The Science Behind Fasting and Blood Viscosity
To understand how fasting affects blood, one must first understand blood viscosity. Blood viscosity is a measure of the thickness and stickiness of blood, which determines how easily it flows through vessels. High viscosity means thicker blood, which requires more force from the heart to pump, while low viscosity indicates thinner blood. A change in blood volume or the concentration of blood components can alter viscosity.
When we fast, especially for prolonged periods and without water, our body's hydration levels drop. This leads to a process called hemoconcentration. As the volume of plasma (the liquid component of blood) decreases due to fluid loss, the concentration of blood cells (like red blood cells) and proteins in the remaining plasma increases. This denser, more concentrated blood becomes more viscous. A study published in the New England Journal of Medicine confirmed that a 24-hour fast led to a significant increase in blood viscosity due to this hemoconcentration.
Short-term vs. Prolonged Fasting Effects
The duration of a fast plays a crucial role in its effect on blood viscosity. The impact is not the same for a short overnight fast compared to a multi-day fast or even time-restricted intermittent fasting.
The Crucial Role of Hydration
Dehydration is the primary driver for increased blood thickness during a fast. During a prolonged fast, the body continues to lose water through breathing, sweating, and urination, especially in hot weather or during physical activity. Without fluid replenishment, the reduced plasma volume concentrates the blood components. This is why staying well-hydrated during non-fasting periods is absolutely critical for anyone engaging in prolonged or even intermittent fasting. Consuming ample water, hydrating foods, and avoiding diuretics like caffeine can help mitigate this effect.
Health Implications of Increased Blood Viscosity
For healthy individuals, a temporary increase in blood viscosity during a short fast may not cause significant problems. However, for certain populations, this can be dangerous. Increased blood thickness puts added strain on the heart, making it work harder to circulate blood. This can be especially risky for elderly individuals or those with underlying cardiovascular conditions like heart disease or vascular insufficiency, potentially impairing blood flow to vital organs. It can also contribute to lower blood pressure, causing dizziness or fainting, particularly during water-only fasts.
Fasting and Viscosity: A Comparative Look
| Feature | Short-Term Fasting (e.g., 12 hours) | Prolonged Fasting (e.g., >24 hours) | Intermittent Fasting (e.g., 16/8) |
|---|---|---|---|
| Effect on Blood Viscosity | Little to no significant adverse effect on viscosity. | Significantly increases blood viscosity due to hemoconcentration and dehydration. | Negligible effect on viscosity if properly hydrated during eating windows. |
| Primary Driver | Body uses stored glucose and maintains hydration. | Dehydration and fluid restriction concentrate blood components. | Depends on hydration and diet quality during the eating window. |
| Associated Risks | Minimal risk for healthy adults. | Potential for impaired organ perfusion and increased cardiovascular strain, especially for vulnerable groups. | Risks are minimal for healthy people but vary based on duration and health status. |
| Safety Considerations | Generally safe for healthy adults. | Medical supervision and IV fluids may be necessary for safety. | Emphasis on mindful hydration and diet quality. |
Intermittent Fasting and Coagulation
The relationship between fasting and blood clotting is complex. While dehydration from prolonged fasting can increase thickness and general risk for some, studies on intermittent fasting (IF) show more nuanced results regarding specific coagulation factors. One 2024 study suggests IF may inhibit platelet activation, a step in blood clot formation, via a gut-produced metabolite, though it notes limitations and the need for more research. Another study found that Ramadan intermittent fasting was associated with improved anticoagulant activity in healthy participants. However, these potential benefits do not negate the viscosity risks associated with dehydration, nor should they be a reason for at-risk individuals to ignore medical advice.
Who Should Be Cautious About Fasting?
While fasting can be a beneficial health practice for many, it is not suitable for everyone. Certain individuals should exercise extreme caution or avoid fasting altogether, and always consult a healthcare provider before beginning any fasting regimen. These groups include:
- Individuals on Blood Thinners: Taking anticoagulants requires careful medication timing. Fasting for prolonged periods can complicate this schedule, as one patient found after experiencing heart issues from a dangerous timing mistake.
- People with Diabetes: Especially those requiring insulin, as fasting can dangerously alter blood sugar levels.
- Individuals with Heart Conditions: Pre-existing heart disease or heart surgery increases the risk of complications from increased blood viscosity and potential electrolyte imbalances. A recent study even suggested a long-term risk of cardiovascular death with time-restricted eating.
- Pregnant or Breastfeeding Women: Fasting can be detrimental to the health of both the mother and child.
- People with a History of Eating Disorders: Fasting can trigger a relapse.
- The Elderly: Older adults may be more susceptible to dehydration and its effects.
Conclusion: The Final Word on Fasting and Blood Thinning
The notion that fasting thins the blood is a misconception. Scientifically, the opposite is true for prolonged fasts, where dehydration concentrates blood components and increases viscosity. While some types of fasting, particularly intermittent forms, may offer cardiovascular benefits like lower blood pressure and improved cholesterol, these are largely dependent on proper hydration and balanced nutrition during eating periods. The risks of dehydration and increased blood thickness, particularly for vulnerable individuals, cannot be ignored. For anyone considering fasting, the most prudent course of action is to stay well-hydrated, listen to your body, and, most importantly, consult a medical professional, especially if you have pre-existing health conditions or take medications.
Medical News Today: Intermittent fasting may help prevent blood clots
