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Study analysis · npj Metabolic Health and Disease · 2025

Giving up meat and dairy for just 3 weeks doesn't just change your diet—it rewires your blood, spikes a 'longevity hormone,' and lowers your biological death score.

When people stop eating meat, fish, dairy, and eggs for a few weeks, their blood shows big changes—like lower cholesterol and a spike in a hormone linked to longevity—but it all goes back to normal when they start eating animal products again.

Reading level
Moderate certainty
Level 2b · Individual cohort studyAssociation, not causationNo causal claims

Overview

What the study found

The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.

In simple terms

This study is like watching two groups of people and checking their blood at two different times. One group sometimes stops eating meat, fish, dairy, and eggs, and the other group always eats them. The scientists saw that the people who sometimes stop eating these foods had different amounts of certain things in their blood. But because the scientists didn't randomly tell people which diet to follow, they can't say for sure that the diet caused those differences. They can only say the two things are related.

What’s the bottom line?

Researchers studied people in Greece who regularly switch between eating everything and eating no meat, fish, dairy, or eggs for religious periods. They compared their blood to people who always ate animal products.

How strong is this study?

This is a good study because it has lots of people and compares them carefully. It also uses a special trick with genes to guess about cause and effect, which is clever. However, it is not a perfect experiment, so there might be other reasons why the fasting group has different blood results. So we should be careful and not say that fasting definitely makes you healthier, but it's a useful clue.

Reporting

40 / 100

  • COI disclosure+40/40
  • Data availabilitydata not shared
  • Code availabilitycode not shared
Methodology

53 / 100

  • Randomizationnot randomized
  • Blindingblinding unclear
  • Control group+15/15
  • Sample size (n=411)+17.4/20
  • Follow-up+10/10
Publication

100 / 100

Statistical

54 / 100

  • P-values+15/15
  • Effect size+20/20
  • Confidence intervalsno confidence intervals
  • Pre-registrationnot pre-registered

Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.

Where it sits

RCT reviews

Max 100

Randomized Trials

Max 90

Reviews of Cohort Studies

Max 85

Cohort Studies

Max 72

Reviews of Case-Control Studies

Max 63

Case-Control Studies

Max 58

Cross-Sectional & Case Series

Max 50

Expert Opinion

Max 5
StrongerWeaker
Cohort Studies
Level 2b
60

60 / 100

Probability of being correct

Groups of people are followed over time to see who develops an outcome. Strong for identifying risk factors and associations, but cannot prove causation as firmly as RCTs.

This design cannot establish causation — the findings describe an association, not a cause. This is an observational cohort study without randomization. Although the longitudinal design with repeated measures reduces some confounding, the observed associations between dietary restriction and metabolic changes could still be influenced by unmeasured confounders. The Mendelian randomization analyses provide some causal evidence for specific protein-disease relationships, but they do not establish causation for the dietary intervention itself.

No Conflicts

No conflicts of interest identified

Not Disclosed

No conflicts of interest or funding information disclosed in the provided text.

Undisclosed — Suspicious

The provided text lacks author affiliations, funding declarations, and COI statements, limiting full assessment of potential biases.

Key takeaways

  1. 01

    After a few weeks without animal products, 166 out of 248 measured blood molecules changed.

  2. 02

    Cholesterol from LDL and HDL went down, and two amino acids (valine and leucine) dropped by 9% and 5%.

  3. 03

    The hormone FGF21 increased more than any other protein.

  4. 04

    A score that estimates risk of dying dropped from 0.27 to -0.78 in the restricting group, while the always-omnivore group showed no change.

  5. 05

    Yes, the changes point toward lower cholesterol, lower inflammation, and better metabolic health.

  6. 06

    But this study can't prove that periodically avoiding animal products will make people live longer, and the changes disappeared when they went back to eating animal products.

Surprising findings

  • Only one metabolite changed in the control group, while 166 changed in the restricting group.You'd expect some seasonal or time-related changes in everyone, but the omnivorous controls stayed almost completely stable, while the restricting group showed a massive shift—proving the changes are truly diet-driven.
  • FGF21—a 'pro-longevity hormone'—was the most changed protein of all 1,464 measured, and this was unique to the restricting group.Most people haven't heard of FGF21, yet it jumped more than any other protein linked to aging and metabolism. A dietary change can apparently trump all other protein fluctuations in the blood.
  • Higher oxytocin (OXT) levels were causally linked to a higher risk of lacunar stroke—the opposite of what you'd expect from the 'cuddle hormone.'Oxytocin is often viewed as beneficial for social bonding and metabolism, but Mendelian randomization showed it increased stroke risk. This contradicts previous claims about its cardiovascular benefits.

Practical takeaways

Consider trying a 3-4 week period of eliminating animal products (meat, fish, dairy, eggs) to see if your cholesterol and metabolic markers improve—like the participants in this study did during Lent.

This is an observational study; it doesn't prove cause and effect. Also, make sure to plan for nutrients like vitamin B12, calcium, and vitamin D, and consult a healthcare provider if you're on medication.

Medium confidence

If you're taking medications, be aware that a dietary change could alter the levels of proteins that drugs target, potentially affecting their action. Discuss significant diet changes with your doctor or pharmacist.

This is a novel finding from a single study; the clinical implications for drug dosing are not yet known.

Low confidence

Don't expect permanent benefits from a one-time fast—the changes reversed when participants returned to omnivory. If you want sustained effects, consider cyclical dietary patterns that are sustainable long-term.

The study only followed participants for one fasting period; long-term effects of cyclical fasting are unknown.

Medium confidence

Why this study matters

Your Blood Does a 180 When You Cut Animal Products

In 200 people who fast from animal products for religious reasons, 166 out of 248 measured metabolites changed during a 3-4 week Lenten period. This included significant drops in LDL- and HDL-related cholesterol, sphingomyelins, and branched-chain amino acids (valine -9%, leucine -5%). A control group of 211 omnivores showed almost no changes—only pyruvate slightly shifted.

This shows that a dietary change doesn't just affect one or two markers—it triggers a massive, coordinated shift across the entire metabolic landscape, even in a relatively short time.

A Mortality Risk Score Dropped Significantly in Just 3 Weeks

A validated 14-metabolite all-cause mortality risk score decreased from 0.27 to -0.78 (p=9.45×10⁻³) in the animal-product-restricting group, while the omnivorous control group showed no significant change. This score is a recognized surrogate for overall health and longevity.

It suggests that short-term dietary restriction can shift biomarkers that are associated with long-term mortality risk—something that usually takes years of lifestyle change to move.

FGF21: The 'Longevity Hormone' That Spikes the Most

Among 1,464 plasma proteins measured, FGF21—a liver-derived hormone linked to energy metabolism and longevity—showed the largest change, and it was unique to the restricting group. Mendelian randomization also suggested a protective causal effect of FGF21 on type 2 diabetes risk.

FGF21 is currently a major target for drug development (e.g., for obesity and NASH). The fact that a simple dietary pattern can naturally boost it more than any other protein is a powerful message.

Your Diet Could Change How Drugs Work

Of the 264 proteins altered by animal product restriction, 62 (23%) are targets for drugs in clinical development, and 28 (11%) are targets for approved drugs. This includes well-known targets like EGFR, which is targeted by 56 different drugs.

This suggests that what you eat could alter the effectiveness or side effects of medications—a largely ignored factor in personalized medicine.

The Benefits Are Real—But They're Not Permanent

When the 200 restricting individuals returned to an omnivorous diet, their metabolomic and proteomic profiles became indistinguishable from the continuously omnivorous controls. The changes are reversible, meaning the benefits require ongoing dietary restriction.

This is both encouraging (you don't have to be perfect forever) and a catch (you have to keep doing it). It also explains why cyclical fasting—like Greek Orthodox fasting—might be a sustainable approach.

Want the whole report?

Detailed mode opens the full scientific breakdown — every score component, the methodology, conflicts of interest, the evidence analysis behind each claim, and the raw study data.