In their paper published in Clinical Nutrition, Ellegård and colleagues report that they can distinguish between individuals on different habitual diets (vegan, vegetarian, omnivore) based on the carbon and nitrogen isotopic ratios of their hair (1). The finding is not new, and has been previously reported in the literature over the last 22 years (2-4). The authors do not properly acknowledge that previous work has shown similar or identical findings framed around the same hypothesis, despite such work being described and discussed in multiple works that they do cite (5-7). I would argue that such findings now come under the heading of common knowledge in many fields, even to the extent of being used as a student practical exercise in many university degrees across the world including in my own teaching – I have a database of hundreds of hair samples with associated dietary records.