Quick answers
Do electrolytes break a fast?
Sugar-free ones are fine. On daily fasts you rarely need them; on longer ones they can help
Why fasting changes your salt balance
Insulin 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 2. During a fast the opposite happens: insulin falls, and your kidneys let go of more sodium and the water that travels with it 1. It's part of why the scale can drop quickly at the start of a long fast, and rise just as quickly when you eat: much of that is water.
When they help
| Situation | What to do |
|---|---|
| Daily fasts of 12 to 16 hours | Water when you're thirsty; your meals replace the rest |
| Fasts of 18 to 24 hours | Keep water nearby; if you get a headache or feel lightheaded when you stand, try a pinch of salt in water |
| Heat, heavy sweating or long workouts | Sugar-free electrolytes can help |
| Fasts longer than a day | Plan them with a doctor, including salt and fluids |
Before you reach for a sachet, start with water. A headache from too little water usually eased within 30 minutes of drinking about half a litre 3, and losing just over 1% of body weight in water was enough to bring headaches and low mood 4. See our answers on headaches and feeling dizzy while fasting.
Reading the label
- Sodium, potassium, magnesium and chloride: no calories, fine.
- Sugar, glucose, dextrose or maltodextrin: these are sugars, so the drink breaks your fast.
- Sucralose, stevia or other sweeteners: a grey zone, see sweeteners.
- Added amino acids, BCAAs or collagen: these are protein, so they break your fast.
The simplest electrolyte is a pinch of salt in a glass of water. In your eating window, vegetables, fruit, legumes, yoghurt and nuts bring plenty of potassium and magnesium.
- MYTH
- Electrolytes give you energy while you fast.
- FACT
- They carry no calories. What they can do is ease the headache or lightheadedness that comes from losing salt and water, which is easy to mistake for running out of energy.
Stop your fast and eat if you feel faint, shaky or confused, or your heart races or flutters, and get medical help if it doesn't settle quickly.
Fasting Tracker lets you log water towards a daily goal, 2,000 ml or 64 oz unless you change it, from your iPhone, a widget or your Apple Watch, so it's easy to see if you've had enough before blaming salt.
Sources
- Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941
- 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
- Blau JN, Kell CA, Sperling JM. Water-deprivation headache: a new headache with two variants. Headache. 2004;44(1):79-83. doi.org/10.1111/j.1526-4610.2004.04014.x
- 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
- 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
Water, salt and electrolytesWhy fasting changes how your body handles water and sodium, and when it's worth doing something about it- 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
- Feeling dizzy while fasting: what to doOften salt and water. Sometimes a sign to eat. Here's how to tell the difference
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.