Why should we dilute pure fruit juices with 50% water?

Dilution is recommended in the case of pure fruit juices. This is because pure fruit juices have concentrated natural sugars.

Fruits are sweet in nature. When they are juiced, the fibre is removed and the natural sugar gets concentrated in the juice. Fibre allows for slow and controlled digestion of sugar. Without the fibre, the sugar will enter the bloodstream too quickly, putting undue pressure on the pancreas to release a lot of insulin to stabilize the high sugar levels.

That is why it is suggested to dilute pure fruit juice with 50% water and then have it. 

In case of diabetes, strictly avoid fruit juice, and stick to only vegetable juices.