Intermittent Fasting and Blood Pressure

What Happens to Blood Pressure

What intermittent fasting trials generally suggest, with the caveat that most are small and last weeks to months:

• Many studies report small-to-modest reductions in systolic and diastolic blood pressure, though some find little or no change
• Reductions tend to be larger in people whose blood pressure was elevated at the start and in those who lose more weight
• When fasting is compared with ordinary daily calorie restriction producing similar weight loss, blood pressure results are usually similar
• Any improvement depends on keeping up the habits (and the weight loss) behind it; it is not a permanent reset

Responses vary widely. For someone with high blood pressure, even a modest reduction can be worthwhile. For someone whose blood pressure is already low-normal, a further drop can cause lightheadedness, particularly when standing up.

Why Blood Pressure May Change

Reduced insulin

Insulin encourages the kidneys to retain sodium. When insulin is lower during fasting, the kidneys excrete more sodium and water, which can slightly reduce fluid volume. This is one reason people often feel lightheaded early on.

Weight loss

Weight loss is one of the best-established ways to lower blood pressure, whatever diet produces it. Research on weight loss generally finds that blood pressure falls roughly in step with weight lost, although individual responses differ. For many people, this is likely the main reason blood pressure changes with fasting.

Improved insulin sensitivity

Improved insulin sensitivity may support the function of the blood vessel lining (the endothelium), which helps vessels relax. How much this contributes independently of weight loss is not clear.

Reduced sympathetic tone

Some studies, mostly in animals, suggest fasting may shift the balance of the autonomic nervous system toward lower resting “fight or flight” activity. Evidence in humans is limited.

Reduced overall sodium intake

People who move from snacking on processed foods (often high in salt) to fewer, whole-food meals may eat less sodium overall. Lower sodium intake can help lower blood pressure, particularly in people who are salt-sensitive.

Typical Timeline

There is no fixed timeline, and some people see no meaningful change. As a rough guide to what people may notice:

First 1–2 weeks

Fluid and sodium shifts happen early. Some people notice lightheadedness on standing in the first days, especially during longer fasts. This is the period when people on blood pressure medication should monitor most closely.

Weeks 2–12

Any change in blood pressure tends to follow weight loss and lifestyle changes, such as eating less processed food or exercising more. Weekly averages of home readings show the trend better than single readings.

Longer term

Readings usually settle once weight stabilises. If fasting stops and weight or old eating habits return, blood pressure tends to drift back toward where it was.

The Medication Adjustment Problem

This is the most important section of this guide. If you take blood pressure medication and your blood pressure falls with fasting or weight loss, the dose that was right before may become too strong. The result can be low blood pressure (hypotension): dizziness, lightheadedness on standing, and sometimes fainting. Fasting can also change fluid and electrolyte balance, which interacts with some drug classes.

Points to discuss with your prescriber, by drug class

• ACE inhibitors and ARBs (for example lisinopril, ramipril, losartan, valsartan): Ask whether your dose may need reviewing as your readings change. These drugs can raise potassium, so check before using potassium-containing electrolyte supplements.
• Diuretics (for example hydrochlorothiazide, furosemide, indapamide): Fasting increases sodium and fluid loss, and diuretics add to it, raising the risk of dehydration, low blood pressure, and electrolyte problems. This class deserves particular attention.
• Beta blockers (for example metoprolol, bisoprolol, atenolol): Can blunt the heart-rate response to standing, which may make dizziness more likely. Some are meant to be taken with food, so ask about timing.
• Calcium channel blockers (for example amlodipine, diltiazem): Can still lower blood pressure too far if your readings fall, so monitoring still applies.
• Combination pills: Contain more than one drug, so any adjustment needs to be worked out with your prescriber.

For a wider overview, see fasting and medications.

Practical approach

• Tell your doctor before starting fasting if you’re on antihypertensive medication
• Use a validated home blood pressure monitor and measure regularly (for example morning and evening) during the first weeks, or as your doctor advises
• Agree in advance with your doctor on the readings or symptoms that should prompt you to call, and contact them promptly if you feel dizzy or lightheaded
• Do not change or stop your medication yourself; bring your readings to your appointments so any change is made by your prescriber
• Start with a gentle schedule (such as a 12-hour overnight fast) rather than long fasts

What and How to Monitor

The basics

• Use a validated automatic upper-arm monitor; wrist monitors are generally less reliable
• Measure at the same times each day for consistency
• Sit quietly for 5 minutes before measuring, back supported, feet flat, arm resting at heart level
• Take 2 readings 1 minute apart; average them
• Record date, time, both numbers, and any symptoms

What to record (simple log)

• Date and time
• Systolic / diastolic / pulse
• Position (seated)
• Notes: any dizziness, fatigue, headache, recent fast length, medication taken

What patterns matter

• Trend over weeks: Single readings vary a lot; weekly averages are more informative
• Drop on standing: If you feel faint on standing, or your standing reading is much lower than your seated one, tell your doctor; this can indicate orthostatic hypotension
• Unusually low readings: Readings much lower than your usual, especially with symptoms, should be discussed with your prescriber
• Pulse: A resting pulse that is unusually slow or fast for you (for example above 100 bpm at rest) deserves attention

Sodium During Fasting

General public-health advice is to eat less salt, and that remains the right default for most people with high blood pressure. Fasting adds a complication because it increases sodium loss.

For people with normal blood pressure

During fasts, the kidneys excrete more sodium because insulin is lower. Some people with normal blood pressure find that a little salt (for example a pinch in water or salted broth) during longer fasts helps with headaches, fatigue, and lightheadedness. There is no need to overdo it; this is mainly relevant to fasts of a day or more.

For people with hypertension

Do not add salt during fasts without talking to your doctor first. Some people with high blood pressure are salt-sensitive, and salt restriction is often part of their treatment. If lightheadedness is a problem, ask your doctor how to handle fluids and electrolytes, and keep monitoring your readings.

Dehydration vs sodium

Some fasting symptoms blamed on dehydration may be related to sodium loss instead. Drinking very large amounts of plain water does not fix this and, in extreme cases, can dilute blood sodium. Drink to thirst, and keep an eye on both fluids and electrolytes.

For full electrolyte detail, see our electrolyte guide.

When to Stop or Slow Down

Some signs suggest your fasting schedule is too demanding or that your medication needs reviewing by your doctor:

• Repeated lightheadedness on standing: Especially if it continues beyond the first week or two
• Readings much lower than usual (for example below about 90/60): Particularly with symptoms
• A racing heart on standing: Can be a sign of dehydration or low blood pressure
• Confusion, blurred vision, fainting, or near-fainting: Stop fasting, sit or lie down, have something to eat and drink, and seek medical advice; call emergency services if you faint, have chest pain, or feel very unwell
• Readings going up rather than down: Fasting does not help everyone. Rising readings warrant a review with your doctor rather than a stricter fast

The firm rule: never stop or reduce blood pressure medication on your own based on home readings. Bring the readings to your prescriber and let them decide on any change.

Frequently Asked Questions