To mix fruits and vegetables (especially the coarser vegetables, such as beets, turnips, onions, boiled cabbage, etc.) at the same meal is never advisable ; not only with respect to the blending of acids with starchy vegetables, but because of the differing periods of time required for their digestion. Fruits, as a rule, are very quickly digested, a sweet apple requiring but a little more than an hour, while a heavy vegetable, such as boiled cabbage, requires from four to six hours. When these are mixed together in the stomach, the lengthy process of vegetable digestion serves to detain the fruit mixture beyond its normal limit, until fermentation naturally results.
“It is better to have the fruit at one meal, and vegetables at another.” White.