Religious partial fasts

What it is

These are partial fasts that restrict certain foods rather than all food. Christian Orthodox Church fasting is a periodic vegetarian-type diet in which seafood and snails are allowed on most fasting days, with main fasting periods of 40 days before Christmas, 48 days of Lent and 15 days in August; monks may fast 180 to 200 days a year. The Daniel Fast is a 21-day plant-based diet without animal products, refined foods, white flour, preservatives, additives, sweeteners, caffeine or alcohol.

What the research says

Evidence is limited to observational studies and short uncontrolled trials. Orthodox fasting was associated with lower total and LDL cholesterol and slightly lower BMI, with a non-significant fall in HDL. A cross-sectional study of Athonian monks found profound vitamin D deficiency with high parathyroid hormone. In two single-arm studies without a control group, the 21-day Daniel Fast was followed by lower LDL, blood pressure and, in one study, weight, but also by lower HDL cholesterol, and in the first study body measurements and insulin did not change significantly.

Evidence grade: Limited evidence.

The schedule

• Pattern: religious

Why people choose it

These fasts are chosen for religious reasons; they limit which foods are eaten, not when, so they are closer to a plant-based diet than to time-restricted fasting. Studies are few and small: a scoping review of 20 publications on Orthodox fasting, a one-year Greek cohort, and two 21-day Daniel Fast studies from one US laboratory. Lower total and LDL cholesterol were reported, but HDL cholesterol also fell, and no study compared these fasts with an ordinary calorie-restricted diet.

Cautions

• People following long or strict Orthodox fasting (for example monastic fasting): A cross-sectional study of monks found profound vitamin D deficiency and high parathyroid hormone. (Source 1)
• People with low HDL cholesterol: In an uncontrolled study, the Daniel Fast was followed by lower HDL cholesterol as well as lower LDL cholesterol. (Source 2)
• People taking insulin, sulfonylureas or meglitinides: An NIDDK clinician interview notes these medicines are the ones associated with hypoglycaemia when eating changes. (Source 3)
• People with a history of eating disorders: A European expert guideline on fasting lists eating disorders among contraindications; a Canadian survey associated intermittent fasting with eating disorder psychopathology (not specific to religious fasting). (Source 4)

What goes wrong

Monks on Mount Athos, who follow strict Orthodox fasting for 180 to 200 days a year, had profound vitamin D deficiency and high parathyroid hormone, with normal blood calcium. (Source 1)

• Survey study, Certainty not rated.
• Size: Vitamin D, PTH and calcium measured in a subsample of 50 monks (mean age 38.7 years); dietary intake recorded in 70 monks (mean age 38.8 years)
• Who: Orthodox monks on Mount Athos, Greece.
• How long: Single assessment.
• Result: Vitamin D 8.8 plus or minus 6.2 ng/ml; PTH 115.5 plus or minus 48.0 pg/ml; energy intake about 1,266 kcal on restrictive days and 1,660 kcal on non-restrictive days.
• Funding: not stated.

Limit of this finding: Cross-sectional with no comparison group; low vitamin D was associated with this setting, and the study cannot separate fasting from other features of monastic life such as sun exposure.

Profound hypovitaminosis D (8.8±6.2 ng/ml), high parathyroid hormone (PTH)

In the first 21-day Daniel Fast study, HDL (protective) cholesterol was lower after the fast than before it, alongside the fall in LDL cholesterol. (Source 2)

• Case series, Certainty not rated.
• Size: 43 adults (13 men, 30 women), aged 20 to 62.
• Who: Adults in Memphis, USA, who self-selected to follow the Daniel Fast.
• How long: 21 days.
• Result: HDL-C 55.65 ± 2.50 vs 47.58 ± 2.19 mg/dL before vs after (p < 0.05)
• Funding: University of Memphis.

Limit of this finding: Single-arm trial with no control group (before-and-after comparison in self-selected adults); very short.

HDL-C (55.65 ± 2.50 vs. 47.58 ± 2.19 mg·dL-1)

What the evidence supports

A scoping review of 20 publications reported healthier blood lipid profiles during and after Orthodox fasting periods, with lower total cholesterol, LDL cholesterol, systolic blood pressure, body weight and BMI after a fasting period. (Source 5)

• Expert review, not systematic, Certainty not rated.
• Size: 1,226 fasting participants in 20 publications.
• Who: Adults following Christian Orthodox Church fasting.
• How long: Fasting periods of up to 48 days; some studies followed people for a year.
• Result: Not pooled; narrative summary.
• Funding: not stated.

Limit of this finding: This is a scoping review (labelled narrative-review here): it maps studies without pooling results or grading certainty. Most included studies were observational.

Fasters have healthier blood lipid profiles during and after COC fasting periods

In a one-year Greek cohort, people who kept all Orthodox fasting periods had lower end-of-fast total cholesterol, LDL cholesterol and BMI than people who did not fast. (Source 6)

• Cohort study, Certainty not rated.
• Size: 120 Greek adults (60 fasters, 60 non-fasting controls)
• Who: Greek adults in Crete.
• How long: One year, covering Christmas (40 days), Lent (48 days) and Assumption (15 days) fasts.
• Result: Fasters vs controls: total cholesterol 12.5% lower (p < 0.001), LDL 15.9% lower (p < 0.001), BMI 1.5% lower (p < 0.001), LDL/HDL ratio 6.5% lower (p < 0.05)
• Funding: not stated.

Limit of this finding: Observational; people chose whether to fast, so other differences between groups may explain results.

Fasters compared to controls presented 12.5% lower end-total cholesterol (p < 0.001), 15.9% lower end-LDL cholesterol (p < 0.001) and 1.5% lower end-BMI (p < 0.001).

A 21-day Daniel Fast was followed by significantly lower total cholesterol, LDL cholesterol, blood pressure, white cell count and blood urea nitrogen than before the fast. (Source 2)

• Case series, Certainty not rated.
• Size: 43 adults (13 men, 30 women), aged 20 to 62.
• Who: Adults in Memphis, USA, who self-selected to follow the Daniel Fast.
• How long: 21 days.
• Result: Total cholesterol 171.07 to 138.69 mg/dL; LDL-C 98.38 to 76.07 mg/dL; SBP 114.65 to 105.93 mmHg; DBP 72.23 to 67.00 mmHg (all p < 0.05); calorie intake 2185 to 1722 kcal.
• Funding: University of Memphis.

Limit of this finding: Single-arm trial with no control group (before-and-after comparison in self-selected adults); very short.

The following variables were significantly (p < 0.05) lower following the fast as compared to before the fast

In a second 21-day Daniel Fast study, LDL cholesterol, fasting glucose, insulin, HOMA-IR, systolic blood pressure and body weight were all lower after the fast than before it; krill oil versus placebo made no difference. (Source 7)

• Case series, Certainty not rated.
• Size: 39 adults (12 men, 27 women), including people with normal weight, overweight, obesity, high lipids, impaired fasting glucose and type 2 diabetes.
• Who: US adults.
• How long: 21 days.
• Result: LDL-C 100.6 to 80.0 mg/dL; fasting glucose 101.4 to 91.7 mg/dL; weight 74.1 to 71.5 kg; SBP 110.7 to 105.5 mm Hg (all p < 0.05)
• Funding: University of Memphis.

Limit of this finding: Single-arm trial with no control group for the Daniel Fast: participants were alternately assigned (not randomised) to krill oil or placebo as they enrolled, and everyone followed the Daniel Fast, so the diet effects are before-and-after comparisons.

Krill oil supplementation had no effect on any outcome measure (all p > 0.05), and so the data from the krill oil group and the placebo group were collapsed

What the evidence does not support

The same review found that key areas, including metabolic syndrome, had not been investigated at all, and said more research is needed before definite conclusions. (Source 5)

• Expert review, not systematic, Certainty not rated.
• Size: 20 publications.
• Who: Adults following Christian Orthodox Church fasting.
• How long: Not applicable.
• Result: No studies on metabolic syndrome as an outcome.
• Funding: not stated.

Limit of this finding: This is a scoping review (labelled narrative-review here): it maps studies without pooling results or grading certainty. Most included studies were observational.

Future research on COC fasting is needed in areas not investigated at all, like MetS, before reaching definite conclusions.

In the same cohort, HDL (protective) cholesterol also fell in fasters, by 4.6%, though this was not statistically significant. (Source 6)

• Cohort study, Certainty not rated.
• Size: 120 Greek adults.
• Who: Greek adults in Crete.
• How long: One year.
• Result: HDL cholesterol 4.6% decline in fasters, not significant.
• Funding: not stated.

Limit of this finding: Observational cohort: the authors' own wording 'contributes to' is causal, but people chose whether to fast, so the study can only show an association.

Adherence to Greek Orthodox fasting periods contributes to a reduction in the blood lipid profile including a non-significant reduction in HDL cholesterol

In that first Daniel Fast study, no body measurement changed significantly, and insulin, HOMA-IR and CRP fell but not significantly. (Source 2)

• Case series, Certainty not rated.
• Size: 43 adults.
• Who: Adults following the Daniel Fast.
• How long: 21 days.
• Result: No significant change in any anthropometric variable (p > 0.05); insulin p = 0.10, HOMA-IR p = 0.10, CRP p = 0.13.
• Funding: University of Memphis.

Limit of this finding: Single-arm trial with no control group (before-and-after comparison in self-selected adults); very short.

No significant difference was noted for any anthropometric variable (p > 0.05).

A common belief, and what the research shows

The belief: Religious partial fasts are well studied and proven to improve heart health.

What the research shows: The evidence is a scoping review, one cohort of 120 people, one cross-sectional study and two 21-day studies from one laboratory; cholesterol fell, but so did HDL, and no randomised trial compared these fasts with another diet.

References

1. European Journal of Clinical Nutrition (Nature). Health benefits and consequences of the Eastern Orthodox fasting in monks of Mount Athos: a cross-sectional study. 2017. DOI 10.1038/ejcn.2017.26. Read the source
2. Lipids in Health and Disease. Effect of a 21 day Daniel Fast on metabolic and cardiovascular disease risk factors in men and women. 2010. DOI 10.1186/1476-511X-9-94. Read the source
3. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), US National Institutes of Health. Fasting Safely with Diabetes (Diabetes Discoveries & Practice blog, interview with Dr. Martin M. Grajower). 2020. Read the source
4. Eating Behaviors (Elsevier). Intermittent fasting: Describing engagement and associations with eating disorder behaviors and psychopathology among Canadian adolescents and young adults. 2022. DOI 10.1016/j.eatbeh.2022.101681. Read the source
5. Nutrition Research Reviews (Cambridge University Press). Impact of Christian Orthodox Church dietary recommendations on metabolic syndrome risk factors: a scoping review. 2021. PMID 34108056, DOI 10.1017/S0954422421000184. Read the source
6. BMC Public Health. Effects of Greek orthodox christian church fasting on serum lipids and obesity. 2003. DOI 10.1186/1471-2458-3-16. Read the source
7. Nutrition & Metabolism. A 21-day Daniel fast with or without krill oil supplementation improves anthropometric parameters and the cardiometabolic profile in men and women. 2012. DOI 10.1186/1743-7075-9-82. Read the source