In general, it is very difficult to do controlled, large population studies over a human life span. Instead, lots of studies rely on self-reported surveys, measurements of various indicators that are believed to correlate with poor heath effects over a very short period, and so on.

Mapping from "we know that if you eat X, the Y level in your blood rises over the next 24 hours, and in a separate study we have shown that Y is correlated with chronic disease Z" is actually a pretty poor argument for "X causes Z", and even more of a poor argument for "thus, you should eliminate X from your diet", and telling people to eliminate "X" from their diet doesn't necessarily always lead to simply the desired effect of eliminating "X", but may lead to them replacing it with "W" which is even worse.

Human bodies and human behavior are quite complex. The actual effects of a particular recommendation are not well understood. By making recommendations and policy from science that was not well enough understood, we may have increased the rate of growth of the obesity epidemic over the past 30 years or so. This has happened, in part, by over-emphasizing "low fat" which encouraged people to replace fat with sugars.

So, food policy recommendation are not food science. They may be influenced by science, but they are different. And I generally take them with a very large grain of salt.

Instead, we can find strategies that, at least anecdotally, seem to work better than adhering to the latest fad diet or the government recommendations. Eat real food; plants and animals that have been minimally processed before getting to you. The more processing that has been done, the more the foods have likely been optimized for something other than nutritional value and flavor (shelf life, profits, etc). Don't eat too much; whatever you do, if you want to lose weight, eating less across the board will likely achieve that. Food that is decadent is likely good to have only in small quantities; if it's sweet or greasy, it's probably good to limit to lower quantities. Get exercise daily; walk, or bike somewhere, hit the gym, do an outdoor activity, or something, don't go from bed to car to chair to car to couch to bed.

Following these kinds of recommendations may not seem as official or authoritative as the government or some author selling a new diet book. But anecdotally, they work pretty well, and they aren't likely to be skewed by various parties monetary interests or simply bad interpretation and over-generalization of good scientific data.