FOOD & DRINK
PHYTATES AND CEREALS Priya Tew Freelance Dietitian, running Dietitian UK, a dietetic and nutritional consultancy business
Priya runs Dietitian UK, a freelance dietetic service. She works with private patients, the media, brands, PR companies and is well known on social media.
For full article references please email info@ networkhealth group.co.uk
Wholegrains are something we often encourage people to eat more of, as they are considered highly nutritious and packed with fibre. Delving deeper, these cereal foods are also phytate-rich. Phytates are known to inhibit the absorption of some minerals, which begs the question how much of the nutrition in wholegrains does the body actually absorb. But there are ways to help increase absorption and to decrease the phytate content. Phytates (chemical name: inositol 6 phosphate) are mainly found in cereals and legumes, but also in seeds and nuts. They serve as the storage of phosphorus and minerals for the plant.1 Around 80% of phytate in cereals is found in the aleurone layers and the rest in the germ; the endosperm is almost phytate-free.1 Phytates form insoluble complexes with the minerals zinc, calcium, iron and magnesium. Thus, when they are present in a food, the full amount of these nutrients will not be absorbed. Phytate also negatively affects the absorption of lipids and protein because phytic acid inhibits enzymes that we need to digest our food, such as pepsin, amylases and trypsin, so it affects the absorption of lipids and protein.2 THE BENEFITS OF PHYTATES
Largely talked about due to their antinutrient, inhibitory effect on mineral absorption, phytates may have some health benefits. There is some research showing antioxidant and anticancer effects, specifically for prostrate, colon and breast cancers.1,3 Phytates can also prevent renal stone formation and some positive effects have been seen on blood glucose.1 Phytates may play a role in reducing the risk of heart disease through their effect on reducing blood cholesterol levels.3 Once phytate levels become too high in a food, these beneficial effects stop and the phytates work as anti-nutrients.3
The daily intake of phytate is estimated to be 0.3-2.6g/d in a Western diet, with a range of 504-844mg/d in the UK.1 It is obviously at the higher end in vegetarian diets. Vegetarians/vegans depend on many foods that are high in phytates for their iron and zinc, so the potential absorption issues that occur with phytates are especially of concern for them. There could also be a risk for infants who are weaned predominantly on cereals. After four months, infants have a reduced amount of minerals such iron in their body stores and breastmilk only provides low levels. If cereals are the main food intake it could potentially leave them deficient.2 CONTENT OF PHYTATES IN CEREALS
The amount of phytates in cereals varies due to differences in the variety of the cereal grown, the climate and environment, the stages of seed maturation and the method used to determine the phytate.3 DIGESTION OF PHYTATES
During digestion, phytase, found in plant foods, breaks down 37-66% of phytate to phytic acid in the stomach and small intestine.1 Phytic acid binds to minerals - iron, zinc and calcium - forming mineral complexes so the minerals cannot then be absorbed. Phytase can be inactivated by cooking and some processing methods, but even then some phytase is available in the gut from microbes in the large intestine and so is still able to degrade phytate.3 www.NHDmag.com May 2017 - Issue 124
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FOOD & DRINK Phytic acid g/100g Maize Maize germ Wheat
0.72-2.22 6.39 0.39-1.35
Wheat bran
2.1-7.3
Wheat germ
1.14-3.91
Rice
0.06-1.08
Barley
0.38-1.16
Sorghum
0.57-3.35
Oats
0.42-1.16
Rye
0.54-1.46
Millet
0.18-1.67
Triticale
0.50-1.89
Wild rice
2.20
Quinoa
0.541
Some substances compete with phytic acid in the mineral binding process, they include organic acids, protein, peptides, beta-carotene and ascorbic acid. Eating these with phytates will, therefore, help increase the absorption of minerals. For example, ascorbic acid prevents the oxidation of ferrous to ferric iron which stops the iron-phytate complexes forming.3 Encouraging people to have a glass of juice, add vegetables, or cook with lemon/lime juice are some easy strategies. Hithamani and Srinivasan found that cooking millet with some lime juice increased the accessibility of polyphenols from grains by 25%.4 REDUCING THE PHYTATE CONTENT OF GRAINS
During food processing, preparation, storage and digestive phytate can be hydrolysed to other forms of inositol phosphate with fewer phosphate groups (lower inositol phosphates). These do not form mineral complexes and so do not have the same effect on mineral bioavailability.1,3 There has been a focus on reducing the phytates in foods before they enter the gut, so that the mineral-phytate complexes do not form. Phytase is extracted from plant foods (wheat bran) and soaking, malting, germination, fermentation and bread making is used to reduce the phytate in foods. To soak grains in order to help breakdown phytates, use one cup of grain in one cup of warm water with one to 34
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Table 1: The content of phytic acid in different cereal grain (adapted from ref 1)
two tablespoons of yoghurt and leave it for at least seven hours, or overnight. Milling can also reduce the phytate content of grains up to 90%, especially when the phytate is contained in the outer layer of the seed. However, this will also lead to other nutrient losses too. Malting is when a grain is allowed to sprout and then quickly dried. This process has been found to decrease phytic acid by 45%.5 Sprouting can increase the levels of phytase, which will then have more action on reducing phytate levels; this varies across cereals. The activity of phytase increases up to 11 times when sprouted. Bartnik and Szafranska found that with wheat, rye and oats, levels of phytase increased 4.5, 2.5 and nine times with soaking and sprouting and the phytate content in these same grains was reduced by 16% in barley, 30% in wheat and rye and 17% in oats.6 Grinding and two-hour soaking of wheat, rye and barley can reduce all the phytate in these grains. Soaking and sprouting of quinoa have been shown to reduce the phytate content by up to 98%.7 Degradation of phytate occurs at different stages of bread making. Decreasing the pH in the bread dough results in phytate breakdown, so sourdough reduces phytate more than yeast fermentation. The longer the fermentation, the more phytate is degraded; one study found that phytic acid was 99% degraded in rye bread with
Adding phytase immediately before a food is consumed has been shown to increase the absorption of some minerals, including zinc. However, this is not going to be something the general public can do. a long fermentation time and 82% degraded in a mix of 50/50 rye and wholegrain.8 So, the choice of bread can be a factor; choosing sourdough or fermenting for longer can reduce phytate levels. Adding phytase immediately before a food is consumed has been shown to increase the absorption of some minerals, including zinc. However, this is not going to be something the general public can do.9 Another option, but only possibly on a larger scale, is using a soil zinc application, or spraying the foliage of crops with zinc. This has been found to increase the zinc concentration and bioavailability in raw and cooked grains.10 SUMMARY
Phytates do decrease the minerals absorbed in grains, but this is not something that would normally be of concern in a meat-eating person with a well-balanced diet. For vegetarians,
vegans and those with a high amount of cereal grains in their diet, it could be worth thinking about some methods to improve the availability of minerals. Infant weaning should not just be carried out using cereals, but a range of foods including a more vegetable focused approach. Adding ascorbic acid to a meal with cereal grains in it will help improve the iron absorption, so simple advice around adding fruit and vegetables to a meal can make a difference. Soaking grains can help breakdown phytates, but may not be feasible for everyone. Eating sourdough bread or bread with a longer fermentation time will help reduce phytate levels too. Fortified foods are of course a good option and an easy one. Looking at a comparison of bioavailability of iron in a foods, fortified cereals with ascorbic acid score well.11 So, a fortified breakfast cereal with fruit and a glass of juice is a winning combination.
References 1 Schlemmer U, Frølich W, Prieto RM, Grases F. Phytate in foods and significance for humans: food sources, intake, processing, bioavailability, protective role and analysis. Mol Nutr Food Res. 2009 Sep [cited 2012 Feb 27]; 53 (Suppl2): S330-75. Abstract available from: www.ncbi.nlm.nih.gov/ pubmed/19774556 2 Abdoulaye Coulibaly, Brou Kouakou and Jie Chen (2011). Phytic acid in cereal grains: structure, healthy or harmful ways to reduce phytic acid in cereal grains and their effects on nutritional quality. American Journal of Plant Nutrition and Fertilization Technology, 1: 1-22. URL: http://scialert.net/ abstract/?doi=ajpnft.2011.1.22 3 Penn Nutrition: Vegetarianism. www.pennutrition.com/KnowledgePathway.aspx?kpid=2709&trid=19294&trcatid=38 4 Hithamani G and Srinivasan K (2017). Bioaccessibility of polyphenols from selected cereal grains and legumes as influenced by food acidulants. J Sci Food Agric, 97: 621-628. doi:10.1002/jsfa.7776 5 Hejazi SN1, Orsat VJ (2016). Malting process optimisation for protein digestibility enhancement in finger millet grain. Food Sci Technol 2016, Apr; 53(4): 1929-38. doi: 10.1007/s13197-016-2188-x. Epub 2016 Apr 18 6 Bartnik M, Szafranska I. Changes in phytate content and phytase activity during the germination of some cereals. J Cereal Sci. 1987 [cited 2012 Mar 26]: 5:23-8. Abstract available from: www.sciencedirect.com/science/article/pii/S073352108780005X 7 Valencia S, Svanberg U, Sandberg AS, Ruales J. Processing of quinoa (Chenopodium quinoa, Willd): effects on in vitro iron availability and phytate hydrolysis. Int J Food Sci Nutr. 1999 May [cited 2012 Mar 26]; 50 (3):203-11. Abstract available from: www.ncbi.nlm.nih.gov/pubmed/10627836 8 Nielsen MM, Damstrup ML, Dahl Thomsen A, Rasmussen SK, Kjærsgård Rasmussen S, Hansen A. Phytase activity and degradation of phytic acid during rye bread making. Eur Food Res Technol. 2007 [cited 2012 Mar 26]: 225: 173-81. Abstract available from: www.springerlink.com/content/ ak65661n70k73410/ 9 Brnić M, Hurrell RF, Songré-Ouattara LT, Diawara B, Kalmogho-Zan A, Tapsoba C, Zeder C, Wegmüller R. Effect of phytase on zinc absorption from a millet-based porridge fed to young Burkinabe children. Eur J Clin Nutr. 2017 Jan;71(1):137-141. doi: 10.1038/ejcn.2016.199. Epub 2016 Oct 19 10 Poblaciones MJ, Rengel Z. Soil and foliar zinc biofortification in field pea (Pisum sativum L): Grain accumulation and bioavailability in raw and cooked grains. Food Chem. 2016 Dec 1; 212: 427-33. Doi: 10.1016/j.foodchem.2016.05.189. Epub 2016 Jun 3 11 Christides T, Amagloh FK, Coad J. Iron bioavailability and Provitamin A from sweet potato and cereal-based complementary foods. Foods. 2015 Sep 18; 4(3): 463-476. doi: 10.3390/foods4030463
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