Are Vegan Diets Low in Certain Minerals?

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My wonderful intern Zachary Wenger has previously written about whether protein or certain vitamins are nutrients of concern for vegans. This article explores whether vegans are at higher risk of specific mineral deficiencies.


Iodine

Traces of iodine are essential for life.

Iodine is needed for the creation of thyroid hormones.

These thyroid hormones regulate body temperature and red blood cell production assists in nerve function, muscle function, as well as assisting in bone and brain development during pregnancy and infancy.

While yoghurt, milk, and white fish are the main sources of iodine in the average UK diet, the richest dietary source of iodine is seaweed. However, the amount of iodine within seaweed is highly variable, due to the differences in species, location, and preparation (1). Researchers investigated the iodine content in 12 different seaweed samples, and found the range to be anywhere from 16 micrograms to over 8,165 micrograms (2)! The researchers also pointed out that dishes that use seaweed, may breach the tolerable upper limit of iodine of 1,100 micrograms per day. An excess of iodine can adversely affect thyroid function, and it is recommended to consume seaweed no more than once a week, especially during pregnancy or breastfeeding. 

Another suitable option for vegans is consuming iodized salt. Iodized salt is an alternative for those trying to improve their iodine status (3). A quarter of a tsp of iodized salt, would amount to 71 micrograms of iodine, which is about 47% of an adult’s daily requirement. Unfortunately, iodized salt isn’t available in all countries and it isn’t a good option for those who need to limit their salt intake. Certain plant-based milks are starting to fortify with iodine, which is very helpful for those who don’t consume dairy.

Prunes and lima beans also contain iodine. However, to meet the RDA for iodine, set at 140 micrograms per day, it would require you to consume roughly 55 prunes or 10 cups of lima beans. 

Lastly, if you aren’t interested in any of the dietary options, there are always iodine supplements. 

Iodine isn’t found in foods vegans typically consume and may not be abundantly available based on one’s location.

Vegans may need to pay extra attention to it, to ensure that they maintain an adequate iodine status. 

If one decides not to supplement iodine, doesn’t regularly consume seaweed, and can’t access iodized salt due to their location, this may be a true nutrient of concern for vegans. However, for those that do have access to any of the options, iodine needs can be met without a problem as a vegan.

Iron

Iron’s major role is carrying and releasing oxygen throughout the body as well as making it available for muscle contractions.

Iron is also essential for cellular function and the utilization of energy. Iron deficiency is also the most common nutritional deficiency. 

Good sources of iron for vegans include legumes, seeds, fortified cereals, tofu, and cashews. Plant sources of iron are often frowned upon for two reasons. Firstly, that plant sources of iron are less bioavailable. Specifically, due to the difference between haem and non-haem iron. Secondly, due to the presence of iron inhibitors, like tannins and phytic acid, which reduces iron absorption further. Let’s investigate the validity of each of these. 

There are two types of dietary iron: non-haem iron, which is found in both plant and animal foods, as well as haem iron, which is found only in animal products, with the exception of a few processed plant products. Specifically, haem iron is found in the popular plant-based burger, the Impossible Burger. They are able to do this by cooking soy leghemoglobin, which produces haem. 

A common misconception is that animal products only contain haem iron. For example, pork liver is promoted as a great source of haem iron. In reality, out of the 12.6mg of iron, only 2.3mg of that iron is haem iron (4). The best source of haem iron that was tested in this study, was cooked blood curds from pork and chicken, in which the average was 9.2mg and 15.4mg of haem iron… yum? Haem iron is estimated to contribute to only 10–15% of total iron intake in meat-eating populations (5). As a reference, the recommended daily intake (RDA) for iron in the US is set at 8 mg per day for men and postmenopausal women, and 18 mg per day for premenopausal women. In the UK, the reference nutrient intake (RNI) for iron is set at 8.7 mg per day for men and postmenopausal women, and 14.8 mg per day for premenopausal women.

Haem iron is the most easily absorbable form of iron.

Absorption varies anywhere from 15% to 35% (6). Non-haem iron is typically much less well absorbed in the body than haem iron. Non-haem iron absorption has been reported to be anywhere from 0.7% to 22.9% (7).

These numbers are individualized and dependent on dietary factors and the body’s iron stores. When iron stores (also known as ferritin)  are low, non-haem iron has a greater effect. This finding is so consistent and robust, that it also can be seen through an equation, using one’s ferritin levels and dietary modifiers (7). 

Dietary iron modifiers refer to the modifications to the diet that either enhances or inhibits iron absorption. Specifically, non-haem iron absorption. The main inhibitor for non-haem iron absorption is phytic acid, also known as phytates. Phytates bind to certain minerals, including iron, making them less bioavailable (8).

If phytates were the deciding factor, there are cooking methods to limit the phytate content (9). However, there are mechanisms supporting the narrative that phytates may actually hold unique properties, including anticancer and antioxidant properties (10). While this is just a mechanism, in humans, the foods highest in phytates, which include nuts, legumes, and seeds — they are all associated with decreased risks of cancer (11-14). 

Another non-haem iron inhibitor is tannins. Tannins are naturally occurring compounds that are found in plants. Foods rich in tannins include tea, wine, coffee, and dark chocolate.

Tannins have been demonstrated to inhibit non-haem iron absorption up to 90%, when consumed with an iron-rich meal (15).

Due to this, it is often advised not to consume tannin-rich foods or drinks for at least one hour when consuming foods which contain non-haem iron. 

Fortunately, there are iron enhancers as well. The most potent and well-understood non-haem iron enhancer is vitamin C (16). Vitamin C has even been demonstrated to overpower the iron inhibitors, phytates and tannins (17).

If you’re looking to optimize your iron absorption from plants, pairing it with vitamin C is the way to go.

This could look like consuming lentils with red bell peppers at the same meal, or tofu and orange juice. 

Vegans and vegetarians have significantly lower iron stores on average than non-vegetarians (18). However, lower iron stores does not equal an outright deficiency. This isn’t surprising though, due to all the inhibitors that we have already discussed. As a result, the Institute of Medicine set the requirement for iron 1.8 times higher for vegetarians, which can be applied to vegans as well (19).

While vegans have lower iron stores than non-vegetarians, iron is the most common nutrient deficiency worldwide. While many want to point fingers at vegans and vegetarians specifically, iron is a nutrient that many struggle with, regardless of any specific dietary pattern, and is a nutrient that many people need to prioritise. 

While iron may be a tedious and tricky mineral for vegans, with the help of iron enhancers and knowledge on non-haem iron inhibitors, it is more than possible to meet the recommendations. 

Calcium

Calcium is a mineral that is primarily found in our bones and teeth, in which it supports their structure.

It also plays a role in muscular control, vasodilation, and much more. 

The RDA in the US for calcium is set at 1,000 mg per day for adults. In the UK, the RNI for calcium is set at 700 mg per day for adults.

There are a variety of plants that naturally contain calcium, including kale, bok choy, and chickpeas. The problem with calcium in a vegan diet isn’t the amount of calcium one can consume, it is how the calcium is absorbed, primarily due to a compound called oxalic acid.

Oxalic acid, also known as oxalates, are compounds that are found in a variety of plants, which bind to calcium, inhibiting its absorption. 

Spinach, swiss chard, almonds, and beetroot leaves are the most concentrated form of oxalates in the diet. When investigated, the absorption rate of calcium from spinach was as low as 5% (22). Nonetheless, vegetable and nut consumption are both associated with a decreased risk of heart disease and other chronic diseases (20-21). In other words, eliminating these foods from your diet, simply due to the calcium absorption, may not be ideal. However, these foods just shouldn’t be your primary source of calcium in the diet. There are a variety of low-oxalate, high-calcium plants with high reported absorption rates, that may be better choices. 

Calcium-fortified soymilk and calcium-set tofu has been reported to have similar absorption rates to dairy milk, at around 30% (23-24). This would extend to soya yoghurt as well. Bok choy and kale have calcium absorption rates even higher, sitting at around 50% (25). Mustard greens and cabbage have calcium absorption rates around 40% (25). White, red, and pinto beans have calcium absorption rates of around 21-26% (25). Foods that weren’t tested in the study, that are a good source of calcium and that are low in oxalates include, tahini, calcium fortified orange juice, chia seeds, and calcium-fortified bread. 

While calcium may seem challenging on a vegan diet, it is more than possible to meet recommendations with a wide variety of plants fortified with calcium, as well as low-oxalate, high-calcium plants! 

Zinc

Zinc has many important roles in our body, for example, it is involved in wound healing, our immune system, DNA synthesis and enzyme functions (26).

In the UK, the RNI for zinc is (27):

  • 9.5mg a day for men (aged 19 to 64 years)
  • 7mg a day for women

The foods which contain the highest amount of zinc are animal-based foods like oysters, beef, crab and lobster (26). However, zinc is also found in plant-based foods like beans, chickpeas, fortified cereals, wholemeal bread, quinoa, nuts and seeds (26).

Some research suggests that long-term vegetarians have lower zinc levels than non-vegetarians (28). This is also likely to apply to vegans.

The National Institutes of Health (NIH) in the US report that suggests that vegetarians “sometimes require as much as 50% more of the RDA for zinc than non-vegetarians” (26).

This may be due to the fact that, similar to iron, the bioavailability of zinc is lower in plant-based foods than in animal-based foods due to the presence of phytates.

Techniques for increasing zinc bioavailability include soaking or sprouting beans, seeds or grains before cooking. It may also be helpful to consume leavened grain products like bread rather than unleavened products like crackers, because this process breaks down phytate.

Overall, an adequate intake of zinc can be consumed on a vegan diet as it is present in many vegan foods. However, it is a nutrient which needs to be prioritised, especially due to the absorption issues related to the presence of phytate. Zinc supplements are also widely available and may be beneficial for some vegans.

Selenium

Selenium is an antioxidant which also plays a vital role in thyroid health, reproduction and DNA synthesis (29).

The UK RNI for selenium is (27):

  • 75μg a day for men (19 to 64 years)
  • 60μg a day for women (19 to 64 years)

Variation in soil impacts the selenium content of both plant-based and animal-based foods, but in general, many animal-based foods are high in this mineral.

Brazil nuts are a great vegan source of selenium and it is thought that adult selenium requirements can be met by consuming two Brazil nuts per day.

Selenium is also found in smaller amounts in the following vegan-friendly foods (29):

  • Brown rice
  • Bread
  • Beans
  • Lentils
  • Oats
  • Spinach

Selenium is included in many vegan-specific multivitamin and mineral supplements, so this is another option for those who don’t regularly consume Brazil nuts.

Conclusion

It is important to note that this is all general information based on the current research that we have available.

Always follow individual advice from your healthcare providers as they know you and your medical history best. 

Furthermore, many fear that by adopting a vegan diet, they will become deficient in key minerals. While it is true that there are certain factors and compounds that play a role in limiting maximal absorption of certain minerals (oxalates, phytates, non-haem iron), there are also many workarounds that are often overlooked that improve mineral absorption!

It’s all about finding the right balance and what works best for you. And if you are struggling to do this by yourself, you should seek support from a Registered Dietitian.

References: 

  1. Zava, Theodore T, and David T Zava. “Assessment of Japanese iodine intake based on seaweed consumption in Japan: A literature-based analysis.” Thyroid research vol. 4 14. 5 Oct. 2011, doi:10.1186/1756-6614-4-14
  2. Teas, Jane et al. “Variability of iodine content in common commercially available edible seaweeds.” Thyroid : official journal of the American Thyroid Association vol. 14,10 (2004): 836-41. doi:10.1089/thy.2004.14.836
  3. Clar, Christine et al. “Iodized salt for iodine deficiency disorders. A systematic review.” Endocrinology and metabolism clinics of North America vol. 31,3 (2002): 681-98. doi:10.1016/s0889-8529(02)00011-7
  4. Kongkachuichai, Ratchanee & Napatthalung, P. & Charoensiri, R.. (2002). Heme and Nonheme Iron Content of Animal Products Commonly Consumed in Thailand. Journal of Food Composition and Analysis. 15. 389-398. 10.1006/jfca.2002.1080.
  5. Hurrell, Richard, and Ines Egli. “Iron bioavailability and dietary reference values.” The American journal of clinical nutrition vol. 91,5 (2010): 1461S-1467S. doi:10.3945/ajcn.2010.28674F
  6. Ems T, St Lucia K, Huecker MR. Biochemistry, Iron Absorption. [Updated 2020 Apr 30]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan. 
  7. Richard Hurrell, Ines Egli, Iron bioavailability and dietary reference values, The American Journal of Clinical Nutrition, Volume 91, Issue 5, May 2010, Pages 1461S–1467S
  8. Hallberg, L et al. “Iron absorption in man: ascorbic acid and dose-dependent inhibition by phytate.” The American journal of clinical nutrition vol. 49,1 (1989): 140-4. doi:10.1093/ajcn/49.1.140
  9. Gustaffson, E‐L & Sandberg, Ann-Sofie. (2006). Phytate Reduction in Brown Beans (Phaseolus vulgaris L.). Journal of Food Science. 60. 149 – 152. 10.1111/j.1365-2621.1995.tb05626.x.
  10. Graf, E, and J W Eaton. “Antioxidant functions of phytic acid.” Free radical biology & medicine vol. 8,1 (1990): 61-9. doi:10.1016/0891-5849(90)90146-a
  11. Schlemmer, Ulrich et al. “Phytate in foods and significance for humans: food sources, intake, processing, bioavailability, protective role and analysis.” Molecular nutrition & food research vol. 53 Suppl 2 (2009): S330-75. doi:10.1002/mnfr.200900099
  12. Lanza, Elaine et al. “High dry bean intake and reduced risk of advanced colorectal adenoma recurrence among participants in the polyp prevention trial.” The Journal of nutrition vol. 136,7 (2006): 1896-903. doi:10.1093/jn/136.7.1896
  13. Lee, Jeeyoo et al. “The relationship between nut intake and risk of colorectal cancer: a case control study.” Nutrition journal vol. 17,1 37. 7 Mar. 2018, doi:10.1186/s12937-018-0345-y
  14. Lowcock, Elizabeth C et al. “Consumption of flaxseed, a rich source of lignans, is associated with reduced breast cancer risk.” Cancer causes & control : CCC vol. 24,4 (2013): 813-6. doi:10.1007/s10552-013-0155-7
  15. Hurrell, R F et al. “Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages.” The British journal of nutrition vol. 81,4 (1999): 289-95.
  16. Hallberg, L et al. “Effect of ascorbic acid on iron absorption from different types of meals. Studies with ascorbic-acid-rich foods and synthetic ascorbic acid given in different amounts with different meals.” Human nutrition. Applied nutrition vol. 40,2 (1986): 97-113.
  17. Siegenberg, D et al. “Ascorbic acid prevents the dose-dependent inhibitory effects of polyphenols and phytates on nonheme-iron absorption.” The American journal of clinical nutrition vol. 53,2 (1991): 537-41. doi:10.1093/ajcn/53.2.537
  18. Haider, Lisa M et al. “The effect of vegetarian diets on iron status in adults: A systematic review and meta-analysis.” Critical reviews in food science and nutrition vol. 58,8 (2018): 1359-1374. doi:10.1080/10408398.2016.1259210
  19. Institute of Medicine (US) Panel on Micronutrients. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington (DC): National Academies Press (US); 2001. 9, Iron.
  20. Guasch-Ferré, Marta et al. “Nut Consumption and Risk of Cardiovascular Disease.” Journal of the American College of Cardiology vol. 70,20 (2017): 2519-2532. doi:10.1016/j.jacc.2017.09.035
  21. Dauchet, Luc et al. “Fruit and vegetable consumption and risk of coronary heart disease: a meta-analysis of cohort studies.” The Journal of nutrition vol. 136,10 (2006): 2588-93. doi:10.1093/jn/136.10.2588
  22. Heaney, R P et al. “Calcium absorbability from spinach.” The American journal of clinical nutrition vol. 47,4 (1988): 707-9. doi:10.1093/ajcn/47.4.707
  23. Yongdong Zhao, Berdine R. Martin, Connie M. Weaver, Calcium Bioavailability of Calcium Carbonate Fortified Soymilk Is Equivalent to Cow’s Milk in Young Women, The Journal of Nutrition, Volume 135, Issue 10, October 2005, Pages 2379–2382, https://doi.org/10.1093/jn/135.10.2379
  24. Weaver, C. M., Heaney, R. P., Connor, L., Martin, B. R., Smith, D. L., & Nielsen, S. (2002). Bioavailability of calcium from tofu as compared with milk in premenopausal women. Journal of Food Science, 67(8), 3144-3147. https://doi.org/10.1111/j.1365-2621.2002.tb08873.x 
  25. Connie M Weaver, William R Proulx, Robert Heaney, Choices for achieving adequate dietary calcium with a vegetarian diet, The American Journal of Clinical Nutrition, Volume 70, Issue 3, September 1999, Pages 543s–548s, https://doi.org/10.1093/ajcn/70.3.543s
  26. NIH ‘Zinc Factsheet for Health Professionals” [accessed November 2020 via: https://ods.od.nih.gov/factsheets/Zinc-HealthProfessional/]
  27. NHS Website “Vitamins and Minerals: Others” [accessed November 2020 via: https://www.nhs.uk/conditions/vitamins-and-minerals/others/]
  28. Foster et al. (2013) “Effect of vegetarian diets on zinc status: a systematic review and meta-analysis of studies in humans”. J Sci Food Agric. 2013;93(10):2362-2371.
  29. NIH ‘Selenium Factsheet for Health Professionals” [accessed November 2020 via: https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/]

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