I don't know about insulin responses and such, but there has been a diet based on a piece of grapefruit in the morning to help in losing weight. I think it was decided (not proven, much of science is actually decided on) that the effect was probably because the acidity of the fruit made you feel very full, so you ate less. In the long run, the weight loss clinical studies had some replication problems and I think a meta-analysis (the effect from the grapefruit, averaged out in a way over several studies) ended up being a wash. I don't know about grapefruit juice; some of them add a lot of sugar.