Disclaimer: This is going to be controversial. If you do not wish to question spiritual beliefs and a possible relationship with mental health issues than please do not read on.
I was inspired to delve into this topic after seeing shows on TV where people were stranded in the wilds and had no food (sometimes also no water). Their health deteriorates and often they start having hallucinations. A second inspiration for me is that I'm not unfamiliar hearing about ascetic idealisms due to doing yoga myself and with other people. Why am I linking this with spirituality? Because many religions and spiritual traditions started out by ascetics that preached sense deprivation. Often there are malnourished leaders described, or seen. The Buddha was said to have been very skinny after meditating too much and forsaking food. Various modern guru's also look rather skinny. Others are bordering on obesity, but as asceticism is such a core tenet of many beliefs, at least in their origins, I will focus on this.
Asceticism is a lifestyle where the follower deprives him or herself of pleasures and lives a minimalist lifestyle. The belief is that this will lead to spiritual insights and uncovering the mystery of the universe. If one rejects the bodily pleasures and comfort than one will come closer to 'higher', spiritual realizations, or so goes the mantra.
This can be rather innocent, from abstaining from a third meal a day to an extreme lifestyle where any hint of self pleasure or acknowledgement is rejected, and one sleeps in uncomfortable places as not to feel any comfort. More: https://en.wikipedia.org/wiki/Asceticism
Jesus had ascetic tendencies and did not preach the worldly life. Buddha was ascetic, as were many other guru's from India.
Now this is where things becomes interesting...
What unites these people isn't just the appeal of asceticism, but also the fact they eat very little and inadequate, and their tradition originates from poor countries. They see a lot of suffering around them and feel that one should ''escape'' the world. In order to do this they think they should live poor and simple themselves, and eat simple food, often heavily dominated by carbs such as rice or bread. Poorer countries are more likely to suffer from nutritional deficiencies. Here are the most common deficiencies in India, which is supposedly the most ''spiritual'' country: https://www.asianage.com/life/health...-in-india.html
Deficiencies common there can lead to hallucination and psychosis: https://www.greatplainslaboratory.co...-schizophrenia
If someone tells you to water fast for a month in order to ''contact your higher self'', or tells you to stay in the monastery and not get outside, think hard and think about your health first. These ascetic traditions could be creating 'truths' that 'work' when one lives on poor nutrition, giving weight that this is about poor health that induces hallucination, which is then written down as 'truth'. I have also scanned through a couple of spiritual retreats online, often there's a focus on little meals (lot of rice and bread) and from reviews online it does not sound adequate.
Could some much 'treasured' spirituality, taught by Indian guru's and end-of-the-world preachers, such as Jesus, early Christians and the gnostics, and other ascetics across the world, be a by-product of mere sense deprivation and nutritional deficiencies? Were they all suffering from health issues? Similar to how a lost person in the wild starts to hallucinate once the body gets into serious danger? Who knows.