Quick answers
Feeling dizzy while fasting: what to do
Often salt and water. Sometimes a sign to eat. Here's how to tell the difference
Why fasting can make you dizzy
Insulin does more than manage blood sugar: it tells your kidneys to hold on to sodium. In a classic experiment, raising insulin cut the sodium people passed in their urine almost by half 3. During a fast the opposite happens. Insulin falls, and your kidneys let go of more salt and the water that travels with it 1. With a little less fluid in your blood vessels, standing up quickly can leave you briefly lightheaded.
Too little water adds to it: even mild dehydration brought more fatigue and headaches in young women 4. In an 8-week trial of alternate-day fasting, 10% of people reported dizziness at the start and 17% at the end, a change too small to be sure it was real 5.
Is it low blood sugar?
In healthy people, rarely on a daily fast. Your liver keeps your blood sugar steady between meals, first from its glycogen and then by making new glucose 6. It's different if you take insulin or tablets that make your body release more insulin, such as sulfonylureas: then skipping meals can push your blood sugar too low 2,7.
| What you feel | What it may mean | What to do |
|---|---|---|
| Brief lightheadedness when you stand up, passing in seconds | Salt and fluid loss | Stand up slowly, drink water, try a pinch of salt |
| Shaky, sweaty, hungry, irritable, a fast heartbeat | Possible low blood sugar 2 | Stop and eat; check your glucose if you have diabetes |
| Confused, blurred vision, can't speak clearly | Possibly very low blood sugar 2 | Eat or drink something sugary now and get help |
| Fainting, chest pain, a racing heart that won't settle | Not a normal fasting symptom | Call your local emergency number |
What to do right now
If you feel lightheaded
- Sit or lie down until it passes. Don't drive.
- Drink a large glass of water.
- On fasts of 18 hours or more, try a pinch of salt in water, unless you've been told to limit salt 8.
- Stand up slowly, in two steps: sit first, then stand.
- If you don't feel better within 15 to 20 minutes, stop your fast and eat a proper meal.
Ending a fast early is fine. There's no prize for pushing through, and your next fast can be shorter. Our guide to water, salt and electrolytes explains how to keep your fluids up, and the water intake calculator gives you a daily target.
When to stop and eat
- You feel shaky, sweaty, weak or confused, or have palpitations.
- The dizziness doesn't pass when you sit down, or keeps coming back.
- You can't keep fluids down.
- You take medication that lowers blood sugar or blood pressure and feel unwell. Talk to your doctor before your next fast 7.
Sources
- Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941
- National Institute of Diabetes and Digestive and Kidney Diseases. Low Blood Glucose (Hypoglycemia). U.S. National Institutes of Health. www.niddk.nih.gov/health-information/diabetes/overview/preventing-problems/low-blood-glucose-hypoglycemia
- DeFronzo RA, Cooke CR, Andres R, et al. The effect of insulin on renal handling of sodium, potassium, calcium, and phosphate in man. J Clin Invest. 1975;55(4):845-55. doi.org/10.1172/JCI107996
- Armstrong LE, Ganio MS, Casa DJ, et al. Mild dehydration affects mood in healthy young women. J Nutr. 2012;142(2):382-8. doi.org/10.3945/jn.111.142000
- Hoddy KK, Kroeger CM, Trepanowski JF, et al. Safety of alternate day fasting and effect on disordered eating behaviors. Nutr J. 2015;14:44. doi.org/10.1186/s12937-015-0029-9
- Rothman DL, Magnusson I, Katz LD, et al. Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR. Science. 1991;254(5031):573-6. doi.org/10.1126/science.1948033
- Grajower MM, Horne BD. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients. 2019;11(4):873. doi.org/10.3390/nu11040873
- He FJ, Li J, Macgregor GA. Effect of longer term modest salt reduction on blood pressure: Cochrane systematic review and meta-analysis of randomised trials. BMJ. 2013;346:f1325. doi.org/10.1136/bmj.f1325
Keep reading
- Headache while fasting: why it happens and what helpsUsually water, caffeine or salt, and usually easy to fix. Here's how to tell, and when to stop
Water, salt and electrolytesWhy fasting changes how your body handles water and sodium, and when it's worth doing something about it- Do electrolytes break a fast?Sugar-free ones are fine. On daily fasts you rarely need them; on longer ones they can help
A note on health
Fasting Tracker is designed for general wellness and is not a medical device. Fasting is not right for everyone. If you are pregnant, diabetic, have a history of eating disorders or take medication, talk to your doctor first, and before any extended fast.