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Often, people say something like

"I'd love to be vegan, but I have low iron levels and a vegan diet would be unhealthy for me."

I'm not a physician and I can't just say

"I assure you, your iron intake will not suffer, but maybe even improve."

I still don't really know whether there are people who can't be vegan for health reasons.

What health conditions can people have such that a change to a vegan diet would be unhealthy?

Some examples I've witnessed (only personally):

  • The iron argument, multiple times, especially about adult women.
  • Someone claiming that he has a special kind of anaemia and he needed to eat beef because of it.
  • A vegetarian with Irritable Bowel Syndrom turning to meat eating because of his doctors recommendation.
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    Please do not answer in comments -- on a question, this space should be used for requesting clarification or suggesting changes :)
    – Erica
    Sep 8 '20 at 12:20
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There are many medical conditions that call for dietary restrictions. For example:

  • irritable bowel syndrome
  • allergies
  • migraines
  • gluten sensitivity

When a person has a combination of medical conditions, it can be very hard to find foods that satisfy all the restrictions.

I have a friend with several allergies and migraines. After removing all allergens and the migraine triggers from his diet, he is very restricted. Considering that he doesn't have much time for complicated recipes, he decided to eat seafood.

Every specific case would need to be evaluated by a combination of a doctor and nutritionist, to determine, if it's really impossible or just very hard to be vegan.

ADDED:

Check the list of Potential migraine triggers

The article in medicalnewstoday says: Any food can cause an allergy, theoretically and As far as foods are concerned, nearly all allergens are proteins

Another good list of food allergens is foodallergy.org

healthline.com lists foods to avoid with irritable bowel syndrome.

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  • I'm interested in know the list of restrictions your friend has found work for him.
    – ecc
    Feb 13 '17 at 16:59
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    actually these conditions make it more difficult, but still possible..
    – Attilio
    Feb 27 '17 at 23:47
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    Some allergies (soy or wheat) would make your CULINARY life far more difficult as a vegan, though :) Mar 13 '17 at 9:52
  • @Attilio has a point. I have two of the example conditions and it just takes a bit more of work
    – istepaniuk
    Jun 27 '19 at 15:21
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Autism. One of the main symptoms of autism is sensory processing disorder. This means that the brain perceives sensory input from the nerves incorrectly. This causes some senses to be very very pleasant, and others to be absolutely horribly unpleasant, and can even be erroneously perceived as physical pain. Every person is different, and thus the things different autistic people perceive as painful vary. Whilst one person can stand loud noise, another is in physical pain because of it. While one likes mac 'n' cheese, another may be in physical pain due to its scent, texture or taste. It's different for everyone, and as you can imagine this restricts some autistic people's diets very very much. There are some autistic people who can survive being vegan, but there are also some whose "safe foods" (foods that don't trigger pain) happen to be mostly non vegan foods. Since the diet is already limited, limiting it further to only the handful of vegan safe foods would have catastrophic affects on their health and safety as they would not have access to variety nor all the nutrients one needs to survive.

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  • I'm an Autistic Person and I definitely find the taste of most vegetables to be problematical. Oct 28 at 22:40
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One possibility: Omega-3 deficiency. Although omega-3 fatty acids can be found in non-meat sources, the best way to get them is by eating fish.

I knew someone once who was an on-again-off-again veg*an for this reason, because her body had trouble absorbing omega-3. She would stick to plant foods for a few weeks, then be forced (feeling horrible about it) back into eating fish for a while. Last I heard, she and her doctor had come up with a way for her to cut out fish on a more permanent basis (although I never found out what that was).

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Besides celiac disease and allergies, the following can make a vegan diet difficult:

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G6PD-Deficiency Disorder and/or Favism

This is a genetic condition in which the red blood cells (RBCs) are inadequately protected from certain proteins commonly found in legumes. The entire legume family of plants must be tightly restricted in the diet of someone with G6PD, which, on a vegan diet, eliminates all of the best sources for lysine, an essential amino acid. If the individual has access to one of the following foods, and does not have a food sensitivity toward it, these foods have high quantities of lysine without being in the legume family:

  • quinoa (a chenopod)
  • Lamb's quarters / goosefoot (a chenopod)

There may be a few more, but most lysine comes from legumes! Most of Asia does not have these chenopods.

G6PD, in the case of Favism, can be deadly. A bowl of fava or Lima beans could kill a G6PD-deficient child due to hemolysis of the red bloods cells and sudden loss of hemoglobin.

Non-vegan, but vegetarian foods (dairy products), including milk and eggs, have lysine in adequate quantities--which is why a G6PD individual may find a vegetarian diet acceptable, although not a vegan diet.

More G6PD Resources

https://hospitals.aku.edu/pakistan/patients-families/Documents/G6PD%20Deficiency-2018.pdf

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5338146/

https://ph.theasianparent.com/g6pd-foods-to-avoid/

https://veganhealth.org/g6pd-deficiency-fava-bean-intolerance/

[As the literature suggests, G6PD is not yet fully understood, and some confusion exists as to what can and cannot be eaten with it. I have close ties to multiple persons with the condition, and can attest that it is best for them not to eat beans generally, except in very small quantities so as to limit the ill effects. One bean won't kill anyone--but a full serving of Lima beans could kill a child with favism.]


Vitamin B12 Deficient

A strictly vegan diet has no reliable source of vitamin B12.

Some argue that certain cultures are vegan, living solely off the land in an agricultural area, but this is not well studied, and they may ingest some bacteria from the plants' soil (assuming the vegetables are not thoroughly washed) or even some insects along with their harvest.

Years ago, it was thought that certain species of seaweed contained vitamin B12; however, it is now known that no seaweed has B12--what they have is, instead, a B12 analogue which is not bioequivalent to actual B12 and which works against vitamin B12 in the body because it takes the place of B12 in the metabolic pathways without performing the necessary functions of B12.

It is said that people produce B12 in the gut. This is true; however, the B12 in the gut is produced well beyond the point at which it could be absorbed in the intestine--too far downstream to have benefit at all.

It is thought that some B12 may be created by the bacteria that forms plaque on the teeth, and that this may help to boost the body's B12 levels. Most people, however, brush that "ugly" plaque away, and do not ingest it. Furthermore, the amounts that this could conceivably produce and supply, even if ingested, are so minimal as to likely be unworthy of consideration.

A B12-deficient individual may have one or more of the following symptoms (listed in approximate order of increasing severity):

  • numbness/tingling in hands or feet
  • loss of taste or smell
  • loss of short term memory
  • weakness / fatigue
  • inability to concentrate
  • weakened nervous system
  • more easily experience motion sickness
  • vertigo / tinnitus
  • permanent brain damage
  • loss of eyesight
  • partial paralysis
  • death

IMPORTANT NOTE: Vegans typically consume high amounts of folate (supplement form is called "folic acid"), a B-vitamin found in green leafy vegetables, which masks the deficiency of vitamin B12. Vegans, therefore, may not experience the early symptoms of B12 deficiency, and be unaware of their lack until permanent brain damage has already been caused.

Please see some official warnings on this grave danger here:

https://ods.od.nih.gov/factsheets/Folate-Health%20Professional/

[Scroll down to "Health Risks from Excessive Folate".]

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Trying to recover from a restrictive eating disorder.

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    Welcome. Maybe you could elaborate to improve this answer. I think a mental health reason is as valid as physical health. Someone with disordered eating may very well use veganism for the wrong reasons, but equally they might use it as a wholesome way back in to eating. Some references would be even better.
    – David S
    Aug 22 '19 at 13:24

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