I haven't read the full article but the LA Times piece discusses the restrictiveness of the diet:

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Whether the Lancet study will change many doctors' minds remains to be seen. In a commentary that accompanied the Lancet study, Ghuman wrote that, while the diet's benefits were impressive, following it for more than five weeks could have detrimental health consequences.

Pelsser is the first to admit that no one can follow such a severe diet for very long. The children in her study have been gradually adding foods back to their diet. "We've followed all these children for about a year, and each child reacts to different foods," she says. For one child, triggers included beets, tomatoes, wheat and bananas; for another, it was fish, pork, eggs and oranges.