What Happens to High Blood Pressure When Fasting?

Blood pressure drops during fasting, and the effect is remarkably consistent across different types of fasts, from a single skipped meal to medically supervised water-only protocols lasting more than a week. A study tracking over 1,600 people during prolonged fasting found average drops of about 7 points systolic and 4 points diastolic, with much larger reductions in people who started with high blood pressure. The mechanisms behind this span several body systems, from shifts in nervous system activity to changes in how blood vessels dilate, and the size of the effect depends on the type of fast, how long it lasts, and how elevated your pressure was to begin with.

How Large Is the Drop?

The size of the blood pressure reduction during fasting scales with how high your pressure is at the start. In a study of 1,610 people undergoing long-term fasting (averaging about a week), those with normal blood pressure saw modest reductions of around 3/2 mmHg. People with untreated hypertension above 140/90 saw drops of roughly 17/9 mmHg. And for those starting above 160/100, the decrease averaged about 25/13 mmHg.¹ That pattern makes intuitive sense: the higher the starting pressure, the more room there is for the body’s regulatory systems to pull it down once the usual dietary triggers disappear.

Even very short fasts produce measurable changes. In one study of borderline hypertensive subjects, just a few days of caloric restriction brought systolic pressure down from about 158 to 146 and diastolic from 96 to 89.² A separate study using 24-hour ambulatory monitoring found that a single day of fasting reduced mean arterial pressure by about 3 mmHg and heart rate by roughly 4 beats per minute compared to a normal eating day.³ These are not dramatic numbers on their own, but they show that the cardiovascular system responds to fasting almost immediately.

At the extreme end, medically supervised water-only fasts produce striking results. In one clinical series, the average blood pressure reduction was 37/13 mmHg, and people with the most severe hypertension (systolic above 180 or diastolic above 110) saw average drops of 60/17 mmHg. By the end of the treatment program, nearly 90% of participants had blood pressure below 140/90.⁴ These numbers are comparable to what you’d expect from prescription antihypertensive medications, though the fasting protocols involved were intensive and required medical oversight.

Your Nervous System Shifts Gears

One of the fastest-acting mechanisms is a change in the balance between the two branches of your autonomic nervous system. During fasting, the sympathetic branch (the “fight or flight” system that, among other things, constricts blood vessels and speeds up the heart) dials down, while the parasympathetic branch (which slows the heart and promotes relaxation) ramps up. A study measuring heart rate variability found that fasting significantly increased a key marker of parasympathetic activity and decreased a sympathetic index, confirming that the nervous system shifts toward a calmer baseline.⁵

Another study confirmed this with ambulatory data, showing that fasting produced a medium-to-large effect on vagal tone (the parasympathetic input to the heart), alongside a significant reduction in resting heart rate.⁶ A slower heart rate and wider, more relaxed blood vessels add up to lower pressure. Interestingly, early research in hypertensive rats suggested that the drop during fasting involves not just sympathetic withdrawal but also the release of endogenous opiates, the body’s own painkillers, which may independently relax blood vessels.⁷ That opiate-mediated effect was seen only in hypertensive animals, which may partly explain why people with higher starting blood pressures benefit the most.

Blood Vessels Open Up

Beyond the nervous system, fasting appears to directly improve how blood vessels function. Healthy blood vessels produce nitric oxide, a molecule that signals the smooth muscle in vessel walls to relax and widen. When nitric oxide availability drops, vessels stiffen and pressure rises. Fasting seems to reverse this to some degree.

Research on Ramadan fasting has found that the month-long practice of eating only between sunset and dawn elevates nitric oxide levels and improves markers of endothelial function.⁸ In a study of hypertensive patients specifically, a test that measures how well an artery dilates in response to increased blood flow showed significant improvement after Ramadan fasting.⁹ And in healthy middle-aged people following an intermittent fasting protocol, researchers used skin blood flow measurements to show that nitric oxide-related vasodilation roughly doubled during the fasting period compared to before it began.¹⁰

This vascular relaxation effect could matter a great deal for long-term cardiovascular health. Stiff, poorly functioning arteries are not just a consequence of high blood pressure; they actively make it worse by forcing the heart to pump harder. If fasting can restore some of that flexibility, the benefits go beyond a temporary dip in the numbers on a blood pressure cuff.

Hormonal Changes That Lower Pressure

Your body has a built-in system for regulating blood pressure and fluid balance called the renin-angiotensin-aldosterone system, or RAAS. When this system is overactive, it causes sodium retention, blood vessel constriction, and higher blood pressure. Fasting appears to recalibrate it in ways that favor lower pressure.

In aging rats with established hypertension, alternate-day fasting reduced blood pressure and shifted the balance of key RAAS receptors toward a pattern associated with vessel relaxation rather than constriction. The fasting also boosted a protective enzyme that counteracts the system’s pressure-raising effects.¹¹ In humans, a weight loss study showed that losing just 5% of body weight through caloric restriction reduced circulating renin by about 43% and aldosterone by about 31%, accompanied by a 7 mmHg drop in systolic blood pressure.¹² This matters because fat tissue itself produces angiotensinogen, one of the starter molecules in the RAAS cascade, so losing fat can directly quiet the system.

The sodium side of the equation plays a role too. When you fast, you stop taking in sodium entirely (or nearly so), and the kidneys continue excreting it. Research has shown a clear positive relationship between sodium excretion and blood pressure, with hypertensive individuals excreting more sodium on average than those with normal pressure.¹³ During a fast, the temporary drop in sodium intake and the continued excretion of stored sodium help reduce blood volume, which lowers the pressure the heart needs to generate.

Different Fasting Styles Produce Different Results

Not all fasting is the same, and the blood pressure effects vary with the approach.

Time-restricted eating, where you confine your meals to a window of roughly 6 to 10 hours each day, is the most popular and mildest form of intermittent fasting. A recent meta-analysis of randomized controlled trials found that time-restricted eating without deliberate calorie reduction lowered systolic blood pressure by about 1.8 mmHg and diastolic by about 1.75 mmHg on average. The effect was more pronounced in people who started with elevated blood pressure.¹⁴ A smaller study using a 10-hour eating window in people with metabolic syndrome found larger drops of about 5 mmHg systolic and 6 mmHg diastolic, and these reductions occurred even though most participants were already on blood pressure medication.¹⁵

Timing matters too. Reviews of time-restricted eating studies have found that earlier eating windows (finishing dinner in the afternoon rather than the evening) tend to produce more consistent blood pressure reductions than late eating windows, and narrower windows generally outperform wider ones.¹⁶ This likely ties into circadian biology: blood pressure naturally dips at night, and eating late may blunt that dip.

Alternate-day fasting, a more aggressive protocol where you eat very little or nothing every other day, has outperformed simple calorie restriction for systolic blood pressure in at least one head-to-head randomized trial involving people with metabolic syndrome.¹⁷ That said, the diastolic pressure difference between the two approaches was not significant, suggesting the advantage may be specific to certain components of blood pressure.

Do the Reductions Last After the Fast Ends?

The most important question for anyone considering fasting as a blood pressure strategy is whether the benefits persist once normal eating resumes. The evidence here is cautiously encouraging.

In a pilot study of medically supervised water-only fasting, systolic blood pressure dropped from an average of 135 mmHg at baseline to 118 at the end of the fast and remained at about 116 through a structured refeeding period.¹⁸ A larger study with longer follow-up tracked hypertensive patients after prolonged water-only fasting and found that the reductions in blood pressure were sustained at both a 6-week follow-up and a 12-month follow-up. At one year, systolic pressure was still estimated to be about 17 mmHg lower than baseline and diastolic about 9 mmHg lower.¹⁹

An important caveat: these longer-term studies often involved a transition to a whole-food, plant-based diet after the fast, not a return to the patient’s original eating pattern. A study that followed participants through fasting and then onto a whole-plant-food diet found that blood pressure reductions at six weeks and twelve months were clinically meaningful, with systolic pressure down roughly 9 to 10 mmHg from baseline and diastolic down about 5 to 9 mmHg.²⁰ Separating the contribution of the fast itself from the dietary change afterward is difficult, and it would be a stretch to assume you could fast for a week, return to a high-sodium processed diet, and keep the benefit.

What Ramadan Fasting Tells Us

Ramadan fasting offers a useful natural experiment because millions of people follow the same basic protocol each year: no food or water from dawn to sunset for about 30 days. A study conducted in London combined its own cohort with a meta-analysis of 33 studies covering over 3,200 participants. In the London cohort alone, systolic blood pressure fell by about 7 mmHg and diastolic by about 3 mmHg after Ramadan. Across all 33 studies, the pooled reductions were roughly 3 mmHg systolic and 2 mmHg diastolic.²¹

What makes this data particularly interesting is that the blood pressure reductions were independent of changes in weight, body water, and fat mass. That challenges the common assumption that fasting lowers blood pressure only because it causes weight loss. Something else is going on, whether it is the nervous system shift, the vascular relaxation, the RAAS recalibration, or some combination. Subgroup analyses found the benefit in both healthy people and those with hypertension or diabetes, though it was absent in patients with chronic kidney disease, which makes physiological sense given the kidneys’ central role in blood pressure regulation.

Ketone Bodies May Play a Direct Role

When you fast long enough for your body to start burning fat for fuel, your liver produces ketone bodies, the most prominent being beta-hydroxybutyrate. These molecules are best known as an alternative energy source for the brain, but recent research suggests they may also have direct blood-pressure-lowering effects through a mechanism that has nothing to do with energy supply.

Work in animal models has shown that beta-hydroxybutyrate triggers a specific chemical modification on proteins that control gene activity, effectively switching on genes involved in fat breakdown and metabolic regulation. Animals given a compound that raises beta-hydroxybutyrate levels showed significantly lower blood pressure, body weight, and blood glucose, with no liver toxicity.²² This is still early-stage research, mostly in animals, but it offers a plausible explanation for why even brief fasts that do not produce weight loss can still reduce blood pressure. Your ketone levels start rising within 12 to 24 hours of your last meal, which lines up with the timeline of the blood pressure changes observed in acute fasting studies.

The Gut Microbiome Connection

An emerging line of research links fasting-induced blood pressure changes to the trillions of bacteria in your gut. A study in people with metabolic syndrome found that a period of fasting altered the composition of the gut microbiome, shifting bacterial populations in ways that affected the production of short-chain fatty acids.²³ Short-chain fatty acids are small molecules produced by gut bacteria when they ferment dietary fiber, and they have been linked to blood pressure regulation through their effects on kidney function and blood vessel tone.

This is one of the less settled areas in the research. The microbiome changes during fasting are real and measurable, but the chain from “different bacterial mix” to “lower blood pressure” involves several steps that are still being mapped out. It is possible that the gut changes are more of a marker than a driver, reflecting dietary shifts rather than causing the blood pressure benefit directly. Still, it adds to the picture of fasting as an intervention that touches multiple physiological systems simultaneously, which is part of why the blood pressure effect is so consistent across very different fasting protocols.

Practical Risks and Who Should Be Cautious

The blood pressure drops during fasting are generally welcome if your pressure is elevated, but they can become dangerous if you are already on antihypertensive medication. Stacking a fast on top of drugs designed to lower pressure can cause hypotension, dizziness, or fainting, particularly when standing up quickly. If you take blood pressure medication, any fast longer than an overnight skip should involve your doctor, who may need to adjust your dose before, during, or after the fast.

People with chronic kidney disease represent another group where caution is warranted. As the Ramadan meta-analysis showed, the blood pressure benefit was absent in kidney disease patients, and fasting can stress the kidneys through dehydration and rapid electrolyte shifts. Similarly, people with diabetes on insulin or sulfonylureas face a real risk of hypoglycemia during fasting, and the cardiovascular stress of low blood sugar can paradoxically raise blood pressure through an adrenaline surge.

Electrolyte depletion is another concern, especially during extended fasts. Sodium, potassium, and magnesium all drop as the kidneys continue excreting them without dietary replacement. Low potassium and magnesium can cause cardiac arrhythmias, which pose a more acute risk than the blood pressure changes themselves. Medically supervised fasting programs monitor blood work for exactly this reason, and attempting a multiday water-only fast without medical oversight is genuinely risky for anyone with cardiovascular disease.

For most people with mildly elevated blood pressure and no complicating conditions, a daily time-restricted eating pattern with an eating window of 8 to 10 hours is the approach with the most favorable risk-to-benefit ratio. The blood pressure reductions are modest but consistent, the safety concerns are minimal, and the protocol is sustainable enough to maintain long-term, which is where any lifestyle-based blood pressure strategy either succeeds or fails.