A bit of porridge for breakfast, with sugar and milk substitutes, maybe some ryvitas for lunch and perhaps a little brown rice or a couple of boiled new potatoes would be safe. If those things spike you, it still won't be as high as marmelade on toast, chips or mashed potatoes. They may want to see how well your body with metformin copes with carbohydrates so starving yourself of them will give false results. Nonetheless, it is not the time to go mad and finishing a meal with a mars bar. Slowly and gently does it and see how things unfold. Try to keep some sort of record so they have some data to go on.