I am 82 years. My blood sugar level is fasting 79 and after meals 219. What is the cause of this wide gap?

Insulin issues. It may be that you are having trouble producing enough Insulin right after a meal to control the blood sugar. But it eventually corrects. Therefore, do not eat large meals and do not eat a diet high in starches or sweets. You do not want to overtax your pancreas because that might push you into full fledge diabetes!