This video is long, but presents the studies, the data, the results, and even traces the “low fat diet” advice back to funding by sugar companies.
Diabetes, metabolic syndrome, fatty liver, coronary issues, inflammation and more. All driven by fructose and the 1/2 of sucrose (table sugar) that is fructose.
In one case, they put a group of kids with fat problems on a constant calories diet, just replacing sugar calories with non-sugar. Their livers showed a big reduction in lipogenisis (fat making) after just 10 days. Their markers for metabolic syndrome improved, as did their cholesterol scores.
Note that starch is a glucose polymer. That it is not the problem strongly implies that you can still sweeten things with glucose. It is sold at beer and wine making supply places since using sucrose changes the flavor and some folks are picky… I’m happy to just avoid sweet things. (I avoid all artificial sweetners due to their basket of problems…)
So you don’t necessarily need to go full on keto or carnivore to fix “issues”. Starting with just avoiding sugar gets you a big win right out the gate.
Then another long one covering some of the same turf but getting into depression and mood issues in the last half.