Could someone get all the nutrients etc they need from liquids and not have to eat foods?
That was the idea behind infant formula and Tang. If you know what is. It is now known that mother's milk is far superior and orange juice is also a better choice. These were man-made products, with a few components derived from whole foods like soy protein and casein.
If you stick with whole foods and run them through a blender, I'd say - yes.
If you are buying powdered whatever with a list of ingredients that take a PhD chemist to interpret, and then adding water, I'd say - no way.
The reason is the so-called vitamin paradox. We've extracted or synthesized things like antioxidants and put them into powders and pills, and they do not have measurable effects like they do when they are consumed as a component in whole foods. Until we get past that problem I'd go for the blender approach.
If you juice fruit and vegetables, make fresh soymilk and milk from nuts you get most of your needs with the exception of fibers.
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