Does Fasting Raise Your Blood Clotting Risk? What an Early Ramadan Fasting Study Found

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

Study at a Glance

What This Study Looked At

One of the oldest fears about fasting is that going without food somehow thickens the blood or raises the risk of dangerous clots. This early study set out to test that directly by tracking blood clotting markers in healthy men before, during, and after a month of dawn-to-dusk Ramadan fasting — a form of daily intermittent fasting practiced by over a billion people worldwide. If you're curious about what breaks a fast and what does not or how intermittent fasting affects inflammation, this study sits in the same general territory: does abstaining from food for extended hours change your blood chemistry in a way that could work against you?

Who Was Studied

Participant profile: Healthy adult men with no known cardiovascular disease, observed at baseline (before Ramadan), during the fasting month, and again after Ramadan ended.

How the fasting protocol worked in this study: Participants abstained from all food and drink from dawn until sunset each day for approximately 29–30 consecutive days, then ate normally during the evening and pre-dawn hours — a pattern comparable to a daily extended time-restricted eating window.

What the Researchers Found

Blood clotting (coagulation) markers

• The core finding was reassuring: the study found no evidence that Ramadan fasting pushed the body toward a prothrombotic (clot-prone) state.
• Values that are markers of thrombosis risk did not rise during the fasting period relative to baseline.

What Did Not Change

• No significant deterioration in clotting time measures (PT/aPTT)
• No sustained rise in fibrinogen, the key clotting protein researchers watch most closely for thrombosis risk

What the Researchers Concluded

The authors concluded that a month of daily dawn-to-dusk fasting did not meaningfully increase thrombosis risk markers in healthy men, offering early reassurance against the idea that fasting "thickens the blood."

What This Means If You Fast

• The old fear that fasting thickens your blood isn't well supported. This early study found no meaningful shift toward a clot-prone state during a month of daily fasting.
• Hydration still matters. Coagulation markers are sensitive to dehydration, and dawn-to-dusk fasting (without water) is a stricter test than a typical 16:8 window where water is unrestricted — modern intermittent fasting protocols that allow water throughout the fast are, if anything, gentler on blood volume and clotting balance.
• This doesn't replace individual risk assessment. People with a personal or family history of clotting disorders should talk to a doctor before starting any fasting protocol, regardless of what population-level data shows.
• Related reading: if you're worried about cardiovascular safety more broadly, see our summary on intermittent fasting and blood pressure and what the research shows about IF and inflammation.

Study Limitations

• Small sample size (approximately 30 participants) limits statistical power
• Men only — no data here on how fasting affects clotting markers in women
• Pre/post observational design, not a randomized controlled trial, so causation can't be firmly established
• Conducted in 1991 — laboratory methods and reference ranges have evolved since publication
• This summary is written from model training knowledge because PubMed and alternate academic databases returned 403 errors at the time of writing; readers should verify specific figures against the original paper where possible

Source

Sajid, K.M., Akhtar, M., & Malik, G.Q. (1991). Ramadan fasting and thrombosis. Journal of Pakistan Medical Association. Search on PubMed →

Frequently Asked Questions

Does intermittent fasting increase risk of blood clots?

Based on this early Ramadan fasting study, there's no evidence that a month of daily extended fasting raised clotting risk markers in healthy men. Modern intermittent fasting research on cardiovascular markers is generally reassuring too.

Does fasting thicken your blood?

No good evidence supports this common claim. This study found stable-to-slightly-lower fibrinogen and no meaningful change in clotting time during a month of fasting.

Is Ramadan fasting the same as intermittent fasting?

Ramadan fasting is a form of daily time-restricted eating, but it's stricter than typical intermittent fasting protocols since water is also restricted during daylight hours — most modern IF plans allow water freely throughout the fast.

Should people with a clotting disorder avoid intermittent fasting?

Anyone with a personal or family history of clotting disorders should discuss fasting with a doctor first — this population-level study doesn't account for individual medical risk factors.

Does staying hydrated matter for blood clotting during a fast?

Yes — dehydration itself can affect blood viscosity and clotting markers, which is one reason typical intermittent fasting protocols (unlike dawn-to-dusk Ramadan fasting) allow unrestricted water intake throughout the fast.

Related Research and Articles

• Can intermittent fasting lower blood pressure?
• Does intermittent fasting reduce inflammation?
• Intermittent fasting and inflammation: the research explained
• Electrolytes and intermittent fasting
• Is intermittent fasting dangerous?

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