There are many medical conditions that call for dietary restrictions.
- irritable bowel syndrome
- 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.
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.