Phytic Acid- The Antinutrient In Your Bread

When it comes to choosing our breads and breakfast cereals, I think it’s fair to say we all know we should be eating more wholemeal/wholegrain products. There are several very good reasons for this- wholemeal flour, which includes more of the outer parts of the grain, is a good source of dietary fibre for one, and the great majority of us in the western world do not eat enough fibre to maintain healthy gut function. These parts of the grain also contain far higher levels of vitamins and minerals than the soft endosperm which goes into making white flour. In fact, almost 90% of some key nutrients are lost in the extraction process, making white flour a food with poor nutritional value. For this reason, flour millers are legally required to supplement these flours with B vitamins once the extraction process is complete in order to prevent widespread nutrient deficiencies. This seems a convoluted way of producing a nutritious product doesn’t it- removing naturally occurring nutrients to later replace them with industrially-produced imitations. So we’re agreed- wholemeal all the way, right?

wpid-20150609_203931.jpgBut there’s a problem. Wholemeal flour has a dirty little secret. It is also laced with an antinutrient called phytic acid. Again, phytic acid is found in the hard outer shell of the wheat grain, but also occurs in legumes and oil seeds. While there is evidence that some dietary phytic acid can have cholesterol-lowering and anti-cancer effects, the levels found in wholemeal flours are sufficiently high as to cause serious dietary imbalances. The molecule is an excellent binder of positively-charged ions such as iron, calcium, magnesium, and zinc, rendering them unavailable for absorption in the human body. So although your wholemeal flour may on the face of it contain a high proportion of your dietary mineral requirements, in reality a very large proportion of these are actually going to pass right through the gut after ingestion.

 

How significant an effect does phytic acid have on mineral absorption? Well, if you were to completely remove it from your flour, your iron absorption from your bread or cereal would increase by a factor of 11.5. That is a staggering difference. In other words, if you were to eat one slice of phytic acid-free bread, you would absorb the same amount of iron as if you consumed 11.5 slices of the same loaf containing the chemical. In Iran, where wholemeal bread constitutes up to a third of the diet, a third of children suffer from iron deficiency anaemia (a low level of red blood cells) because of phytic acid in their food.

The structure of phytic acid, and the nutrients released after its degradation by phytase (adapted from Gobetti et al, Food Microiology 37(2014) 30-40)
The structure of phytic acid, and the nutrients released after its degradation by phytase (adapted from Gobetti et al, Food Microiology 37(2014) 30-40)

So what can we do about phytic acid? Surely there must be some way we can remove it from our supposedly healthy wholegrain foods? Yes, there is, and the answer lies in proper fermentation of our breads. Phytic acid is broken down by an enzyme called phytase, which also occurs in nature.  While our friendly lactic acid bacteria in sourdough do have some phytase activity, higher and more significant amounts are found in the wholegrains themselves. However, these endogenous phytases are not active in flour at neutral pH. They only really begin to have a significant effect when the pH of the flour drops to around 5.0, which would be just moderately acidic for sourdough. At this level, phytase will degrade around 70% of phytic acid within 6 hours, releasing all its bound minerals and thereby making them available to the human body. Studies in animals have documented very clear improvements in red blood cell parameters in those fed sourdough versus other breads.

Rushed baking using large amounts of baker’s yeast does not allow for this acidification or slow fermentation, and so loaves produced in this way are on an entirely different and inferior nutritional plane to our beloved sourdough. Just one more reason to take your time when baking your daily bread.

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