Electrolytes and Dehydration During Intermittent Fasting
Do you need electrolytes during intermittent fasting? Not for an ordinary fast, per official sources. Learn the dehydration signs to watch and when to end a fast.
- Written by
- Blimbi Editorial
- Reading time
- 4 min
In 30 seconds
Drink on purpose. Food normally supplies some of your fluid, so a long fasting window can lower your intake without you noticing, and a 12-month trial encouraged plenty of water. Water and sugar-free tea or coffee count, pale-yellow urine is a practical check, and dizziness that doesn’t settle means it’s time to end the fast.
- Some of your daily fluid normally comes from food, so a fasting window can lower your intake.
- Water, lower-fat milk in the eating window, and sugar-free drinks such as tea and coffee all count.
- Drink more in the heat, during long exercise, when you are ill, or when you are pregnant or breastfeeding.
- End the fast if you are vomiting, have diarrhoea, or feel dizzy, confused or unusually tired.
Does fasting change how much you drink?
It can, and you might not notice. Food carries some of your water, so a fasting window that removes a meal can also remove some of your usual fluid, and a busy morning makes it easy to forget to drink. A 12-month fasting trial didn’t limit water. It encouraged plenty of it.
NHS guidance notes that water is a healthy way to stay hydrated, that other drinks also count, and that we get some fluids from the foods we eat.Source 1
In a 12-month time-restricted eating trial, participants were encouraged to drink plenty of water during the 16-hour fasting window.Source 3
Which drinks fit the fasting window is covered in water and lemon water during intermittent fasting.
How much should you drink?
There isn’t a fasting-specific number in these sources. Use everyday NHS guidance, and drink more in the heat, during long activity, when you are ill, or if you are pregnant or breastfeeding.
NHS guidance suggests aiming for pale-yellow urine and about 6 to 8 cups or glasses of fluid a day, with more when pregnant or breastfeeding, in a hot environment, physically active for long periods, or ill.Source 1
According to the NHS, dehydration happens more easily with diabetes, vomiting or diarrhoea, heat, heavy sweating after exercise, a high temperature, drinking too much alcohol, or medicines that make you pee more, and older adults are more at risk.Source 2
| Situation | What NHS guidance suggests |
|---|---|
| Everyday | About 6 to 8 cups or glasses of fluid a day, aiming for pale-yellow urine |
| Pregnancy or breastfeeding | More fluid than usual |
| Hot environment or long physical activity | More fluid than usual |
| Illness | More fluid than usual |
| Vomiting or diarrhoea | Replace lost sugar, salts and minerals; a pharmacist can recommend oral rehydration solutions |
What signs should you watch?
| What you notice | What to do |
|---|---|
| Thirst, dry mouth or a mild headache | Drink water now and keep sipping |
| Dark yellow, strong-smelling urine or peeing less | Drink more; end the fast if it does not improve |
| Feeling dizzy, light-headed or unusually tired | End the fast, drink and eat |
| Dizziness on standing that does not go away, fast breathing or heartbeat, confusion | Get urgent medical help |
Do you need electrolyte drinks?
These sources don’t support them for an ordinary daily fast. Electrolyte drinks are heavily marketed to people who fast, but official guidance mentions them for vomiting or diarrhoea. If that is happening, the fast should end anyway.
NHS guidance says that people losing too much fluid through vomiting or diarrhoea need to replace lost sugar, salts and minerals, and that a pharmacist can recommend oral rehydration solutions.Source 2
Mixed evidence
There is no fasting hydration formula
Hydration advice here comes from NHS guidance for everyday life and from dehydration symptom guidance. The fasting trial encouraged water but did not test specific fluid amounts or electrolyte products, so there is no evidence-based fasting hydration formula.Source 1Source 2Source 3
When should you get urgent help?
Talk to a qualified health professional before fasting if you are pregnant or breastfeeding, under 18, living with or recovering from an eating disorder, have diabetes, take insulin or a medicine affected by meal timing, are an older adult, or are acutely ill. Do not change the dose or timing of any medicine without your clinician. See Blimbi’s safety guide.
Frequently asked questions
How should I handle hydration while fasting?
Drink water through the fasting window. A 12-month trial encouraged plenty of water, and these sources do not support routine electrolyte drinks for an ordinary daily fast. Oral rehydration solutions are for replacing losses from vomiting or diarrhoea, and a pharmacist can advise.Source 2Source 3
Does coffee or tea count toward hydration while fasting?
Yes. NHS guidance counts sugar-free drinks, including tea and coffee, toward daily fluid. Keep caffeine moderate, and if you are already dehydrated, the NHS advises avoiding caffeinated drinks and alcohol while you rehydrate.Source 1
Is a headache during a fast a sign of dehydration?
It can be. Headache and light-headedness are NHS symptoms of dehydration, so drink water first. If it persists, or it comes with dizziness or confusion, end the fast and seek advice.Source 2
Sources
Water, drinks and hydrationNHS. NHS, May 17, 2023.Official guidance
DehydrationNHS. NHS, May 1, 2026.Official guidance
Time-Restricted Eating Without Calorie Counting for Weight Loss in a Racially Diverse Population: A Randomized Controlled TrialShuhao Lin et al. American College of Physicians, July 1, 2023.DOI: 10.7326/M23-0052 · PMID: 37364268Human randomized trial
Fasting Safely with DiabetesMartin M. Grajower. National Institute of Diabetes and Digestive and Kidney Diseases, August 26, 2020.Official guidance