Hydration During Intermittent Fasting: What Breaks a Fast and What Doesn't

First: food is a fluid source. In a normal mixed diet, food supplies somewhere around 20 to 30 per cent of total water intake. Skip breakfast and lunch and you have removed that contribution silently — nothing about it feels like a change in drinking, because you have not changed your drinking at all. On a 16:8 schedule that can easily be several hundred millilitres a day unaccounted for.

Second: low insulin makes your kidneys excrete sodium. This is the part almost nobody is told. Insulin promotes sodium retention; as insulin falls during a fast, the kidneys dump sodium, and water follows it out. This is why the first few days of fasting often come with rapid weight loss that is mostly water, and why the symptoms people attribute to 'carb withdrawal' — headache, fatigue, lightheadedness, muscle cramps — map so neatly onto mild sodium depletion.

Drinking more plain water without replacing sodium can make this worse, not better, because it dilutes the sodium you have left.

For the purposes of a fast aimed at metabolic benefit — not a religious fast, where the rules differ entirely — the practical test is whether a drink provokes a meaningful insulin response or supplies meaningful calories.

Safe: • Water, still or sparkling • Black coffee • Plain tea — black, green, herbal • Water with a pinch of salt, or a zero-calorie electrolyte mix

Breaks a fast: • Anything with sugar, including juice and soft drinks • Milk and milk alternatives, including a splash in coffee • Bone broth (useful during longer fasts, but it is food) • Smoothies and anything with protein in it

Contested: • Diet drinks and artificial sweeteners. No calories, but evidence on cephalic-phase insulin response is mixed and they can amplify appetite in some people. Fine for most; worth dropping if your fasts feel unusually hard. • A splash of milk in coffee. Technically breaks a fast. Whether ten millilitres matters for your goals is a judgement call — for weight management, generally not much; for a strict fast, yes. • Lemon in water. A squeeze is negligible. Half a lemon's juice is not nothing.

Once a fast passes roughly twelve hours, sodium becomes the limiting factor for how you feel far more often than water does.

The symptoms of mild sodium depletion during fasting are specific and recognisable: headache, lightheadedness when standing up, muscle cramps, unusual fatigue, and a foggy, low-grade irritability. If those arrive on a fast day and not on a normal day, sodium is the first thing to try.

The intervention is unglamorous — a quarter to a half teaspoon of salt in water across the fasting window is enough for most people, or a commercial zero-calorie electrolyte sachet if the taste of salt water is unbearable. Magnesium and potassium matter too, particularly for cramps, and are worth adding for longer fasts.

One important exception: if you have high blood pressure, kidney disease or heart failure, or take medication affecting sodium or potassium, do not start adding salt without asking your doctor. This advice is written for healthy adults.

Fasting 20:00 to 12:00, as an example:

On waking — 400 to 500 ml of water. You have just gone eight hours without fluid; this is the largest single deficit of the day. Morning — black coffee or tea as usual. It counts toward your fluid. Mid-morning — 300 to 500 ml, with electrolytes if you are past hour twelve or if the headache usually lands here. Just before breaking the fast — a glass of water 20 to 30 minutes ahead. It takes the edge off and stops the first meal being eaten at speed. Eating window — drink with meals as normal; food is now contributing fluid again. Evening — taper an hour or two before bed so you are not up in the night.

The general principle is to front-load. Most people under-drink in the morning and then try to catch up after their eating window opens, which is the least useful order.

Drinking litres of plain water to suppress hunger. It works briefly and then makes the sodium problem worse. Volume is not the lever here.

Treating coffee as the whole hydration plan. Coffee counts, but three espressos is not two litres of fluid, and people who fast on coffee alone are usually well behind by midday.

Ignoring heat. A fast on a 33 °C day is a materially different proposition from the same fast in winter. Sweat losses stack on top of the fluid your missed meals would have supplied.

Breaking the fast on a large volume of liquid. Drinking a lot immediately before or with the first meal after a long fast tends to cause discomfort. A glass beforehand, then eat.

If you would rather not track two moving numbers by hand, MyWaterCalendar raises your daily goal automatically while a fast is running, tightens reminders during the window, and flags drinks that would break the fast — which removes most of the guesswork at hour fourteen.