Background and aim: The proportion of vegetarian diets has increased in recent years. The beneficial effects of a lacto-ovo vegetarian diet on various diseases such as type 2 diabetes and cardiometabolic disorders are well documented. In contrast, there are also potentially critical nutrients such as vitamin B12, D, and iron. The popularity of vegetarian diets is increasing in endurance sports as well. However, previous studies investigating the relationship between vegetarian diets and sports are outdated, questionnaire-based and did not include nutritional or sports medical diagnostics, had a very low sample size or included only vegetarians and omnivores. There are no significant data on vegan athletes. In addition, data on exercise-induced metabolic changes do not exist. The validity of the current knowledge is therefore low. As a result, the aim of this thesis was to study the nutrient intake, status of selected biomarkers, exercise capacity and exercise-induced changes in energy metabolism (sirtuins), oxidative stress (malondialdehyde), amino acid profile (AA profile), and nitric oxide (NO) (nitrate, nitrite).Methods: In a cross-sectional study 81 ambitious male and female recreational runners (18-35 years, BMI 20-25 kg / m², 2-5 training sessions per week) with an omnivorous (OMN, n=27), lacto-ovo vegetarian (LOV, n=26) or vegan (VEG, n=28) diet were included in the study and were matched according to age and gender. Initially, anthropometric examinations, recording of nutrient intake (incl. supplements) based on 3-day dietary records as well as fasting blood samples to determine the status of vitamin B12, D, and iron, and folic acid as well as of zinc, magnesium, and calcium, were carried out. In addition, performance diagnostics and measurement of the lactate and glucose concentration by means of an incremental step test until exhaustion took place. The short-term food intake was conducted on the day of the performance diagnostics using a 24-hours dietary recall. In addition, blood samples were taken before and after exercise to analyze exercise-induced changes in sirtuins, malondialdehyde, AA profile, nitrate, and nitrite.Results: On average, dietary nutrient intake was adequate for all three groups, which was in part due to supplementation. Despite partially quantitatively different nutrient intake, a comparable and adequate status of vitamin B12, D, and iron biomarkers could be determined. The performance diagnostics revealed no differences between the groups in maximum power output related to body weight (OMN: 4.15 ± 0.48, LOV: 4.20 ± 0.47, VEG: 4.16 ± 0.55 watt/kg BW, p=0.917) and lactate and glucose concentrations. In addition, the study showed that the groups differ in terms of oxidative stress, NO metabolism, and AA profile. Exercise induced an increase in MDA in all three groups, no significant changes in nitrate and nitrite, and comparable increases in alanine, while there were differences in AA metabolism as well.Conclusion: Ambitious recreational runners with vegetarian diets have an adequate status even with otherwise critical nutrients and thus show a high health awareness. Especially in VEG, there seem to exist adaptational mechanisms in energy metabolism, as shown by sirtuin activity and AA metabolism. However, these only partially affect performance. Factors such as exercise and genetic aspects seem to have a greater impact on performance than avoiding meat. In addition, it can be assumed that increased antioxidant intake has no additive effects on exercise-induced oxidative stress. The study shows that both vegetarian and vegan diets are an adequate alternative for ambitious recreational athletes. Further studies are needed to examine the influence of vegetarian diets on health and physical performance.