From my experience, Google Scholar (https://scholar.google.com/) is a great place for research. You can just enter some key words (e.g. hba1c) in combination with plant-based, vegan, vegetarian, omnivore etc. and it will show you relevant scientific papers and studies. Still be sceptical with everything you find. Not every study or paper is independent and reliable. As a rough indicator you can look at the number of citations. Typically, good studies and articles are quoted more often.