Polyphenols in dark chocolate lower blood sugar levels in rodents; human studies suggest dark chocolate is safe for many diabetics
by Craig Weatherby
It would seem safe to assume that chocolate is a bad choice for people with diabetes or at risk of the disease.
But, combined with prior human studies, the surprising results of a new animal study from Japan turn conventional wisdom on its head… at least with regard to raw cocoa and to chocolates dark enough (like ours) to be very rich in antioxidant polyphenols.
A team led by Dr. Makoto Tomaru of Tokyo Medical and Dental University tested the effects in female mice of diets supplemented with a cocoa “liquor” rich in the polyphenols called flavanols (catechins and procyanidins).
They did this by adding various amounts of a cocoa “liquor” high in flavanols to the diets of healthy and diabetic mice (Tomura M et al 2007).
(Note: The term “cocoa liquor” refers to finely ground cocoa beans, which liquefy during grinding. Cocoa liquor has the same flavanol content as unsweetened baking chocolate, and not many more flavanols than our 85-percent-cocoa Organic Extra-Dark Chocolate bars.)
Study tests various levels of cocoa flavanols
The scientists used two groups of mice: normal, healthy mice and obese, diabetic mice, and divided each of these main groups into three subgroups:
- Control Group received a standard lab-mouse diet.
- Group A received a standard diet containing 0.5 percent flavanols.
- Group B received a standard diet containing one percent flavanols.
They measured the animals' blood levels of glucose and fructosamine at the beginning of the study, and after three weeks on the test diets. (Fructosamine is a metabolic byproduct of dietary sugars: doctors measure its blood levels to help gauge diabetics' sugar-control status.)
Compared with the healthy mice, the diabetic mice started the study with higher levels of blood levels of glucose and fructosamine.
Chocolate flavanols moderate blood sugar in diabetic mice
After three weeks, the blood levels of glucose and fructosamine in the healthy animals did not change significantly, regardless of which diet they were on.
But the blood levels of glucose and fructosamine dropped substantially in the obese, diabetic mice fed flavanol-supplemented diets (Their body weights and food consumption were unaffected).
And the flavanols reduced blood sugar levels in a dose-dependent manner, with the one-percent flavanol diet lowering them more than the 0.5-percent flavanols diet.
As the investigators said, “To our knowledge, this is the first study to report that flavanols can prevent aggravation of type 2 diabetes...” (Tomura M et al 2007).
Based on the result of prior human and animal research, the Tokyo-based team hypothesized that the antioxidant effects of flavanols reduced insulin resistance in the mice, thereby lowering the animals' blood levels of glucose.
What about people?
A prior study showed positive blood-sugar-control-effects in people with high blood pressure who ate three ounces (100 grams) of dark chocolate per day (Grassi D et al 2005 and 2003).
We also found a Swedish study in diabetic adolescents, in which replacement of a “diabetic” snack with milk chocolate actually produced a lower blood glucose response.
As the Swedes said, “We conclude that an occasional exchange of a regular diabetes afternoon snack for an isocaloric [calorie-equivalent] amount of milk chocolate bar… has no negative impact on… blood glucose.” (Cedermark G et al 1993)
Together with the new findings in mice, the outcomes of these human studies indicate that moderate enjoyment of dark chocolate—especially extra-dark chocolate—does not promote diabetes.
Dark chocolate doesn't spike blood sugar
Intrigued by the results of these studies, we took a look at the “glycemic index” and “glycemic load” of dark chocolate.
- Glycemic Index (GI) ranks carbohydrates on a scale from 0 to 100, according to the extent to which they raise blood sugar levels after eating. Foods with a high GI are those which result in large rises in blood sugar levels.
- Glycemic Load (GL) combines both the GI and quantity of a carbohydrate-rich food in one ranking, and is the best way to gauge the blood sugar impacts of different types and amounts of food.
The only dark chocolate in The University of Sydney's GI Database had these rankings (1.3 oz / 37 gm serving):
Glycemic Index = 23 (low)
Glycemic Load = 4.4 (low)
This is how The University of Sydney's GI website characterizes various GI and GL ranges (GI Database 2007):
Low GI = 55 or less
Medium GI = 56 - 69
High GI = 70 or more
Low GL = 10 or less
Medium GL = 11- 19
High GL = 20 or more
Today's welcome research news is not a license to overindulge in dark chocolate. As in all things, moderation is wise. But it is heartening to learn that extra-dark chocolate may be a healthful treat… even for many diabetics.
Diabetics with serious blood-sugar control issues should consult with a doctor concerning dark chocolate. And because everyone is different, all diabetics should monitor blood sugar to see how it affects them.
- Tomura M et al. Dietary supplementation with cacao liquor proanthocyanidins prevents elevation of blood glucose levels in diabetic obese mice. Nutrition. Volume 23, Issue 4 , April 2007, pages 351-355. Published on-line ahead of print. doi:10.1016/j.nut.2007.01.007
- Grassi D, Necozione S, Lippi C, Croce G, Valeri L, Pasqualetti P, Desideri G, Blumberg JB, Ferri C. Cocoa reduces blood pressure and insulin resistance and improves endothelium-dependent vasodilation in hypertensives. Hypertension. 2005 Aug;46(2):398-405. Epub 2005 Jul 18.
- Grassi D, Lippi C, Necozione S, Desideri G, Ferri C. -term administration of dark chocolate is followed by a significant increase in insulin sensitivity and a decrease in blood pressure in healthy persons. Am J Clin Nutr. 2005 Mar;81(3):611-4.
- Shively CA, Apgar JL, Tarka SM Jr. Postprandial glucose and insulin responses to various snacks of equivalent carbohydrate content in normal subjects. Am J Clin Nutr. 1986 Mar;43(3):335-42.
- Cedermark G, Selenius M, Tullus K. Glycaemic effect and satiating capacity of potato chips and milk chocolate bar as snacks in teenagers with diabetes. Eur J Pediatr. 1993 Aug;152(8):635-9.
- GI Database. Accessed online April 5, 2007 at http://www.glycemicindex.com/