Not eating can temporarily raise your blood pressure, but over longer periods, it typically lowers it. The short-term spike happens because your body treats low blood sugar as an emergency and floods your system with stress hormones that constrict blood vessels and speed up your heart. The longer-term drop happens because fasting causes your kidneys to flush out sodium and water, reducing blood volume. Which effect dominates depends on how long you go without food, your overall health, and whether you’re also skipping fluids.
Why Skipping Meals Can Spike Blood Pressure
When your blood sugar drops low enough, your body activates its fight-or-flight response. The adrenal glands release adrenaline (epinephrine) and noradrenaline, along with cortisol. These hormones exist to mobilize stored energy, but they also act directly on your cardiovascular system. Adrenaline increases heart rate and raises systolic blood pressure (the top number) while causing a small dip in diastolic pressure (the bottom number). The net effect is a heart working harder and blood vessels under more tension.
Cortisol amplifies this. Extended fasting, even as short as overnight, raises cortisol levels. Five days of fasting dramatically elevates cortisol and shifts its natural peak from morning to afternoon, disrupting the hormone’s normal rhythm. Even a modest four-day time-restricted eating pattern slightly but significantly increases morning cortisol. Cortisol promotes what researchers call “cardiovascular hypertonicity,” meaning stiffer, more constricted blood vessels. For someone who skips meals regularly or goes long stretches without eating, these repeated cortisol surges can keep blood pressure elevated during the fasting window itself.
How Fasting Lowers Blood Pressure Over Time
Paradoxically, the same fasting that triggers short-term spikes tends to reduce blood pressure when practiced consistently. A meta-analysis of ten randomized controlled trials involving 694 patients found that time-restricted eating lowered systolic blood pressure by an average of about 4 mmHg. Diastolic pressure dropped too, though the effect was smaller and became clearly significant only in studies lasting at least 12 weeks (about a 2 mmHg reduction).
A 4 mmHg systolic drop may sound modest, but at a population level, reductions of that size meaningfully lower the risk of stroke and heart disease. The mechanism involves several pathways working together: weight loss, improved insulin sensitivity, and a fascinating kidney response called fasting natriuresis.
Fasting Natriuresis
Within hours of your last meal, your kidneys begin excreting more sodium and water than usual. This response is especially pronounced in people who are obese or already have high blood pressure. Scientists have studied it for decades, and it kicks in quickly, sometimes after just an overnight fast. The moment you eat carbohydrates again, the effect reverses almost immediately, with sodium and water retention snapping back within about 20 minutes of glucose intake. This cycle of sodium flushing during fasting and sodium retention during feeding, repeated over weeks, appears to gradually reduce blood pressure and improve insulin sensitivity.
What makes this process unusual is that it defies the body’s normal salt-retention reflexes. Aldosterone, the hormone that tells kidneys to hold onto sodium, actually rises during early fasting. Yet the kidneys keep dumping sodium anyway, suggesting the flushing happens in a part of the kidney that aldosterone doesn’t strongly control.
Dehydration Changes the Equation
If you’re not eating and also not drinking enough water, the picture shifts. Dehydration activates a separate arm of the stress response. As blood becomes more concentrated (higher osmolality), your brain ramps up sympathetic nerve activity to keep blood pressure from collapsing. Animal studies show that blocking adrenaline receptors during water deprivation significantly drops blood pressure, confirming that the sympathetic nervous system is doing the heavy lifting to maintain pressure when fluids are low.
Your body also releases more vasopressin (an antidiuretic hormone) and angiotensin II during dehydration, both of which constrict blood vessels and tell kidneys to retain whatever water remains. So while mild dehydration may keep your blood pressure propped up or even elevated, severe dehydration eventually overwhelms these compensatory mechanisms, and blood pressure can plummet dangerously.
When Blood Pressure Drops Too Low
For some people, not eating causes the opposite problem. Low blood pressure, generally defined as a reading below 90/60 mmHg, can develop during fasting due to reduced blood volume from sodium and water losses. A drop of just 20 mmHg in systolic pressure, say from 110 to 90, can cause noticeable symptoms.
Signs that fasting has pushed your blood pressure too low include dizziness or lightheadedness (especially when standing up), blurred or fading vision, fatigue, trouble concentrating, and nausea. These symptoms are most common in people who are already lean, take blood pressure medications, or fast for extended periods without adequate fluid intake. Nutritional deficiencies from prolonged undereating can also contribute: low levels of vitamin B-12, folate, and iron impair red blood cell production, leading to anemia, which further lowers blood pressure.
What This Means Practically
If you skip a meal or two, you’ll likely experience a temporary bump in blood pressure from the stress hormone response, followed by a gradual decline as your body sheds sodium and water. For most healthy people, these fluctuations are modest and self-correcting once you eat again. The people most likely to notice problems are those on blood pressure medications (where the combined blood-pressure-lowering effects of fasting and medication can cause dangerous drops) and those with diabetes or reactive hypoglycemia, where the adrenaline-driven spike can be more intense.
If you take blood pressure medication and need to fast for blood work, Harvard Health notes that you can generally continue your medications unless specifically told to take them only with food. Water, plain coffee, and black tea are fine during most diagnostic fasts. For longer intentional fasting, staying hydrated blunts the dehydration-driven pressure swings and helps your kidneys manage sodium balance more smoothly.