This study aimed to evaluate the effect of a vegan diet (supplemented with vitamin B12) and an mediterranean diet on performance, cardiorespiratory fitness, metabolic health, immune status, and environmental impact in healthy adults.

OMNIVEG is a controlled crossover trial. Initially, registered dietitians conducted an assessment of participants' lifestyles in a preliminary session. Qualified nutritionists then developed personalized diets for each participant in accordance with the recommendations of the International Society of Sports Nutrition (ISSN). These diets followed specific guidelines, including a daily intake of 3-5 g of carbohydrates per kilogram of body weight, 1.4-2.0 g of protein per kilogram of body weight, and 0.5-1.5 g of fat per kilogram of body weight.

Both the mediterranean diet and vegan diets prescribed to the participants were isocaloric, with similar distributions of macronutrients. The primary difference between the two interventions was the source of food: the vegan diet exclusively comprised plant-based foods, while the omnivorous diet included foods of both animal and plant sources. In the case of the omnivorous diet, 60% of the total protein intake was derived from animal sources (mainly from fish, white meat, low-fat dairy and eggs).

As part of the vegan diet, participants were instructed to take 1000 µg of cyanocobalamin (a form of vitamin B12) twice a week from Harrison Sport Nutrition, Granada, Spain. This supplementation aimed to ensure adequate vitamin B12 intake, which is essential for individuals following a vegan diet.

Throughout the study period, participants were advised to maintain their initial physical exercise frequency and volume to maintain consistency and avoid confounding variables. To ensure standardized measurements, all tests were conducted in the same laboratory, employing identical testing devices, and supervised by the same group of researchers. This approach aimed to minimize experimental variability and improve the reliability of the study's results.

Inclusion Criteria:

• Men.
• Aged between 18-40 years.
• Physically active according to the recommendations of the World Health Organization (WHO).
• Body Mass Index (BMI) between 18.5-24.9 kg/m2.
• No tobacco use.
• No or low alcohol consumption and no orthopedic limitations that would interfere with the performance of the study tests.

Exclusion Criteria:

• Chronic diseases that impair athletic performance (cardiovascular, metabolic, gastrointestinal, respiratory) within the last six months.
• Musculoskeletal disease within the last six months.