The Effects Of Sourdough On Blood Glucose & Insulin

By nature and by training I’m a scientist, and while I do love the flavour and texture of my sourdough, it’s the science that really gets my juices flowing.

WARNING – The following contains references and links to genuine, peer-reviewed information on the nutritional and physiological effects of eating sourdough. If science bores you, LEAVE NOW!!

As I alluded to in my earlier summary on the Real Bread Movement, sourdough offers numerous proven advantages over mass-produced bread. Possibly the most exciting and relevant of these for the average consumer are the reductions in blood glucose and insulin spikes after a sourdough meal versus those seen after consuming breads produced using baker’s yeast.

Good, simple food. Nature's medicine.
Good, simple food. Nature’s medicine.

I think most of my readers will be familiar with the concept of the Glycaemic Index, which measures the effects of a food on blood sugar levels compared to the effect of ingesting an equivalent amount of pure glucose. Research conducted by several teams of respected academics has proven beyond doubt that sourdough bread has a lower glycaemic index than other breads, including supposedly healthy options such as wholemeal products. This means that eating a slice of sourdough results in a lower surge in blood glucose (and thus insulin) when compared to an equivalent amount of ‘regular’ bread. Terry Graham was a member of one such team of researchers, and his interview with Vanessa Kimbell of BakeryBits, linked below, makes for fascinating listening.

So how does sourdough, which starts off containing the same carbohydrate constituents as mass-produced bread, manage to end up as a product which has less of an impact on blood sugar levels? And why should we care? I’ll answer my second question first. Spikes in blood glucose and insulin are a major risk factor for developing type II diabetes, and promote weight gain and increased body fat percentage. As to my first question, the answer lies in the process of fermentation. Slowly fermented bread contains far lower levels of simple sugars which can be absorbed unchanged in the gut and therefore cause an almost immediate spike in blood sugar. By allowing our lovely microbes time to properly ferment our bread, we reduce the amounts of sugars such as glucose and fructose in our dough. By ingesting more complex sugars and starches, we extend the period of time over which the bread is providing us with energy; we avoid the detrimental sugar ‘high’ associated with eating mass-produced bread and other junk foods.

Sourdough-leavened bread improves postprandial glucose and insulin plasma levels in subjects with impaired glucose tolerance” – a seminal paper featured in Acta Diabetologica

Interestingly, another reason for the reduced glycaemic index is the production of acetic acid (vinegar for all you fast food afficionados). Acetic acid is a product of glucose metabolism in sourdough. In addition to this metabolism (breakdown) of glucose resulting in less available sugar in the dough, it has been shown that simply adding vinegar to a ‘normal’ dough results in much less of a glycaemic surge after eating. Incredibly, it also increases the satiety value of a bread meal, meaning that you feel fuller for longer, and are less likely to overeat.

Vinegar supplementation lowers glucose and insulin responses and increases satiety after a bread meal in healthy subjects” – European Journal Of Clinical Nutrition

So- who’s for a bread that makes you less hungry, reduces blood sugar and insulin surges, and tastes a hell of a lot better than the rubbish on your supermarket shelves? Well, me for one….


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