The average U.S. adult drinks about two 8-ounce (240-milliliter) cups of coffee a day, which can contain around 280 milligrams of caffeine. For most young, healthy adults, caffeine doesn't appear to noticeably affect blood sugar (glucose) levels, and having up to 400 milligrams a day appears to be safe.

Some studies suggest that drinking coffee — whether caffeinated and decaffeinated — may actually reduce your risk of developing type 2 diabetes.

If you already have diabetes, however, the impact of caffeine on insulin action may be associated with higher or lower blood sugar levels. For some people with diabetes, about 200 milligrams of caffeine — or the equivalent of one to two 8-ounce (240-milliliter) cups of plain, brewed coffee — may cause this effect.

Caffeine affects every person differently. If you have diabetes or you're struggling to control your blood sugar levels, limiting the amount of caffeine in your diet may provide a benefit.