If you stop having dairy for a few years, your body stops producing the lactase enzymes that are necessary for digesting the lactose sugars in dairy. So when you start eating dairy again, you’ll get the runs like mad, and perhaps even vomiting and other digestive upsets. However, if you persist and keep consuming dairy despite the discomfort, your gut bacteria will adapt and break down the lactose for you(it helps that your native bacteria intermingle with the bacteria brought in on the dairy) so that you can happily return to consuming dairy. I’ve done this myself. So yes, the body can re-adapt to some extent to its former diets.
As for nutrients that may be missing from Soylent unbeknownst to us…well, I used to think of Soylent as something like kibble for humans, in liquid form. But there’s a key difference between Soylent and pet kibble: kibble is made primarily from various “real” foodstuffs mixed together. Yes, Soylent has some “real” stuff in there, but the majority of its nutritional value is synthetic, so it’s far more likely to be missing some unknown essential nutrient than kibble is, since kibble contains a variety of unintended nutrients in addition to what pet food companies actually measure. 100%Food is more like kibble than Soylent, but even so it does have a pretty limited ingredients list compared to kibble. I’d like to see a liquid meal that resembles kibble more, as odd as that sounds.
EDIT: I should specify that I’m talking about the good high quality kibble brands out there, not the cheap ones that are basically corn plus vitamins lol