Intermittent Fasting in the Bible: Faith and Health

Intermittent fasting in the Bible is a topic that bridges ancient spiritual practice and modern nutritional science. Fasting appears throughout both the Old and New Testaments as a deliberate act of abstinence from food, and sometimes water, undertaken for prayer, repentance, or preparation for significant events. From Moses fasting 40 days on Mount Sinai to Jesus fasting in the wilderness, scripture presents fasting as a meaningful discipline. This article explores what the Bible actually says about fasting, how it compares to contemporary intermittent fasting approaches, and what UK health guidance says about safety.

Does the Bible actually mention fasting, and what does it say?

Yes, the Bible explicitly mentions fasting across both Testaments as voluntary abstinence from food and sometimes water, with Yom Kippur being the only formally commanded fast and Jesus assuming his followers would fast as a personal spiritual practice.

Yes, the Bible mentions fasting explicitly and repeatedly across both the Old and New Testaments, describing it as a deliberate spiritual discipline involving abstinence from food, and sometimes water, for a defined period. The Hebrew word 'tsom' and the Greek 'nesteia' both refer to voluntary abstinence, and fasting appears in contexts of mourning, repentance, petition, and preparation for significant events.

In the Old Testament, fasting is woven into Jewish religious law and practice. The Day of Atonement (Yom Kippur), described in Leviticus 16:29 to 31, is the only fast formally commanded in the Torah, requiring complete abstinence from food and drink for approximately 25 hours. Beyond this single mandated fast, numerous voluntary fasts are recorded throughout the historical and prophetic books.

In the New Testament, Jesus assumes his followers will fast rather than commanding them to do so. In Matthew 6:16 to 18, he instructs: 'When you fast, do not look sombre as the hypocrites do,' implying fasting is an expected personal practice rather than a public performance. Jesus himself fasted for 40 days in the wilderness (Matthew 4:2), and the early church fasted before making significant decisions (Acts 13:2 to 3).

It is important to note that biblical fasting was primarily a spiritual act, not a dietary or health strategy. The motivations, duration, and context differ substantially from contemporary nutritional approaches, and the two should not be conflated without careful consideration. Some religious fasts, including Yom Kippur, prohibit fluids as well as food; for people with certain health conditions this can carry additional risk, and individual medical and religious guidance may be appropriate. The safety section below discusses this further.

Which biblical figures fasted, and for how long?

Moses, Elijah, and Jesus each fasted for 40 days, presented as miraculous events, while shorter fasts of one to three days are more common in scripture, including Esther's three-day fast and Daniel's three-week partial fast.

Several prominent biblical figures fasted for notable periods, with Moses, Elijah, and Jesus each recorded as fasting for 40 days, a duration that holds particular symbolic significance throughout scripture. These extended fasts are presented as extraordinary, spiritually sustained events rather than templates for ordinary practice.

Moses fasted twice for 40 days and 40 nights on Mount Sinai (Exodus 34:28 and Deuteronomy 9:18), abstaining from both food and water, which from a physiological standpoint would be incompatible with survival without divine intervention, as the text implies. Elijah, after fleeing Jezebel, fasted for 40 days and nights while travelling to Horeb (1 Kings 19:8), sustained by food provided by an angel before the fast began. Jesus fasted for 40 days in the wilderness (Matthew 4:2), after which he was described as hungry, suggesting a genuine physical fast.

Shorter fasts are also recorded. Queen Esther called a three-day fast for herself and all the Jews in Susa before approaching the king (Esther 4:16). Daniel and his companions abstained from certain foods for ten days (Daniel 1:12 to 15) and later Daniel undertook a partial fast of three weeks, avoiding meat, wine, and rich food (Daniel 10:2 to 3). The Ninevites fasted for an unspecified period following Jonah's warning (Jonah 3:5).

These accounts vary considerably in duration and strictness. The majority of fasts recorded in scripture outside the extraordinary 40-day examples appear to have lasted one to three days, which is more comparable in length to patterns examined in modern fasting research, though the spiritual context and physiological conditions differ substantially.

How does biblical fasting compare to modern intermittent fasting?

Biblical fasting was a spiritually motivated, event-driven practice, whereas modern intermittent fasting is a structured nutritional strategy; both involve food restriction, but their purpose, timing, and expected outcomes differ substantially.

Biblical fasting and modern intermittent fasting share the common feature of deliberate food restriction, but they differ substantially in purpose, structure, and expected outcomes. Biblical fasting was fundamentally a spiritual and communal act, oriented towards prayer, repentance, or seeking divine guidance, whereas modern intermittent fasting is primarily a nutritional strategy aimed at metabolic health or weight management.

Modern intermittent fasting encompasses several structured approaches. The 16:8 method involves eating within an eight-hour window and fasting for sixteen hours each day. The 5:2 approach involves eating normally for five days and significantly restricting calories on two non-consecutive days per week. Alternate-day fasting alternates between unrestricted eating days and fasting or very low-calorie days. The British Dietetic Association (BDA) provides an overview of these patterns in its Intermittent Fasting Food Fact Sheet, which notes that the evidence comparing different dietary approaches shows broadly similar outcomes when total calorie intake is matched.^[2][3] Research suggests these protocols may influence insulin sensitivity and energy balance, though the evidence for some proposed mechanisms, including cellular repair processes such as autophagy, is largely based on animal studies and early-phase human research; established clinical benefit in humans has not been confirmed.^[14][3]

Biblical fasts, by contrast, were rarely structured around a repeating weekly or daily cycle. They were typically event-driven, beginning and ending in response to a specific spiritual need. The partial fast described in Daniel, avoiding rich foods and meat, bears some resemblance to modern calorie-restriction approaches, and has attracted interest from researchers and faith communities alike.

One meaningful parallel is the practice of Ramadan fasting observed by Muslims, which involves complete abstinence from food and drink between dawn and sunset for one lunar month. Research has examined the metabolic effects of Ramadan fasting, providing some evidence relevant to time-restricted eating more broadly, though direct extrapolation to biblical fasting patterns is limited by the differences in structure, context, and the prohibition on fluids during daylight hours.^[11][12]

Is intermittent fasting safe to try if you are inspired by religious practice?

Intermittent fasting is generally safe for healthy adults but carries particular risks for people with diabetes, pregnant women, those with eating disorders, and people on certain medicines, who should consult their GP before starting.

Intermittent fasting can be safe for many healthy adults, but it is not appropriate for everyone, and anyone inspired by religious practice to try it should first consider their individual health circumstances and, where relevant, speak with their GP or pharmacist before starting.

Groups for whom fasting carries particular risk include:

• People with type 1 or type 2 diabetes, especially those on insulin or sulfonylureas, where fasting can cause dangerous hypoglycaemia. People with diabetes should plan any fast with their clinical team and monitor their blood glucose as advised. Those taking SGLT2 inhibitors (such as dapagliflozin, empagliflozin, or canagliflozin) face an additional risk of euglycaemic diabetic ketoacidosis during prolonged calorie restriction and should seek specific medical advice before fasting.^[4]

• Pregnant or breastfeeding women, for whom adequate calorie and nutrient intake is essential

• People with a history of eating disorders, as fasting can trigger or worsen disordered eating patterns

• Children and adolescents, whose nutritional needs during growth make prolonged restriction inadvisable

• People who are underweight or malnourished

• Frail older adults, who may be at increased risk of dehydration, falls, and muscle loss

• People with advanced chronic kidney disease or significant cardiovascular disease

• Those taking certain medicines that require food for absorption or to avoid gastrointestinal side effects. Do not alter the timing or doses of any medicine without first discussing this with your prescriber or pharmacist.

For healthy adults without these risk factors, shorter intermittent fasting protocols such as 16:8 or 5:2 are generally well tolerated. Common side effects during the adjustment period include headaches, fatigue, irritability, and difficulty concentrating. These often improve as the body adapts, though the timeframe varies between individuals.

Some religious fasts, including Yom Kippur and Ramadan during daylight hours, prohibit fluids as well as food. Where a fast permits fluids, staying well hydrated is an important precaution. Where fluids are restricted for religious reasons, people with health conditions should seek both medical and religious guidance, as many faith traditions provide exemptions for those whose health would be put at risk.

Stop fasting and seek help promptly if you experience confusion, fainting, persistent vomiting, inability to keep fluids down, signs of severe dehydration, or chest pain. Call 999 for symptoms that are severe or life-threatening. For non-urgent concerns, contact your GP or call NHS 111.

It is worth noting that the 40-day fasts described in scripture are not safe to replicate. Complete abstinence from food for this duration would cause severe malnutrition and organ failure. These accounts are presented in their religious context as miraculous events, not as health guidance.

If you think a medicine you are taking has caused an unwanted effect, you can report this to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

What do UK health guidelines say about fasting and your wellbeing?

NICE and the BDA do not formally endorse intermittent fasting as a first-line strategy, but acknowledge it can support weight loss when nutritionally complete, sustainable, and tailored to the individual.

UK health guidelines do not formally endorse intermittent fasting as a first-line dietary strategy, but neither do they prohibit it for healthy adults. NICE guidance on obesity management acknowledges the role of various dietary approaches, including calorie restriction, in supporting weight loss when clinically appropriate. The NHS Eatwell Guide focuses on balanced nutrition across food groups rather than meal timing, and there is no specific NHS protocol for intermittent fasting.

NICE guidance on obesity recommends that any dietary approach used for weight management should be sustainable, nutritionally complete, and tailored to the individual.^[8] Intermittent fasting regimens that result in a meaningful calorie deficit can support weight loss, but NICE emphasises that behavioural support and long-term adherence are as important as the specific dietary pattern chosen. Readers should check the NICE website for the most current guideline, as guidance is periodically reviewed and updated.

The BDA has noted, in its Intermittent Fasting Food Fact Sheet, that while some people find intermittent fasting easier to maintain than continuous calorie restriction, the evidence comparing different dietary approaches shows broadly similar outcomes when total calorie intake is matched.^[2][3] The BDA advises that no single dietary pattern suits everyone and that individual preference, lifestyle, and health status should guide the choice.^[10][2]

For those exploring fasting within a faith context, spiritual motivation can be a powerful driver of adherence, but it does not remove physiological risk. Breaking fasts gently with balanced meals and monitoring for symptoms such as dizziness, palpitations, or significant weakness are sensible precautions. Healthcare professionals and patients with concerns about their health during fasting should speak with a GP or contact NHS 111 for advice.

Scientific References

1. The Influence of Time-Restricted Eating on Weight Management and Metabolic Health.
2. All Food Fact Sheets – British Dietetic Association.
3. Intermittent fasting for adults with overweight or obesity.
4. SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis.
5. Low blood sugar (hypoglycaemia).
6. Hypoglycaemia | Treatment summaries – BNF.
7. Is Time-Restricted Eating Safe in the Treatment of Type 2 Diabetes? A Review of Intervention Studies.
8. Overweight and obesity management (NG246).
9. Overweight and obesity management (NG246) – Behavioural overweight and obesity management interventions.
10. Diabetes – Type 2 – British Dietetic Association.
11. Cardiometabolic and obesity risk outcomes of dawn-to-dusk, dry intermittent fasting: Insights from an umbrella review.
12. An umbrella review and meta-analysis of meta-analyses of the impact of Ramadan fasting on the metabolic syndrome components.
13. Effects of Time-Restricted Feeding and Ramadan Fasting on Body Weight, Body Composition, Glucose Responses, and Insulin Resistance: A Systematic Review of Randomized Controlled Trials.
14. Fasting-mimicking diets as a strategy to reprogram tumor metabolism: a systematic review.

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