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Plant‐based diets reduces the risk of chronic conditions. The interaction between protein source and other macronutrients—fat (F) and carbohydrate (C)—has yet to be investigated. The aim was to assess the main and interactive effects of protein‐source (plant vs. animal) and F:C (high or low) and the transition from an Australian diet to a whole food diet on various health markers in older individuals. This single‐blinded, parallel, randomised experimental trial used a 2 × 2 factorial design to compare pro‐vegetarian (70:30 plant to animal) versus omnivorous (50:50 plant to animal) diets at 14% protein and varying fat‐to‐carbohydrate ratios (high fat ~40% vs. low fat ~30%) over 4 weeks. Study foods were provided, alcohol consumption was discouraged, and dietary intake was determined through food records. Analysis included both RCT and observational data. Changes in appetite, palatability of diets, and dietary intake were assessed. Body composition, muscle strength, function, gut microbiome, and cardiometabolic health parameters were measured. Data from 113 (of the 128 randomised) individuals aged 65–75 years were analysed. Pro‐vegetarian diets reduced diastolic blood pressure, total cholesterol and glucose levels. Moreover, the overall sample exhibited increased short‐chain fatty acids and FGF21 levels, as well as improvements in body composition, function, and cardio‐metabolic parameters irrespective of dietary treatment. Transitioning to a diet rich in fruit, vegetables, fibre, and moderate protein was associated with improved health markers in older age, with added benefits from pro‐vegetarian diets. Further research on long‐term effects is needed. [ABSTRACT FROM AUTHOR]