I received an interesting question from a vegan who is concerned that her manganese intake on a vegan diet is 4 times the DRI of 1.8 mg for women. I thought this was going to be an easy question to answer – probably in 10 minutes or less. Seven solid hours later, I have added a new page to VeganHealth.org, Manganese. It is reproduced here:

Manganese is a mineral that is essential for humans. It is part of the antioxidant system in the mitochondria, and is also involved in metabolism, bone development, and the creation of collagen for wound healing.

Although manganese is an essential nutrient, manganese toxicity has been relatively common in places where workers are accidentally exposed to large amounts of manganese. Manganese in drinking water is the biggest concern since manganese in food is not as easily absorbed.

Manganese toxicity symptoms tend to be neurological problems. Headaches, muscle cramps, fatigue, and aggressiveness are early signs of manganese toxicity, which can then proceed into Parkinson’s disease-like symptoms such as tremors (6). People with manganese toxicity have more of a tendency to fall backwards than do those with Parkinson’s (6). Studies have been mixed on whether chelation therapy, the only therapy currently available, is successful at treating overt manganese toxicity (6).

According to the Linus Pauling Institute, “A single case of manganese toxicity was reported in a person who took large amounts of mineral supplements for years, while another case was reported as a result of a person taking a Chinese herbal supplement. Manganese toxicity resulting from foods alone has not been reported in humans, even though certain vegetarian diets could provide up to 20 mg/day of manganese.”

The DRI for manganese if 1.8 mg for adult women and 2.3 mg for men. One study has measured the manganese intakes in vegans and it found an average intake of 4.1 mg for women and 5.6 mg for men (not including supplements) (1), though some vegans get much higher amounts. The upper tolerable limit for adults is 11 mg/day, although this level is considered “very conservative” by the Linus Pauling Institute.

Despite the fact that overt manganese toxicity from food rarely occurs, iron deficiency has been shown to increase the risk of manganese accumulation in the brain (6). Because iron and manganese share similar absorption mechanisms, iron deficiency can increase manganese intake (from the body trying to increase iron absorption but getting manganese instead) and high manganese intakes can depress iron absorption (3, 4). A study in miners whose drinking water was contaminated with manganese showed that manganese toxicity decreased iron status which returned to normal after the manganese was minimized in their environment (2).

People with liver damage are also at risk for manganese deficiency (6).

Manganese absorption is very low, from 1 to 5% (4). The phytic acid in a soy formula was shown to cut manganese absorption in half (from 1.6 to .7%) in adults (5). Phytic acid has a similar effect on iron absorption, but vitamin C can overcome phytic acid’s effect on iron, whereas it does not do so for manganese (5).

From this research, it seems possible that long-term iron deficiency (not necessarily anemia, but low iron stores) could increase manganese absorption on a vegan diet high enough to cause problems, though I have not heard of any long term vegans getting Parkinson’s-like symptoms or manganese toxicity.

Vegans with iron deficiency should make sure they eat vitamin C at meals so that iron is absorbed instead of manganese. Including some soy with meals for phytic acid might also be a good idea for such people.

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References

1. Haddad EH, Berk LS, Kettering JD, Hubbard RW, Peters WR. Dietary intake and biochemical, hematologic, and immune status of vegans compared with nonvegetarians. Am J Clin Nutr. 1999 Sep;70(3 Suppl):586S-593S. | link

2. Boojar MM, Goodarzi F, Basedaghat MA. Long-term follow-up of workplace and well water manganese effects on iron status indexes in manganese miners. Arch Environ Health. 2002 Nov-Dec;57(6):519-28. (Abstract) | link

3. Kim Y, Lee BK. Iron deficiency increases blood manganese level in the Korean general population according to KNHANES 2008. Neurotoxicology. 2011 Mar;32(2):247-54. doi: 10.1016/j.neuro.2010.12.006. Epub 2010 Dec 21. | link

4. Finley JW. Manganese absorption and retention by young women is associated with serum ferritin concentration. Am J Clin Nutr. 1999 Jul;70(1):37-43. | link

5. Davidsson L, Almgren A, Juillerat MA, Hurrell RF. Manganese absorption in humans: the effect of phytic acid and ascorbic acid in soy formula. Am J Clin Nutr. 1995 Nov;62(5):984-7. | link

6. Aschner M, Erikson KM, Herrero Hernández E, Tjalkens R. Manganese and its role in Parkinson’s disease: from transport to neuropathology. Neuromolecular Med. 2009;11(4):252-66. doi: 10.1007/s12017-009-8083-0. Epub . Review. Erratum in: Neuromolecular Med. 2009;11(4):267. | link

This is Part 1 of a two-part series. In Part 2, I will discuss research on whether soaking beans reduces their propensity to produce intestinal gas.

Question:

I’m a mom of three vegan kids (13, 8, and 5) so I’m always tweaking our diet to ensure we are eating well. The kids eat a variety of nuts (usually on a daily basis) and I was wondering if we should be soaking them first to aid in digestion and absorption of nutrients. I know raw foodists suggest soaking, but I’ve never seen anything about this from a reputable source.

Answer:

It’s not easy to find much information on soaking nuts, but here is what I came up with…

Phytic acid is a molecule found in many seeds of plants. Legumes and grains tend to have large amounts of phytic acid, which often attach to calcium, magnesium, iron, or zinc and prevents their absorption.

I see many articles claiming that nuts are also high in phytic acid, but I could not confirm this to my satisfaction and so it should come as no surprise that I couldn’t find any research on whether soaking nuts could reduce their phytic acid content.

I did find two papers discussing the research on soaking legumes and grains:

The first review said that ten hours of incubating California small white beans at 140°F (60°C) resulted in an almost complete loss of phytic acid, with 75% being hydrolyzed (broken into phosphorus and inositol) and 25% being diffused into the water. Germination reduced phytic acid by over 60% in garbanzo beans and over 40% in soybeans. Boiling reduced phytic acid in soybeans by 40%. Soaking for 12 hours in room temperature water reduced phytic acid by 7.7%, 8.1%, 13.2%, and 19.1%, respectively, for black-eyed beans, red kidney beans, mung beans, and pink beans. Soaking for 18 hours reduced phytic acid by 52.7%, 69.6%, and 51.7% in pinto, Great Northern, and red kidney beans. So, as far as legumes go, soaking for 18 hours appears to be fairly effective at reducing phytic acid levels (1).

Another review (2) showed that soaking maize for 24 hours reduced phytic acid by about 50%, with most of the reduction occurring in the first hour. It said that soaking also removes other anti-nutrient factors such as saponins, trypsin inhibitors, and polyphenols.

Given the above, I would say that if nuts are high in phytic acid, soaking is likely to reduce it. And because vegans’ zinc intakes tend to be marginal, increasing the zinc absorption from nuts would be beneficial. Personally, I prefer to take a supplement with zinc rather than worrying if I’m absorbing enough from the food I eat.

One final note about soaking nuts:

In the comments section of the article, Go nuts for better health, in The Sydney Morning Herald (July 13, 2012), Lisa Yates, Program Manager and Dietitian of the organization Nuts for Life (established by the Australian nut industry), points out that the many studies showing nuts to reduce the risk of heart disease and type-2 diabetes use unsoaked nuts, so, there is no reason to think that you must soak them to receive the benefits.

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1. Urbano G, López-Jurado M, Aranda P, Vidal-Valverde C, Tenorio E, Porres J. The role of phytic acid in legumes: antinutrient or beneficial function? J Physiol Biochem. 2000 Sep;56(3):283-94. | link

2. Mensah P, Tomkins A. Household-level technologies to improve the availability and preparation of adequate and safe complementary foods. Food Nutr Bull. 2003 Mar;24(1):104-25. | link

A reader commented on Try it Again, Dr. Kim!:

I’m in such a quandry over beans, nuts and grains. I have a grown son who has been reading books/articles/websites about how our bodies are not made to absorb the nutrients from nuts and beans. Grains are the cause of inflammation and such things as ‘leaky gut’. When I shop now, I’m not sure what to buy. … What is the truth about beans, nuts and grains?

Phytates are the main reason for the concerns about mineral absorption in plant foods. Soy tends to be the highest food in phytates and I have a run down of mineral absorption from soy in Soy: What’s the Harm?

As for inflammation and leaky gut, I cannot speak much to that, though I’m not aware of any research on humans that shows grains to cause either of those conditions in people who do not have an allergy to such foods.

Here are two posts I made on whole grains that shows they are generally associated with good health:

jacknorrisrd.com/?cat=56

Should we, as vegans, be concerned about the oxalic and phytic acid content of foods? I personally avoid eating rhubarb, buckwheat and starfruit, and only occasionally eat spinach, but it would be nice to know if my worries about mineral loss were accurate.

I would not worry much about oxalic acid as long as you are getting plenty of calcium and you are not prone to oxalate kidney stones. I think it’s fine to eat spinach every day.

Phytic acid is a more difficult problem for me to answer. My guess is that it doesn’t present that much of a problem for vegans, but I also heard that in the Vegan Health Study, Dr. Michael Klaper found that many vegans were low in trace minerals (I was not able to find anything on it in a quick Internet search just now). It might, therefore, be prudent to take a multivitamin which would most likely overcome any problems caused by phytic acid.