The Adventist Health Studies are a series of long-running cohort studies run out of Loma Linda University, tracking members of the Seventh-day Adventist Church, whose religious teaching favors a largely vegetarian diet, discourages tobacco and alcohol, and emphasizes rest and physical activity. The first study, AHS-1, began in 1974; the current phase, AHS-2, enrolled roughly 96,000 participants across North America starting in 2002 and has followed them for years since, recording diet, lifestyle and health outcomes at a scale few voluntary religious populations offer.
The appeal to researchers is that Adventists provide a large population sharing one faith but a wide range of actual dietary practice within it, from strict vegans to regular meat eaters, all living in the same countries and subject to the same medical systems. That internal variation lets the studies compare health outcomes by diet without also having to control for the much larger differences between entirely different national populations, which is a harder comparison to draw cleanly.