It's the caffeine, not the coffee -- coffee is good!
It's the caffeine -- it's not the coffee. Actually, caffinated coffee is slightly better than a straight caffeine source because it contains quinides which may actually increase glucose disposal and improve insulin sensitivity (this topic needs to be studied further). Coffee is well studied for its other compounds that exert antioxidant effects.
While the quinide content of caffinated coffee isn't strong enough to counter the effects of the caffeine, it's better than drinking straight caffeine because there may at least be some inhibition of the negative insulin effect propogated by caffeine -- additionally, the quinides in decaf coffee may actually increase glucose and insulin tolerance. So, if you're a coffee lover, decaf is the way to go if you are taking in carbs...
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Accumulated evidence has implicated the antioxidant effects of dietary phenols and flavonoids in the prevention of various diseases, and in particular colorectal cancer, atherosclerosis, and neurodegenerative disorders (Hollman 1996). The antioxidant potential of two plant catechol-carrying compounds found in roasted coffee was tested in blood plasma and human erythrocytes using ICS Pilot Study Program funds awarded to James May, M.D., Professor of Medicine and Molecular Physiology and Biophysics and Jason Morrow, M.D., Professor of Medicine and Pharmacology. Results suggest that the catechol moiety of chlorogenic acid derivatives conveys antioxidant activity in cells and plasma. The detailed structure-function activity of these antioxidant effects remains to be elucidated.
Hollman, P. C. H., Hertog, M. G. L., & Katan, M. B. (1996). Role of dietary flavonoids in protection against cancer and coronary heart disease. Biochemistry Society Trans., 24, 785-789.
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MD/PhD and researcher Wim Saris:
1) Caffeine intake (in all of its forms) decreases whole body glucose disposal (carbohydrate uptake) by 15-30%.
2) Caffeine intake decreases skeletal muscle glucose disposal by 50%.
3) When consumed with a standard carbohydrate breakfast, caffeine decreases insulin sensitivity, leading to large increases in blood insulin. But even in the face of this insulin surge, blood glucose doesn't disappear at a normal rate. When the body can't take up carbohydrates properly (as when drinking caffinated coffee), it releases loads of insulin to help out. However, the caffinated coffee actually prevents the insulin from doing this job and you end up with high insulin and glucose. That, my friends, is the serum profile of the obese, type II diabetic.
4) Caffeine decreases insulin sensitivity for at least three hours (this is the duration of the longest study they've performed), but the true duration of the effect isn't known.
Maybe 5 hours, the half-life of caffeine...