Caffeine and kidney stones:
45,289 men without previous renal stones were followed in the Health Professionals Follow-up Study for formation of stones. Increasing intake of fluid reduced kidney stone formation. Specific types of beverages were found to decrease risk of stone formation. For each 8 ounce serving of the following beverages consumed per day, stone formation risk decreased by the following amounts on average: coffee with or without caffeine, 10%; tea, 14%; beer, 21%; and wine, 39%. Also, for each 8 ounce serving of apple juice, stone formation risk increased by 35%, and grapefruit juice increased risk by 37%. (7)
Coffee consumption and diabetes:
Drinking caffeinated coffee with meals may decrease insulin sensitivity and exacerbate control of blood glucose, according researchers. Ten healthy men 5 mg/kg of caffeinated coffee one hour before eating a meal (75-g oral glucose load) with either a high glycemic index (Crispex) — comprised of foods that raise blood sugar levels quickly, such as processed carbohydrates — or a low glycemic index meal (All Bran). Caffeinated coffee before a high glycemic index meal resulted in significant insulin sensitivity reduction of 40% compared to decaffeinated coffee before the same meal. Caffeinated coffee prior to a low glycemic index meal led a 29% decrease in insulin sensitivity than the same meal eaten after consuming decaffeinated coffee. (8)