I don't know the mechanism of the flu thing (and i have heard it, after you get infected you're more social until the virus confines you to bed), but the craving sugar thing is 100% gut biome, the grass-fed gut flora release opioids when they eat grass, which get sent, via the blood, to our brains, which make us feel good. not eating grasses means we go through opiate withdrawal!
This is fascinating. I wanted to know more about that but my cursory searches didn’t turn up anything except for connections between opioid use and gut microbes. It certainly feels like withdrawal!
I'm not sure where all of my research PDFs wound up over the last 20 years, but start with that DOI ( 10.1016/0014-5793(92)80414-C ) from 1992(!).
There's a trend to looking into the gut for causes of certain diseases. Prior to the pandemic, there were publications that Alzheimer's, for example, was linked to gut biome.
I get the downvotes, what i say goes against conventional wisdom, that all you gotta do to lose weight is "not eat so much" - but the human body is not just your brain and heart and some tooling to allow intake of fuel as food.
There's a growing (and, depending on your views of "life", alarming) sense that our guts are in more control than we think. This can cause a lot of "free-willers" to get upset and be reactionary.
