There isn't a fruit and vegetable industry that would make money off of "scientific" studies, so of course there are none.
I would limit a juice fast to a couple days, and don't make it a "fast" - make it a feast, meaning get as many calories as your body needs.
I find most people opposed to healthier diets just haven't tried them, are Western conditioned to think that processed foods are part of a healthy diet, but don't worry - raw fruits and vegetables are the best source for nutrition. People who base their diet off of these as opposed to processed and cooked foods often have no idea what PMS is, take no medications, and obesity is rare while exercise and glow is prevalent. I don't need a study to see how other countries live and eat. I honestly wouldn't take dietary advice from anyone who follows the "norm" (including a normal plant-based diet unless the person bases their diet predominantly on vegetables and fruit and other items AFTER those).