Why does intermittent fasting without carbs or fats cause hypotension?

Intermittent Fasting and Hypotension: Mechanisms and Implications

Intermittent fasting without carbs or fats causes hypotension primarily through decreased calcium intake and alterations in the renin-angiotensin system and autonomic nervous system function. 1, 2

Physiological Mechanisms of Blood Pressure Reduction

Intermittent fasting affects blood pressure through several pathways:

Renin-Angiotensin System Changes

• Recent evidence shows that intermittent fasting lowers angiotensin II levels and angiotensin-converting enzyme (ACE) activity 2
• Decreased angiotensin II is a predictive factor for blood pressure improvement during fasting 2

Autonomic Nervous System Effects

• Fasting increases high-frequency power and RMSSD (root mean square of successive differences) in heart rate variability measurements 2
• These changes indicate improved parasympathetic tone, contributing to blood pressure reduction 2
• However, earlier research found that short-term fasting (48 hours) actually increased muscle sympathetic nerve activity despite blood pressure reduction 3

Dietary Composition Changes

• During fasting periods, especially when carbs and fats are restricted:
  • Calcium intake significantly decreases 1
  • Low calcium intake is associated with impaired blood pressure control 1
  • Meta-analyses show that calcium supplementation (400-2000 mg/day) can reduce systolic BP by 0.9-1.4 mmHg 1

Sodium and Volume Effects

• Fasting reduces 24-hour urinary sodium excretion 4
• Weight loss from caloric restriction (1-8% from baseline) contributes to BP reduction 5

Blood Pressure Variability During Fasting

Intermittent fasting affects not only absolute blood pressure values but also blood pressure variability:

• Fasting increases blood pressure variability as measured by the smoothness index from ambulatory blood pressure monitoring 4
• Blood pressure variability is particularly high in individuals who wake up to eat before sunrise during religious fasting 4
• Higher BMI is associated with lower blood pressure variability during fasting 4

Clinical Implications

The hypotensive effect of intermittent fasting has several important clinical considerations:

• Office and ambulatory blood pressure measurements show significant decreases during intermittent fasting 4
• Home and central blood pressure measurements may not show significant changes 4
• Fasting periods of 15-16 hours for 30 days have demonstrated improvements in blood pressure in hypertensive patients 2
• Individuals with borderline hypertension can experience significant reductions in both systolic (12 mmHg) and diastolic (7 mmHg) blood pressure after just 48 hours of fasting 3

Potential Pitfalls and Considerations

When interpreting blood pressure changes during intermittent fasting:

• Blood pressure variability increases during fasting, which may mask the true hypotensive effect 4
• The timing of meals during intermittent fasting affects blood pressure variability 4
• The acute hypotensive response to fasting appears to involve different mechanisms than long-term blood pressure adaptation 3
• Individuals on antihypertensive medications may need dose adjustments during fasting periods to prevent symptomatic hypotension

In conclusion, the drop in blood pressure to 82 after a day of intermittent fasting without carbs or fats likely results from multiple mechanisms including reduced calcium intake, alterations in the renin-angiotensin system, changes in autonomic nervous system function, and reduced sodium excretion. These effects are well-documented across multiple studies examining various fasting protocols.