Blood sugar spikes may occur for the first 2-3 days when reintroducing carbohydrates after a long period of zero-carb or very low-carb intake due to physiological adaptation.

“And what we know is that when we don't eat carbohydrates for long amounts of time, the body does develop glucose sparing. Now, this is not the same as pathologic insulin resistance, but we could technically call it physiologic insulin resistance at the level of the muscles. I think a more accurate descriptor is glucose sparing, which means that if you're doing a carnivore diet, if you're doing a zero carb diet, if you're doing a long-term very low carb ketogenic diet and you reintroduce carbohydrates, you're going to see a spike in your blood sugar for the first two to three days because your muscles aren't really used to seeing this glucose. It's going to take some time for your body to switch.”