One of the problems with science is how it is reported in magazines and newspapers. Also how it is reported on the web can be a problem. This problem came to light for me when I was reading the free magazine “Better Nutrition”. In the February issue there was a short article on “The best weight management diet” which talked about a New England Journal of Medicine article which showed that high protein-low glycemic index diets were better for maintaining weight loss. This sound realistic and was confirmed by reading the article, but what peeked my interest was the table of glycemic index values in the Better Nutrition article because apparently sourdough bread has a lower GI (54) than white bread (100).
This did not seem possible as sourdough bread is essentially made from the same ingredients as white bread with a different starter is added instead of yeast for proofing. There is nothing in the process of making sourdough bread that should change the carbohydrates, which are from wheat flour.
So I looked up how glycemic index was measured. What I found was that glycemic index (GI) ranks foods by how quickly they increase blood sugar (glucose) levels. Foods that increase blood sugar rapidly after being consumed have a high GI. For example, honey has a GI of 85 and sucrose, table sugar, has a GI of 70. Conversely foods which are slowly digested and absorbed have a low GI. Examples of these foods are green vegetables (GI = 15) and dark chocolate with greater than 70 % cocoa solids (GI = 22).
GI is measured by feeding measured portions of the test food containing 10 – 50 grams of carbohydrate to 10 healthy people after an overnight fast. Blood samples are taken at 15-30 minute intervals over the next two hours and used to construct a blood sugar response curve. The area under the curve (AUC) is calculated to reflect the total rise in blood glucose levels after eating the test food. The results for a test food is divided by the results of the standard containing the same amount of carbohydrate, either glucose or white bread are used as standards, and multiplied by 100. The result gives a relative ranking for each tested food. There is some concern, firstly that the standards used are different and secondly two hours after a meal is too short. Food is known to stay in the stomach for over 4 hours, so longer term blood glucose monitoring might be better.
The glycemic index was developed at the University of Sydney (Australia) originally to aid people with diabetes control their blood sugar levels. Low GI diets are useful for people with diabetes as it allows them to regulate their blood sugar levels and this in turn helps with insulin levels and may reduce insulin resistance for people with Type II diabetes.
So the more I read, the less likely it seemed that sourdough bread could have a lower glycemic index than white bread, which by the way, in some measurements of GI is set as the reference with a GI of 100 and in others, where glucose is the reference, white bread has a GI of 70. Yes, not even the measurements of GI are standardized.
Interestingly it seems that the reason the high protein/low glycemic index diets work is that protein fills you up and after eating a meal that is high in protein you are more satisfied.
References
http://heartscanblog.blogspot.com/2010/02/is-glycemic-index-irrelevant.html
http://en.wikipedia.org/wiki/Glycemic_index
http://voices.washingtonpost.com/checkup/2010/11/in_theory_losing_weight_and.html
http://articles.latimes.com/2010/nov/26/news/la-heb-diet-20101126
http://www.extension.iastate.edu/publications/n3450.pdf
Thomas Meinert Larsen, et al, Diets with High or Low Protein Content and Glycemic Index for Weight-Loss Maintenance N Engl J Med 2010; 363:2102-2113 doi:10.1056/NEJMoa1007137
Lab Cat 
This might explain it: http://www.jacn.org/cgi/reprint/28/1/30.pdf
Aha, thanks. I hadn’t thought about the effects of organic acids. It is amazing what can affect the body.
Italian study shows that sourdough fermentation with same ingredients produced more ‘resistant starch’:
https:/docs.google.comfileviewid=0B21SlJ7mbE4fYzM4YWVlNDYtYzI3MC00ZDJiLTk5MTktOTNhZDdmYzkzNzRj&hl=en&pli=1
The different culture medium and method has different effects on the dough: If you do a proper sourdough cultures (which takes hours and pre-digests the starch), of course it will have a different effect than a quick-rise saccharomyces cerevisiae yeast that takes 15 minutes to make the bread rise.
The junk bread produced in modern times (and especially in the US) is problematic and yet one more reason why American are far more fat and diabetic than europeans, who generally eat naturally leavened breads.
Ronald
Thanks for your comment. It is very helpful. I think the problem I had that the article didn’t state why the difference or which type of sour dough bread. I still haven’t seen any direct data on the glycemic index and sour dough bread.
Now we have two related hypotheses:
1) formation of organic acids
2) formation of resistant starch
Hi Cat, the GI mesurements aren’t standardized? It’s interesting. But I think it depends on kind of food. Some bread has higher GI than another one. B.
There appear to be two GI measurements, one in comparison to glucose and one in comparison to white bread.