Water, salt and electrolytes

Some of the most common complaints on longer fasts, such as a dull headache, feeling flat or a little dizzy when you stand up, are often blamed on hunger. Many of them have more to do with water and salt. Fasting changes how your kidneys handle sodium, and on longer fasts that can add up. The good news is that it's easy to manage once you understand what's happening.

Why fasting makes you lose salt

Insulin is best known for managing blood sugar, but it also talks to your kidneys. In a classic experiment, raising insulin while keeping blood sugar steady cut the amount of sodium people passed in their urine almost by half, and potassium too ¹. In other words, after a meal, insulin tells your kidneys to keep salt and water in the body.

What a little dehydration feels like

You don't have to be very dehydrated to notice it. When young women lost just over 1% of their body weight in water, they reported more headaches, more fatigue, worse mood and found tasks harder to concentrate on, even though most tests of their thinking were unaffected ⁴.

Coffee counts

So your morning coffee isn't working against you. It just shouldn't be your only drink: keep some water nearby too.

What to do, depending on your fast

• Fasts of 12 to 16 hours: drink water when you're thirsty and a little more in hot weather or when you exercise. Normal meals provide the salt and potassium you need.
• Fasts of 18 to 24 hours: keep a bottle of water with you. If you get a headache or feel lightheaded when you stand up, some people find a pinch of salt in water helps.
• Fasts longer than a day: sodium and fluid losses add up. Plan them with a doctor, especially if you take any medication.
• In your eating window, include vegetables, fruit, legumes, yoghurt and nuts, which are rich in potassium and magnesium.

MYTH

Everyone who fasts needs electrolyte supplements.

FACT

For daily fasts of 12 to 16 hours, there's no evidence that supplements are needed. You eat every day, and your meals replace what you lose. They become more useful on fasts of a day or longer, in the heat, or with heavy exercise.

Sources

1. 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
2. Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941
3. 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
4. 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
5. Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One. 2014;9(1):e84154. doi.org/10.1371/journal.pone.0084154
6. 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