Fasting

Nearly every tradition fasts, and nearly every tradition built in exemptions for the people it would harm — which is more medical prudence than most modern fasting content manages.

What it is

The deliberate abstention from food, and sometimes drink, for religious or contemplative purposes.

The forms vary enormously:

Ramadan — no food, drink, smoking or sexual activity from dawn to sunset, for a lunar month. Because the Islamic calendar is lunar, it moves through the seasons, and a summer fast at high latitude can exceed eighteen hours.
Yom Kippur — a full twenty-five hours without food or water.
Lent and the Orthodox fasts — extended periods of dietary restriction rather than total abstention, typically excluding meat, dairy, eggs, and in stricter observance oil and wine.
Buddhist monastic practice — no solid food after midday.
Ekādaśī — twice-monthly fasting in Vaiṣṇava practice.
The Bahá'í fast — nineteen days, sunrise to sunset.
Jain fasting, the most severe, including sallekhanā — a formal religious fast unto death, undertaken by a small number of practitioners at the end of life, and the subject of genuine legal contest in India over whether it constitutes suicide.

Where it came from

It is close to universal, and the stated purposes recur: purification, discipline, mourning, solidarity with the hungry, atonement, and preparation for vision or revelation.

The link between fasting and altered states is old and deliberate. Vision quests in several Native American traditions involve fasting; Christian desert asceticism used it heavily; Mikao Usui's founding experience of Reiki came during a twenty-one day fast. Extended food deprivation reliably alters cognition, and the traditions that used it for that purpose were not mistaken about the effect.

How it is done

Prescribed by the tradition rather than chosen: fixed days, fixed hours, fixed exclusions. Communal, in most cases — Ramadan and Yom Kippur are observed simultaneously by whole communities, and the shared quality is a substantial part of what they are.

And every major tradition builds in exemptions. Islamic law excuses the sick, the travelling, the pregnant and nursing, the menstruating, the elderly and the very young from Ramadan, with provision for making up or feeding the poor instead. Jewish law not only permits but requires breaking the Yom Kippur fast where life or health is at risk — the principle of pikuach nefesh, the preservation of life, overriding almost all other obligations. Christian fasting rules traditionally exempt the ill, the pregnant, children and the elderly.

That is worth pausing on. Traditions with no medical science built structured exemptions for the groups most likely to be harmed, and made them obligatory rather than optional.

What we can and cannot say

We can say religious fasting as practised is safe for most healthy adults, and the large Ramadan research literature broadly supports this — while identifying real risks in specific groups.

The documented risks are specific and worth stating. People with type 1 diabetes and insulin-treated type 2 diabetes face hypoglycaemia and ketoacidosis, and diabetes organisations publish detailed Ramadan guidance for exactly this reason. Dehydration is the main risk in fasts that exclude water, particularly in heat and at long day lengths. Kidney disease, pregnancy and some medication schedules require adjustment or exemption. And prolonged fasting followed by eating can cause refeeding syndrome, an electrolyte disturbance that can be fatal and requires medical management.

There is also an interaction the traditions could not have anticipated: religious fasting can mask, legitimise or trigger disordered eating, and clinicians working with eating disorders treat it as a genuine complication rather than a hypothetical one.

We can say the popular health claims outrun the evidence. Intermittent fasting is a real research area with real findings on metabolic markers, largely from animal work and short human trials, and it is not the same thing as religious fasting in schedule or purpose. The claim that fasting triggers autophagy and thereby confers broad health benefits is frequently attached to Yoshinori Ohsumi's 2016 Nobel Prize — awarded for work on autophagy mechanisms in yeast. The extrapolation from yeast cell biology to human health outcomes is very large and is not established.

We cannot give guidance, and will not. What can be said is that anyone with a chronic condition, taking regular medication, pregnant, or with a history of disordered eating should treat a fast as a medical question as well as a religious one — and that most religious traditions would agree, because they wrote that in.

What it is

Where it came from

How it is done

What we can and cannot say

Further reading