A story about how your uncle or your college roommate eats loads of butter and still stays fit is an anecdote.
But if you want to be taken seriously, you need to use data, the kind that's arrived at through peer-reviewed studies with large sample sizes.
Better yet, go for consensus.
"Scientists often use 'consensus' as the ultimate argument-winner, and for good reason," Jacquelyn Gill writes on Contemplative Mammoth. "Scientific consensus is the collected opinions of all scientists, and not just the one you're arguing with. There can be one or two scientists who disagree (just like there are a handful of people who don't believe the Holocaust happened), but if the vast majority of scientists have reached consensus, it means that there is so much evidence in support of an idea that it's basically a guaranteed thing, based on state-of-the-art knowledge."