Hi, While I Mostly follow Atkins, I have a couple copies of South Beach.

Changing food intake affects the foods for the microbiome of the gut. In livestock we change foods over a two week period. It take that long to gentl change the profile n quantites adapting to the new foods.

Fr me, taking probiotics helps ease a fast change over. And actually keeping up a daily allotment helps with if you fall off the wagon.

When feeding cattle and sheep, we focus on feeding the microbes in the rumen. Ours is not a big vat but a thin tube located after the stomach...but functions the same. Eating bread after a long hiatus gave me horrible cramps for 24 hours. Since keeping up with probiotics, GI i happy no matter what goes down.

Hope your GI settles quickly an you have an easier time of it.