Coffee contains many biologically active components, the most important of which is caffeine. Caffeine can affect how your body responds to insulin, the hormone that allows sugar to enter your cells and turn into energy.
It only takes about 200 milligrams of caffeine to affect blood sugar. This is the amount found in about one or two cups of coffee or 3 or 4 cups of black tea. So is caffeine harmful to diabetics? It depends on a person's body and how well it handles caffeine. For example, people who drink coffee regularly do not suffer from higher blood sugar levels compared to those who do not regularly drink it.