Fibre versus Phytate: Who's the True Mineral-Blocking Baddie?
Bran, the fibrous part of grain products, is very nutritious - on paper. The tiny problem is that its high mineral content comes along with an anti-nutrient - phytate or phytic acid - that prevents the vast majority of those minerals from being absorbed. Unleavened or unfermented grain products contain all the minerals but all the phytate; bran-depleted products such as white bread contain very little phytate but also very little in the way of minerals; most "wholemeal" or "wholegrain" products are produced commercially using quick-rise yeasts which reduce the phytate content only slightly.

Interestingly, it appears that phytate is one of the main factors responsible for the lower glycaemic index of certain grain products (leguminous foods produced a lower glycemic index than grain foods with very similar phytate contents, indicating that other anti-nutrients, such as polyphenols, have an impact as well). "Wholemeal" products, which provide all the components of "wholegrains" but more separated from each other and with less phytate, have quite a high glycaemic index (i.e. they tend to result in quicker and more radical rises and falls in blood glucose/sugar levels after being fed in unnaturally isolated 50g-carbohydrate portions to overnight-fasted subjects). "Wholemeal" wheat and barley flour breads had approximately the same glycaemic index as white breads, and the index was lower when the proportions of whole wheat or barley grains in the test bread were increased - although the index was only lower than 50 when the 75%-wholegrain barely bread was fed.

In prehistoric and pre-industrial times, when many people habitually faced strenuous undertakings that resulted in a high carbohydrate (glycogen) turnover, an infusion of easily digested carbohydrates (when available) was probably welcomed rather than seen as a source of angst. Hunter-gatherer tribes who use starchy foods certainly seem intent on making the starch as rapidly digestible as possible. Therefore, the ideal grain-based product (if it's in any way permissible to use the word "ideal" in relation to a grain-based product) would have been one that provided readily available carbohydrate and decent quantities of absorbable minerals. Accordingly, grain products were traditionally subjected to long-winded fermentation processes (e.g. sourdough fermentation) that reduced the phytate and left a decent amount of the minerals (e.g. calcium, magnesium, iron, zinc) intact. This method is extremely rare nowadays because, needless to say, it is not conductive to commercial mass-production.

Because fibre itself is noted for increasing fecal excretions, and because it occurs alongside phytate naturally, the two are often assumed to have the same effect. But they can be separated, including by natural fermentation methods, and the results of that separation are interesting.

One experiment compared the absorption of calcium, magnesium, iron and zinc after unnaturally adding the said minerals along with phytate to "white", "brown" and "wholemeal" breads. The subjects (5 males and a female aged 25-55) left the facility to work but were fully provided with three 12-day weight-maintenance diets (preceded by 12-day washout periods) that differed only in the bran (but not phytate or mineral) content of the "white", "brown" and "wholemeal" breads (all contained as much bran as typically found in "wholemeal" bread, but the phytate and mineral contents were equalised). Urine and fecal samples indicated positive iron and zinc balances, neutral or mildly positive calcium balances, and negative magnesium balances, in all groups. The only possible difference between the groups was a tendency for iron balance to be not-quite-so-positive as the bran content increased. It therefore appears that any independent mineral-blocking effect of bran is very mild.

A slightly later experiment, using natural bread-leavening methods to reduce the phytate while retaining the bran and minerals, was carried out on a larger number of subjects (17 males and 25 females aged 20-52) but over a shorter term and only assessing zinc absorption. 80-85% was the greatest phytate reduction achieved by leavening, and was achieved after two days, but 60% was lost after two hours. In the feeding study, carried out after an overnight fast, two 20g wheat-flour rolls were given alongside deionised water, 200g fermented milk and 10g butter and 10g, 16g or 30g of wheat bran (in most cases added to the rolls) that had been leavened (if at all) for 15 or 45 minutes or 3 or 16 hours (phytate was predictably lowered with leavening fermentation time). The 30g-bran meal fermented for 16 hours had about twice as much zinc as the corresponding 10g one but nearly five times as much phytate and a phytate:zinc ratio nearly twice as high, and therefore the percentage of zinc retained (judged via an administered zinc radioisotope that was monitored for 10-14 days) was only a third as high and the absolute amount retained was a little lower. Phytate was clearly the main factor affecting absorbability, and, because only 80-85% of it was destroyed at most, a higher initial bran content meant a greater absolute amount of phytate not broken down. The authors considered the importance of the ratio of phytate:zinc and that of calcium:phytate. A calcium:phytate ratio of 6:1 or greater was said to result in trace minerals being bound to a complex with phytate, but apparently amino acids from protein recruit the minerals from that complex and allow absorption, and therefore the protein from the milk in this study would have cancelled out the calcium. Citrate is another element of milk that binds to zinc, but again protein appears to cancel it out.

A later experiment (12 females aged 22-28 taking lunch on-site and other meals packed-for-home in three 3-week periods with 3-week washouts) measured calcium, magnesium, iron and zinc balance with and without the addition of 23g of "barley-fibre concentrate" derived from the kernal rather than the husk. The three diets were deemed "high-protein, low-fibre", "high-protein, high-fibre" and "low-protein, high-fibre", the 23g of the said product being added to the two high-fibre diets. The "high protein" diets were really barely-sufficient-protein diets giving 115-130g of meat and 80g of cheese per day, this animal protein providing two-thirds of the 70-75g total. Animal protein provided half the protein on the (catastrophically) "low protein" diet, the total being 55.7g per day. All mineral balances were positive (though not greatly above neutral for calcium and magnesium) on the "high-protein, low-fibre" diet, and all were negative on the "low-protein, high-fibre" diet. The "high-protein, high-fibre" diet resulted in positive balances for calcium and iron, neutral ones for magnesium but negative ones for zinc. Although phytate was said not to be an important factor here, the phytate content was higher with greater fibre and lesser protein in the diets. The authors were unable to explain the greater fecal weights observed on the "low-protein, high-fiber" diet compared to the "high-protein, high-fibre" one, but it fits in nicely with what the authors of the previously cited study said about amino acids from protein recruiting minerals from phytate complexes in the gut. Once again, therefore, phytate looks like the main offender against mineral absorption.

Phytate seems much more troublesome than fibre, and long-fermented grain products much less troublesome than short- or especially non-fermented ones. I nominate phytate as the true mineral-blocking baddie in grain products - but bear in mind that there are other anti-nutrients in other foods (e.g. polyphenols and oxalate).