Given that the experts in the field are precisely those learning from and producing that same literature, the fact that the literature is generally low-quality doesn't make me more inclined to trust them.

In terms of statistics and data, yes, the papers they produce are fairly low-quality. In terms of domain-specific knowledge, however, I'd trust an expert over pretty much anyone else. That being said, I do agree with you here:

It does make it exceptionally hard to gain a good understanding of the field yourself, though. Unlike Lumifer, I'd say the correct move, unless you are yourself a nutritionist or a fitness nerd or otherwise inclined to spend a large portion of your life on this, is to reserve judgment.

Although I prefer trusting expert authority to making my own judgments on unfamiliar topics, gaining a good-enough understanding to figure out which experts to trust is still hard, especially with so many conflicting conclusions out there. This being the case, the strategy you propose--reserve judgment--is precisely what I do.