Antinutrients: phytates and oxalates

With this entry I start a new series of videos about antinutrients. Antinutrients are natural or synthetic compounds that interfere with nutrient absorption. In this series I’m going to focus on the natural and exogenous ones (those who come from the food) which usually are in legumes, grains and seeds. There are a few types, so I’ll talk about them separately in each post. Today I’m going to talk about phytates and oxalates.

While I was doing research for the post What did our ancestors ate? - Part 2, I stumble upon the word phytate and that’s when I decided to talk about antinutrients. In the scientific article where I saw the word phytate (1), they were blaming this compound for being the reason why Neolithic humans had a worse health compared to the paleolithic ones. Let’s see why the author believed that.

By the way, I already have a post about an antinutrient, lectins. Since there's a lot to say about lectins, I made a full post talking about them, you can read that here: Lectins, a possible cause of diverse digestive issues

Phytates

A phytate is the anionic form of the phytic acid, at physiological pH this acid loses a hydrogen resulting in a phytate. It's found in legumes, seeds and grains between a 0.1 to 6%. Due to its high density in phosphates negatively charged, phytates form salts with cations like potassium (K⁺), magnesium (Mg⁺) and in a lesser extent with calcium (Ca⁺), zinc (Zn⁺), iron (Fe⁺) and copper (Cu⁺).

As you know from the post The wonderful digestive system, the nutrient absorption happens in the intestinal wall, both for the macro and the micro (vitamins and minerals) nutrients. For a mineral to be absorbed, it cannot be bound to another molecule, for this reason when a mineral is bonded to a phytate, it cannot be absorbed.

According to the World Health Organization (WHO), a third of the population suffers from zinc deficiency (2). Zinc is strongly bound to phytates in the intestinal tract (3), lowering its absorption and reabsorption. The following factors affect zinc absorption:

• The more phytates you eat, the worse zinc absorption.
• The less zinc is consumed, the less absorption (obviously).
• Another factor that lowers the zinc absorption in the presence of phytates is the amount of calcium. Phytates, calcium and zinc form a stronger salt than the phytate-zinc salt, therefore, less zinc is available to be absorbed.

The two first statements are especially important in vegetarian and vegan diets since those diets are high in phytates (legumes, seeds and grains) and poor in zinc (4). A 15 to 1 ratio of phytates to zinc is considered below the optimal ratio for humans (4).

Regarding iron, according to WHO, a 29% of the population suffers from iron deficiency. If you haven’t read the post Iron deficiency and/or anemia I invite you to read it so you can understand better what I'm going to say next.

The evidence suggests that phytates do inhibit iron absorption, but only when it's non heme iron, the one that comes from plants (5). Once again, here vegetarian and vegans are more at risk since the iron they consume is non-heme iron. There is a way to counteract the phytate effect regarding iron which is adding food rich in vitamin C like peppers, tomatoes, or lemon juice.

Magnesium is not a source of concern since the phytate rich products are also high in magnesium. The evidence for calcium says it's not affected by phytates and for copper the evidence is not conclusive, so we don't know how phytates affect copper absorption.

Here you have a list with the milligrams of phytates by 100 grams of some foods in dry weight (6):

• Corn: 720 – 2220
• Wheat: 390 – 1350
• Rice: 60 – 1080
• Oats: 420 – 1160
• Different kinds of beans: 610 – 2380
• Green peas: 220 – 1.220
• Chickpeas: 280 – 1600
• Lentils: 270 – 1510
• Peanuts: 170 – 4470
• Almonds: 350 – 9420
• Walnuts: 200 – 6690

Oxalates

Oxalates are found in some crops and in some of them in large quantities. Humans also produce oxalates since it's an end product of the digestion of ascorbate, glyoxylate and glycine (a non-essential amino acid).

This antinutrient does the same as phytate, it forms salts with cations like calcium (Ca⁺), making harder the absorption of these micronutrients. Oxalates have another negative effect for our health since they can form kidney stones.

Mineral absorption

There are several studies about mineral absorption in presence of oxalates, but some of them are not well planned.

I’ll give you an example, spinaches have a high content of oxalates and iron. It’s believed that the high oxalate content in this vegetable is what causes the poor iron absorption from it. There are several studies that conclude that oxalates are the ones to blame, but spinach has other compounds that could cause the malabsorption. Remember that correlation doesn’t imply causation.

There are newer studies that simulate the action of oxalates but without the spinach in the middle (7) and they conclude that oxalates are not guilty for the iron malabsorption in the spinach. Now, spinach is still a poor source of iron because it’s non-heme iron that we already know from the post about anemia that it’s absorbed poorly.

Regarding calcium absorption here we can see a clear poor absorption in the presence of oxalates.

There is an article (8) that doesn’t use vegetables to test that oxalates lower the calcium absorption. In this article they decided to use the calcium oxalate by itself. By using just, the salt and not an oxalate rich food, what they do is to remove any substance present in that food that could interfere with the calcium absorption. To compare the oxalate salt, they used milk as a calcium source. The experiment results tell us that the calcium absorption from milk was 36%, while in the presence of oxalates was 10%. We can conclude that oxalates lower the calcium absorption. One interesting thing about this article is that they also tested what would happen if you mixed the oxalate salt with milk. What happened is that it doesn’t matter if they are together or separated the absorption is the same. So, don’t worry to mix that spinach with cheese and cream to make a sauce, you’ll totally get the calcium from the dairy even though it’s mixed spinach and its oxalates.

In another study they compared the calcium absorption in different vegetables (9) and those with a high oxalate content like spinach or rhubarb had a lower calcium absorption, 9.3 and 9.2% respectively. These absorptions are similar to the ones in the previous study.

Once again, those that are at high risk of suffering from a calcium deficiency are vegans, simply because they eat more products high in oxalates. Moreover, to absorb calcium we need to have enough vitamin D and vegans don’t eat any source of it and although humans generate their own vitamin D from sun exposure, we need cholesterol to do that and vegans don’t eat that either, so they solely rely on their own ability to produce it.

Kidney stones

Some of the salts generated from oxalates, can solidify in the kidneys giving place to kidney stones. The chance of having these stones depends on the percentage of oxalates absorbed in the intestine. This percentage of absorption depends on the quantity of ingested oxalates, the nature of the oxalate molecule, diet and genetics and it goes from 2 to 22% (10).

A curious detail about kidney stones is that this affects more man than women and most of these stones are made of calcium oxalate. Those who have suffered from these kidney stones should ask for an analysis of the stones to know from what they are made of. Depending on the salt, they should change their diet to avoid the formation of the stones in the first place. If the analysis shows oxalate stones, then it would be helpful to lower the consumption of food rich in oxalates.

Here you have a list with the milligrams of oxalate in 100 grams of some foods (10):

• Beets: 121-450
• Spinach: 320-1260
• Coffee grains: 50-150
• Cashew: 231
• Cocoa: 500-900
• Tea: 300-2000
• Parsley: 140-200
• Potato: 20-141

Nutritionist Sally Norton does a lot of campaign against oxalates in social media. For years she suffered a bunch of health problems that went away when she started a low oxalate diet. You can read her story in her website: https://sallyknorton.com/

How to lower phytates and los oxalates

I'm sure you are thinking, what the h***, now we can't eat a thing, there's always a new scientific paper saying we can't eat this or that. There are a few ways to lower the phytates and oxalates content in the food through germination, soak and cooking, this is why legumes need to be soaked the night before cooking them.

Conclusions

As you can see antinutrients are a real thing and in some cases, they can be really harmful for our health. Now, we are all different from each other so what's beneficial for someone can be detrimental for others. What I always say, what is important is to pay attention to the signs our bodies give us. If you eat something and it gives you diarrhea, bloating, heartburn or whatever stomach upset, then there's something wrong with that food, and you should not eat it.

Sex is something to consider, since women are prone to suffer from calcium and iron deficiencies (due to pregnancy and periods). Unfortunately, women are the ones who are also prone to do stupid diets to lose weight or be vegetarian or vegan because they are more sensitive towards animals.

Be gentler with your body and think a little bit more about the quality of the fuel you are giving to it to function properly. We are all different and maybe that spinach that everybody says is so healthy is what's making you sick.

Note: Spinach is a crappy food if you are looking for minerals.

References

1. Garn, S. M., & Leonard, W. R. (2009). What Did Our Ancestors Eat? Nutrition Reviews, 47(11), 337–345. doi: 1753-4887.1989.tb02765.x
2. https://www.who.int/whr/2002/chapter4/en/index3.html
3. Flanagan, P.R. (1984). A model to produce pure zinc deficiency in rats and its use to demonstrate that dietary phytate increases the excretion of endogenous zinc. Journal of Nutrition, 114, 493–502. doi: 10.1093/jn/114.3.493
4. Gibson, R.S., Donovan, U.M. & Heath, A.L.M. (1997). Dietary strategies to improve the iron and zinc nutriture of young women following a vegetarian diet. Plant Foods for Human Nutrition, 51, 1–16. doi: 10.1023/a:1007966104442
5. Lopez, H. W., Leenhardt, F., Coudray, C., & Remesy, C. (2002). Minerals and phytic acid interactions: is it a real problem for human nutrition? International Journal of Food Science and Technology, 37(7), 727–739. doi: 10.1046/j.1365-2621.2002.00618.x
6. S Storcksdieck genannt Bonsmann, T Walczyk, S Renggli and RF Hurrell (2008). Oxalic acid does not influence nonhaem iron absorption in humans: a comparison of kale and spinach meals. European Journal of Clinical Nutrition 62, 336–341. doi: 10.1038/sj.ejcn.1602721
7. Heaney, R. P., & Weaver, C. M. (1989). Oxalate: effect on calcium absorbability. The American Journal of Clinical Nutrition, 50(4), 830–832. doi: 10.1093/ajcn/50.4.830
8. Weaver, C. M., Heaney, R. P., Nickel, K. P., & Packard, P. I. (1997). Calcium Bioavailability from High Oxalate Vegetables: Chinese Vegetables, Sweet Potatoes and Rhubarb. Journal of Food Science, 62(3), 524–525. doi: 10.1111/j.1365-2621.1997.tb04421.x
9. Noonan SC, Savage GP. (1999). Oxalate content of foods and its effect on humans. Asia Pac J Clin Nutr., 8(1), 64-74. doi: 10.1046/j.1440-6047.1999.00038.x