I was sure I had replied to this post... hmmm, must have written something and never pressed post! I'd have to say I can't think of anything that has NO nutritional value. But of course sometimes things are used for other purposes, such as softening meat, food colouring, sweetener. And of course some plants might have side effects. I'd heard it often that manioc leaves have a decalcifying effect, so if you eat a lot then it can weaken your bones. Although that's not to say that it has no nutritious value. Here where I've been studying in Brazil though people don't have the custom of eating the leaves anyway. I visited Madagascar before and it's an important traditional dish there. An example from back home, Galium aparine, or cleavers / goosegrass / robin-run-the hedge, interestingly it has a negative calorific effect, so it speeds up the metabolism and you end up using more energy to digest it than what you get from it. Apparently Victorian ladies used to eat it as a dieting tonic. I'm surprised it's not used in that way today to be honest considering society's obsession with weight loss.

There's still lots of expensive laboratory work to be carried out to detail the nutritional contents of wild plants. The general pattern though is that the wild edible plants that have had extensive lab work to map out their nutritional qualities, usually they are extraordinarily nutritious. Many of them send down tap roots to eek out the maximum of nutrients from the soil.

And yep, there is astounding loss of ethnobotanical knowledge all around the world. It can disappear rapidly. Sometimes it seems that people think that Google knows everything and that we don't need to take any proactive action ourselves to preserve knowledge, but there is just so much info that you cannot get from a Google search....and, so much knowledge that doesn't particularly lend itself to being transmitted in modern media formats. Never mind the more philosophical / spiritual aspects of some of that "wisdom".

The general pattern seems to be this, there is rapid loss of traditional knowledge in rural communities where people are aspiring to achieve a more modern life-style, or are simply being forced into a more modern life-style and either no longer value that knowledge or simply don't have time, or have moved away to the city and broken the inter-generational transmission pathways....whilst meanwhile the middle-class of the city generally have little traditional knowledge within their own immediate families but are actively seeking to reconnect with ancestral imaginative. That is happening in Europe and Brazil and no doubt lots of other countries. It's interesting to see in Brazil a lot of people putting energy into reviving and allocating value to traditional knowledge, it's still fresh, many city-dwellers have grandparents who lived in the "roça".

Went on a bit of a ramble there, hope I answered your question!