I've been independently investigating this very issue for myself lately, as the subject of a written assignment for my current studies.

What I've personally found is that there have definitely been differences between the DGAC's review processes for 2020 vs 2015. The end results for low carb in 2020 don't look to be much different than for 2015, but the reasons for their exclusion are definitely different this time around.

For example, in 2015 the review committee carried out a number of searches across various databases of literature. The dietary patterns that they searched for in their review on body weight/obesity were primarily Mediterranean, DASH, vegan and vegetarian, with additional searches for "prudent", "Western" and "plant based" diets also appearing. So, little wonder that they couldn't find anything for low carb in 2015; they never even searched for low carb in the first place. This was actually the biggest shock to me in my own research, that as bad as the 2020 review is turning out to be, their internal process for 2015 was actually even worse.

In 2020 meanwhile, the list of dietary patterns for which the committee searched for their review on body weight/obesity, got a lot longer: this time they searched for Mediterranean, DASH, "prudent", paleolithic, vegetarian, vegan, plant based, "Western", "healthy", low carbohydrate, high carbohydrate, ketogenic, low fat, high fat, high protein and low sodium. So at least this time they actually returned a bunch of low carb studies from their initial searches.

But this time, practically all of those studies were excluded on the basis of some interesting eligibility criteria. For example, for studies of diets based on macronutrient distribution (such as low carb), one criteria is that all three macronutrients be defined in the study. Some of the individual studies I looked at specified the amount of carbs and fat for example, but not protein, so the study is excluded. Even though the protein could still be calculated by the reader as being a logical subtraction of the carbs + fat.

I'm still putting the finishing touches on my own course assignment since it's not due for several more weeks, but from what I've put together to this point, LCAN is right on the button here. When it comes to low carb acceptance in the guidelines, the specific nature of the barriers this time around may have changed, but they're still there nonetheless.