It is really hard to do a good studies on diet. You end up with one of two conclusions: “Despite our best efforts we were unable to get our test subjects to follow the required diet”; or “These results may not generalize to the general population who isn’t confined to [a prison cell/hospital bed]”.

We can study how one meal effects your body, but that isn’t really helpful - Does it matter if some diet causes cholesterol to go up/down for a bit and then it returns? And cholesterol is one of those markers where we have enough studies to conclude that high is bad, but that doesn’t necessarily mean that high for a bit and returning to low is also bad. Some things like smoking are such large effects that we can look at general population and make conclusions, but often the effect isn’t that large and so it is believed that some diet is good/bad, but we cannot prove it from data we can collect.

The above is about actual science. Most mainstream diet books at best cherry pick some fact and then take it to an extreme to create some eating plan ignoring all evidence of other facts that might limit how far you can take this one. (that is assuming they start with a fact - just making up facts is common as well) The news media doesn’t care to figure out what is real science and what is made up facts.