There are many studies on this subject. Natural ground coffee is safer for health because it does not have chemical components that are added to a soluble drink, it has less effect on the acidity of the stomach. You can find it here https://myfriendscoffee.com/best-ground-coffee/

But it also contains more caffeine. On average, the amount of caffeine in instant coffee is 60-80 mg per cup, while in the natural ground it contains from 80 to 150 mg.