and liver and to uphold body weight it is necessary to adapt the carbohydrate intake throughout periods of heavy physical activity (Rodriguez et al. 2009). A high supplementation about 60-65% of total energy seems to have a positive effect on athletic performance, due to the ability to prolong aerobic and anaerobic activity (Borrione et al. 2009). These number should however be treated carefully, since the intake and adaptation mechanisms are always individually dependant, e.g. body size, sport-specific energy requirements, and gender (Venderley and Campbell, 2006).
Concerning overall carbohydrate consume, especially complex carbohydrates vegetarians appear to have a slight advantage over omnivores due to their focus on legumes, cereals, and pastas. Nieman (1999) for example could recognize among >75% of 347 marathon runners a higher intake of fruit, vegetables, and whole grains instead of red meat or eggs compared to their prerunning nutrition. Furthermore, the probably most interesting fact is whether and how a vegetarian diet in terms of carbohydrate intake may influence anaerobic and aerobic performance. For instance, Craddock et al. (2016) investigated four studies regarding the effect mentioned above. None of the studies was able to show significant improvements or differences in aerobic or anaerobic performance. However, one of the investigations recorded a significantly higher oxygen consumption by virtue of a low protein vegetarian diet.
Even long-term vegetarians (mean duration vegetarianism 46 years) do not seem to have increased cardiorespiratory fitness. Moreover, the examined individuals displayed a significantly lower blood glucose level compared to non-vegetarians (Nieman 1999). As the theory indicates and according to Rodriguez et al. (2009) a restriction of carbohydrates though, seems likely to detrimentally affect athletic performance. Nieman (1999) reviewed several more studies which cannot support the hypothesis of an increased endurance performance due to the possibly higher intake and higher quality of carbohydrates during a vegetarian diet. (Table 1)
Table 1: Total endurance time to exhaustion related to time on diet
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Nieman (1999) Physical fitness and vegetarian diets: is there a relation? S. 571, American Society for Nutrition
All in all, there is no evidence that vegetarians who do easily fulfil carbohydrate needs appear not to gain an advantage over omnivores concerning endurance activity and cardiorespiratory fitness. But when not controlling the diet carefully a decrease of carbohydrate intake may lead to negative effects on athletic performance.
Protein is the macronutrient which is responsible for muscle and strength improvements. It is proven that an enlarged protein oxidation and breakdown exists during physical activity, followed by heightened muscle synthesis and further protein breakdown while recovery (Fuhrman and Ferreri, 2010). A widespread assumption, particularly among amateur and hobby athletes, proposes higher protein needs for athletes than for normally active persons, but there is still continuing discussion about this controversial topic. For endurance athletes, characteristical suggestions are 1.2 to 1.4 g/kg/d (gram protein per kilogram bodyweight per day) and for heavy strength and resistance training athletes up to 1.7 g/kg/d. The Institute of Medicine however, illustrated that the recommended dietary allowance (RDA) of 0.8 g of protein is adequate for exercising adults (Barr and Rideout 2004). Moreover, the Institute of Medicine showed that protein requirements are commonly met or even exceeded by vegetarians who include dairy products or eggs in their dietary pattern and consider a balancing composition of high-quality plant proteins (Barr and Rideout 2004). Nieman (1999) supports this opinion, although he notes that a strict vegan diet often provides less protein than non-vegan diets.
Resuming this point, Barr and Rideout (2004) suggest a 10% higher consumption of protein for vegan athletes in order to be completely safe not to fall below requirements. The thought process behind this recommendation is that plant proteins are less well digested than animal proteins. However, Haub et al. (2005) for example, were not able to find differences in improvements of upper and lower body strength and power between two groups after a 12-week dietary pattern of either beef-containing food or texturized vegetable protein.
Based on carefully planning a vegan diet, containing protein-rich plant foods, protein requirements can be met easily and athletes do not have to fear decreased muscle and strength gain. Further important facts to be noticed are quality and timing of protein intake to guarantee optimal digestion and conversion.
 Note. Adapted from “Beyond Meatless, the Health Effects of Vegan Diets: Findings from the Adventist Cohorts,” by Lee & Sabate, 2014, Nutrients, 6, p. 2133. Copyright 2014 by the authors; licensee MDPI, Basel, Switzerland.