Phytic acid: the zinc counterpart
The mineral zinc belongs to the essential trace elements, it occurs in very small quantities in the human body and can only be stored for a short time. Nevertheless, it is indispensable: with insufficient zinc intake, numerous metabolic processes cannot take place correctly or not at all. Many foods contain zinc, so that the zinc requirement can often be met through food. But caution is advised with some plant foods: they contain particularly high levels of phytic acid. What is phytic acid and how does it influence zinc absorption?
Zinc in food
Many foods contain the trace element zinc. Animal products such as liver, red meat or dairy products have a high zinc content; just 200-300 g of these foods already cover the daily requirement. Vegetable products such as pulses, wholemeal products or nuts have a similarly high zinc content, but: the high content of phytic acid or phytate prevents zinc absorption in the intestine.
What is phytic acid?
Phytic acid is a bioactive substance, it serves plants, such as legumes, grains and oilseeds as a store for phosphate and binds various minerals that the germinating plant needs for growth. Phytic acid is therefore mainly found in foods, which can also serve as seeds. In nature, phytic acid occurs as an anion, phytate, it is the salt of phytic acid (hexaphosphoric acid ester of myo-inositol). The terms phytic acid and phytate are used synonymously.
Influence of phytic acid on zinc absorption
Due to its complexing properties, phytic acid binds various minerals. This is desirable in plants, as it stores the minerals necessary for germination. Phytic acid also binds minerals in the human body. Here, however, the formation of complexes is not desirable because the bound minerals, mainly zinc and iron, can then no longer be absorbed by the body. In addition, calcium, magnesium and zinc are released back into the small intestine in a recycling process via the digestive secretions of the pancreas and are reabsorbed from there. A high phytate intake can reduce zinc absorption by up to 45 percent.
Degradation of phytic acid
Preparation methods such as soaking, germination, sourdough fermentation or fermentation break down phytate and increase the bioavailability of zinc. Phytic acid sits in the marginal layers of grains and nuts, and soaking for several hours dissolves it. During the soaking time, the water should be changed several times, and finally the grains should be rinsed under running water. Even more phytic acid is broken down when the grains subsequently germinate. In the process, the phytic acid is used up for the actual purpose, namely providing minerals for the germination process. There is also a reason for letting a yeast dough rise for a long time: in this way, phytic acid is broken down and rendered harmless.
Bioavailability describes the percentage amount of a substance that can be absorbed unchanged in our body and whose concentration is also detectable. This applies to all biologically available substances, in particular also medicines, which we ingest.
Reference value of the DGE
The German Nutrition Society (DGE), together with the nutrition societies of Austria and Switzerland, publishes reference values for nutrient intake. These D-A-CH reference values provide reference values for all nutrients and are continuously updated. Until summer 2019, the DGE recommended a daily intake of 7 mg (women) and 10 mg (men) of zinc. In July 2019, the D-A-CH reference values were updated by taking into account the level of daily phytate intake. There are now 3 different recommendations for adults.
Recommended zinc intake in mg per day for adults aged 19 and over depending on phytate intake according to DGE
| - | Männer | - | - | Frauen | - | - |
|---|---|---|---|---|---|---|
| Phytatzufuhr | niedrig | mittel | hoch | niedrig | mittel | hoch |
| Zinkzufuhr mg/Tag | 11 | 14 | 16 | 7 | 8 | 10 |
The table distinguishes between a low, medium and high phytate intake, which are defined as follows according to the DGE:
- Low phytate intake (330 mg per day): Is present in a diet in which few whole grain products and legumes are consumed and the protein sources are primarily of animal origin. This diet is nutritionally unfavourable because little dietary fibre is consumed from cereals.
- Medium phytate intake (660 mg per day): Corresponds to a complete diet in which the protein requirement is covered by animal products, wholemeal products and pulses. The phytate intake of vegans or vegetarians who primarily consume highly milled, germinated or fermented cereal products is also in the medium range.
- High phytate intake (990 mg per day): In this diet, the consumption of - non-germinated, non-soaked and non-fermented - whole grain products and legumes is high, and protein sources are primarily or exclusively of plant origin (e.g. soy). With such a diet, zinc absorption is limited.
Phytic acid in food
Phytic acid is mainly found in foods that can also serve as seeds. It is located in the marginal layers of grains and nuts, which is why the bran of various cereals has a particularly high value. High concentrations are found in maize, soy, wheat, rye and barley bran, unpolished rice and peanuts.
The following table shows the phytic acid content of selected foods:
| Food | mg phytinsäure per 100g |
|---|---|
| Wheat bran | 3610 |
| Wheat germ | 1470 |
| Peanuts | 1336 |
| Soybean seeds,dry | 1250 |
| Barley hulled,whole grain | 1070 |
| Rye,whole grain | 970 |
| Maize,whole grain | 940 |
| Wheat,whole grain | 906 |
| Oats,hulled,whole grain | 900 |
| Rice,unpolished | 890 |
| Quinoa | 541 |
| Whole wheat bread | 330 |
Every person can estimate their personal phytate intake based on their diet with the help of the table and thus determine their reference value for daily zinc intake.
People who follow a strict vegetarian or even vegan diet should pay particular attention to how much phytic acid they consume daily. 990 mg per day is quickly reached:
| meal | meals | phytinsäurecontent |
|---|---|---|
| breakfast | Müsli with 10 g wheat bran | 361 mg phytinsäure |
| lunch | 50 g unpolished rice | 445 mg phytinsäure |
| Dinner | 2 slices of wholemeal bread | 330 mg phytinsäure |
| Snack | 2 slices of wholemeal bread | 267 mg phytinsäure |
| Total | 1403 mg phytinsäure |
Of course, this sample calculation is not meant to discourage you from going vegetarian or vegan. It is meant to inform and inspire you to analyse your personal diet. Appropriate measures such as omitting wheat bran, eating sourdough bread, or replacing peanuts with sultanas or dried cranberries, reduce the phytate intake considerably. If you don't want to give up your habits, you should make sure you get enough zinc and, if necessary, use dietary supplements.
Conclusion
Plant foods with a high zinc content often also have a high concentration of phytic acid. Phytic acid reduces the bioavailability of zinc, so that there is an increased risk of zinc deficiency despite a high intake of zinc-rich foods. Soaking, germinating and fermenting cereals significantly reduces the phytate content.
An adequate supply of zinc has a positive effect on the health of the heart. This is because the heart has the highest metabolic activity in the entire body, which means that a particularly large number of free radicals are produced there. When the balance between free radicals and protective antioxidants is disturbed, the body is exposed to oxidative stress. In addition to vitamins A, C and E, zinc and selenium provide good protection against free radicals and thus protect the heart from oxidative stress.
High doses of zinc can lead to zinc overdose, which results in gastrointestinal symptoms such as nausea, vomiting or diarrhoea. It is hardly possible to take in a high dose of zinc through the diet; it is more likely to be taken in through incorrectly dosed zinc supplements or food supplements. The consumption of foods containing zinc, which have been stored in zinc-containing containers, can also cause an overdose.
The daily amount of zinc up to which no side effects occur with prolonged intake is 25 milligrams. If the guideline value is rarely exceeded, there are generally no side effects. If the guideline value is exceeded continuously, an overdose can result, which manifests itself in nausea, vomiting or diarrhoea. Chronic overdosage can cause copper and iron deficiency with the corresponding side effects up to anaemia in the body. A zinc level of 200 to 400 milligrams of zinc in the blood or more is an acute zinc poisoning; symptoms include nausea, loss of appetite, headache, fever and shivering. If an overdose or zinc poisoning is suspected, a doctor should be consulted.
A sufficient supply of zinc is necessary for healthy and firm nails. The trace element contributes to the strength and growth of the nails because it is involved in the metabolism of the amino acid cysteine, which is important for nail growth. In the skin, where the nails originate, zinc is also responsible for the formation of keratin, the building material of the nails. Weak and brittle nails with severe furrows and grooves are typical symptoms of a zinc deficiency.
The mineral zinc is just as important for dogs as it is for humans. The trace element must be fed to the dog in the diet, dry and wet food usually contain added zinc. Zinc is only harmful to dogs in high doses and can even have a fatal effect.
Iron and zinc are absorbed through the intestinal mucosa in the same way. When high-dose iron is taken together with zinc from food supplements, the absorption of zinc is impeded. If iron is taken as a supplement together with food containing zinc, zinc absorption is not inhibited.
Most manufacturers of zinc tablets advertise that taking their product can positively influence acne. Those who are already undergoing medical treatment for a zinc deficiency will have a product recommended by their doctor. If you are looking for a suitable product on your own, you should consider the following criteria: Organically bound compounds, for example zinc glycinate or gluconate, have a high bioavailability. The combination with the amino acid histidine improves zinc absorption. A dosage of 10 to 15 milligrams of zinc per daily dose is sufficient for people who do not have an increased zinc requirement; higher-dose products can lead to overdosing. High-quality products are free of additives and allergens.
Iron and copper make the red blood cells in the body. A copper deficiency often accompanies an iron deficiency, as copper facilitates the absorption of iron. If both elements are not present in sufficient quantities, anaemia can develop. Copper deficiency increases the susceptibility to infections and can lead to pigmentation disorders of the skin and hair. Other symptoms include skeletal changes, growth disorders, depression and a disturbed nervous system.
A horse weighing 500 kilograms needs 1.1 milligrams of selenium per day, which usually requires an additional mineral feed. If the supply is insufficient, a deficiency occurs which can be detected in the blood. A supplement feed with a combination of vitamin E and selenium replenishes the reserves, but the supplementation should be monitored, as selenium in high doses can lead to selenium poisoning.
