Fasting lowers blood pressure, and the effect is strongest in people who start with elevated readings. Across clinical studies, intermittent fasting reduces systolic pressure by about 3 to 5 mmHg and diastolic pressure by 2 to 4 mmHg on average. For people with untreated hypertension, the drop can be far more dramatic, reaching 16 to 24 points systolic in extended fasts. The size of the effect depends on your starting blood pressure, the type of fast, and how long you maintain it.
How Much Blood Pressure Drops, and for Whom
A large study of 1,610 people tracked daily blood pressure during fasting periods lasting 4 to 41 days. The results split cleanly by starting blood pressure. People with untreated hypertension (above 140/90) saw their blood pressure fall by an average of 16.7/8.8 mmHg. Those with more severe hypertension (above 160/100) experienced even steeper drops, averaging 24.7/13.1 mmHg.
People with normal blood pressure saw only a modest decrease of about 3.0/1.9 mmHg. And those with truly healthy readings (between 100/60 and 120/80) saw essentially no change at all, going from an average of 110/72 to 109/71. This is reassuring: fasting doesn’t push already-normal blood pressure into dangerous territory.
There’s one notable exception. A subgroup of 69 women who started with low blood pressure (below 100/60) actually saw a small increase of about 6.3/2.2 mmHg during fasting. This suggests the body has a corrective mechanism that nudges blood pressure toward a healthy middle range rather than simply pushing it lower.
Why Fasting Lowers Blood Pressure
Several overlapping mechanisms explain the drop, and they kick in at different points during a fast.
Your Kidneys Flush Sodium
Within roughly 48 hours of beginning a fast, your kidneys start excreting significantly more sodium than usual, a phenomenon known as the natriuresis of fasting. This sodium loss is substantial. In studies, the average cumulative sodium deficit during the first week of fasting was around 203 milliequivalents, far more than what would happen from simply cutting salt out of your diet. The sodium leaves your body because, as you shift to burning fat and protein for fuel, your metabolism generates acidic byproducts that must be neutralized. Sodium gets pulled along to balance these acids in the urine. Less sodium in your body means less water retention, which means lower blood volume and, consequently, lower blood pressure.
Insulin Levels Fall
Insulin does more than regulate blood sugar. It directly tells your kidneys to hold onto sodium. In studies using insulin infusions, sodium excretion dropped by about 50% when insulin was elevated, even without any change in kidney blood flow or filtration rate. High insulin also ramps up the sympathetic nervous system (your “fight or flight” response), which constricts blood vessels, and promotes the growth of smooth muscle cells in artery walls, making them stiffer. When you fast, insulin drops. The kidneys release sodium, sympathetic activity calms, and blood vessels relax.
Blood Vessels Produce More Nitric Oxide
Fasting boosts your body’s production of nitric oxide, a molecule that relaxes and widens blood vessels. One pathway involves an enzyme called arginase, which competes with nitric oxide production for the same raw material (the amino acid L-arginine). During fasting, arginase activity drops by about 16%, freeing up more L-arginine to be converted into nitric oxide. The result is better blood flow and more flexible arteries. Research on Ramadan fasting has consistently shown improvements in endothelial function, the ability of blood vessel walls to expand and contract in response to blood flow.
When the Drop Begins
The timeline depends on your starting point. In the large cohort study, people with untreated hypertension showed a significant decrease in both systolic and diastolic pressure starting from the very first day of fasting. For people with normal blood pressure, the change took longer to appear, becoming statistically significant only after about 8 days of fasting. The sodium flush that drives much of the early drop begins around 48 hours into a fast and accelerates through the first week.
Time-Restricted Eating vs. Extended Fasting
You don’t need a multi-day water fast to see blood pressure benefits. Time-restricted eating, where you confine all your food to a set window each day, also works. In a 12-week trial, people with metabolic syndrome who ate within a 10-hour daily window experienced lower blood pressure alongside weight loss and improved cholesterol. This was true even though most participants were already taking blood pressure or cholesterol medications. The American Heart Association has cited evidence on meal timing to suggest that a longer overnight fast may reduce cardiovascular risk.
The tradeoffs are straightforward. Extended fasting produces larger, faster blood pressure drops but is harder to sustain and carries more risk. Time-restricted eating produces more modest reductions but fits into daily life and is easier to maintain for months or years. For most people interested in blood pressure management, a daily eating window of 8 to 10 hours is the more practical approach.
How Long the Effects Last
In the large fasting study, blood pressure remained lower during a four-day food reintroduction phase after the fast ended. Measurements on the last day of fasting and the fourth day of refeeding showed no difference, meaning the drop held even as calories were gradually reintroduced. For people who had stopped their blood pressure medications during the fast, the lower readings persisted through reintroduction as well.
The longer-term picture is less clear. Follow-up questionnaires at 2 weeks, 3 months, and 11 months had response rates too low to draw conclusions (as few as 12% responded at 11 months). It’s likely that without ongoing dietary changes, blood pressure gradually returns toward its previous level. The best evidence suggests that fasting works as a blood pressure intervention when it becomes a recurring pattern, whether that’s a regular time-restricted eating schedule or periodic longer fasts, rather than a one-time event.
Risks if You Take Blood Pressure Medication
Fasting while on blood pressure medication creates a real risk of your pressure dropping too low. Dehydration from not eating (and sometimes not drinking enough) reduces blood volume on its own. When that combines with medication designed to lower pressure further, the result can be dizziness, fainting, dangerously low readings, and electrolyte imbalances that affect heart rhythm and kidney function.
This risk is amplified if your medication is a diuretic, since diuretics cause additional fluid and electrolyte loss on top of what fasting already produces. Symptoms to watch for include lightheadedness when standing, persistent headaches, unusual fatigue, and heart palpitations. People on blood pressure medications who want to try any form of fasting need their dosing adjusted and monitored rather than simply skipping meals while continuing their usual prescriptions.
The Bottom Line on Blood Pressure and Fasting
Fasting lowers blood pressure through a combination of sodium flushing, reduced insulin signaling, and improved blood vessel function. The effect is proportional to how high your blood pressure is to begin with. If your readings are already normal, fasting won’t push them meaningfully lower. If they’re elevated, fasting can produce clinically significant reductions, with extended fasts showing the largest drops and time-restricted eating offering a more sustainable, moderate benefit.