Within a few hours your blood glucose will reflect what you ate. Within about an hour after a meal. If the meal is very low in carbohydrates your blood sugar should not go up much after a meal. Meat protein will only give a modest rise in both glucose and insulin secretion in a healthy person. Fat has almost no effect. Carbohydrates will cause the most blood sugar rise and associated insulin secretion from the pancreas.

Sugar is a carbohydrate. Potatoes or any starchy root vegetable is high in carbohydrate. Rice is high. Anything made of flour like bread is high.

Green vegetables that grow above ground are generally low in carbohydrates.

Here is a good source of information that shows the association of carbohydrates and the equivalent in teaspoons of sugar.

https://phcuk.org/sugar/#mce_temp_url#

Or just do a google search for a specific food like "carbs in potato" and a bunch of information will come up.

A change in diet in the amount of carbs consumed has an almost immediate result in the amount of glucose going into the bloodstream. If you don't eat enough carbohydrates to meet your body's energy glucose needs the liver will convert fat into glucose and maintain blood glucose levels.

More than you ask for.

Be aware if you happen to be on diabetic or blood glucose lowering medicines a drastic change in diet will affect how that medicine reacts. It could cause dangerously low blood sugar levels. If you are not on medication the liver will make up for any glucose levels lowered by not eating carbohydrates. I am definitely not a doctor and this is not medical advice. I am a high school educated retired dirt farmer that has an interest in metabolic health.