Water for intermittent fasting: how much to drink and when water alone is not enough

1. Work out how much water you need for intermittent fasting

The National Academies set an adequate intake of about 3.7 liters (125 ounces) of total water a day for men and 2.7 liters (91 ounces) for women, counting the water in food. About 80 percent of it usually comes from drinks, roughly 13 cups a day for men and 9 for women. Heat and exercise push your own number higher.

Food supplies more water than most people expect. In US survey data from 2005 to 2006, adults reported about 3.2 liters of total water a day, with plain water making up 33 percent and other drinks 48 percent. That leaves roughly a fifth from food such as oatmeal, fruit, soup and salad, and when you skip a meal, its water goes with it.

Your fast	What you lose	Rough adjustment
12 to 14 hours overnight	Almost nothing	Your usual drinks
16:8, skipping breakfast	The water in breakfast	One extra 16-ounce glass
18:6, 20:4 or one meal a day	The water in one or two meals	16 to 24 extra ounces over the fast
24 to 36 hours	All water from food, plus extra urine early on	2 to 3 extra cups, with salt (step 5)
Any fast with heat or a workout	Sweat	Replace what the scale shows (step 6)

2. Spread your water across the fasting hours

A big amount at once mostly ends up in the bathroom within a few hours, so sip steadily instead. A day on a noon to 8 pm eating window might look like this:

• 7 am, on waking: a 16-ounce glass of water.
• 8 am: black coffee or plain tea, which counts toward your fluids (step 4).
• 9 am to 3 pm: a 32-ounce bottle at your desk, sipped steadily.
• Noon: a 16-ounce glass of water before your first meal.
• 7 pm: a glass with dinner, then taper off an hour or two before bed.

That adds up to roughly 80 to 90 ounces. Add or drop a glass to fit your size and your day.

The glass before lunch has a small trial behind it. Forty-eight adults aged 55 to 75 on a reduced-calorie diet were split into two groups, and one group also drank 500 ml (about 17 ounces) of water before each main meal. That group lost about 2 kg (4.4 lb) more over 12 weeks. The water trimmed a test meal by about 40 calories at the start, a gap that was no longer clear by week 12. Treat it as a habit that may help some people, with no promise for the scale.

If you log each glass in FastTrack, your fasting history shows that day's water next to each fast, which makes a low-water day easy to spot when a headache turns up.

3. Check your urine and the other signs you are short

Thirst works well for most healthy adults, though it fades with age, so people over 65 do better with a routine. Urine color is the easiest second check. Pale straw is fine; dark yellow or amber means you are behind. Your first pee of the morning is always darker, and a multivitamin with B vitamins can turn urine bright yellow whatever you drink.

The color also tracks changes. In pooled data from 238 healthy adults, drinking about 1.1 liters (37 ounces) more a day lightened 24-hour urine by about two shades on an eight-shade chart, and drinking that much less darkened it by two. A 32-ounce bottle added to your day is roughly that change.

What you notice	Likely cause	What to do
Dark urine, fewer bathroom trips	Not enough fluid	A 16-ounce glass now, then sip through the day
Dry mouth and a dull headache late in the fast	Low fluid, sometimes low salt	Water first, salted water if it lingers an hour
Pale urine, yet cramps or a flat feeling on a long fast	Low sodium	A pinch of salt in water, or broth (step 5)
Light-headed when you stand up	Low fluid or sodium	Stand slowly and sip salted water; end the fast if you faint
Nausea or puffy fingers after drinking a lot	Too much water too fast	Stop drinking; get medical help if confused or vomiting

Headaches have other causes too, from caffeine withdrawal to a short night, and our guide to headaches while fasting goes through them in order.

4. Choose drinks that count and keep your fast

Plain still water, sparkling water, seltzer and mineral water have zero calories and fit every kind of fast. Black coffee and plain tea count toward your fluids too. In a UK trial, 72 fasted men drank a liter of still water and of three other common drinks on separate days, and urine over the next four hours after coffee, hot tea or sparkling water was no different from still water.

Flavored water is where labels matter:

• Fine on any fast: a slice of lemon or cucumber, a few mint leaves, and seltzers that list 0 calories and 0 g sugar.
• Breaks the fast: tonic water, sweetened vitamin waters and seltzers with added juice, all of which contain sugar.
• Your call: zero-calorie flavor drops and diet soda. Drop them if sweet tastes make you hungrier.

Bubbles bloat some people on an empty stomach, so switch to still water if they bother you. Our page on whether water breaks a fast covers enhanced and flavored waters in more detail.

5. Add sodium when water alone is not enough

On 16:8, your two meals bring enough salt and you can skip this step. Past about 24 hours it starts to matter. In the first days without food your kidneys pass more sodium and water, and the water stored with glycogen goes too. Extra plain water replaces volume while diluting the sodium you have left, which is why someone can have pale urine on day two and still feel light-headed and flat.

Sodium also helps you keep what you drink. In the same UK trial, an oral rehydration solution containing sodium left the men with less urine over four hours than plain water. For a long fast:

• Salted water: 1/8 to 1/4 teaspoon of salt in a 16-ounce glass, sipped over half an hour, once or twice a day.
• Broth: a mug of salted chicken or vegetable broth on day two, a few calories most people accept.
• Pickle brine: two or three tablespoons from a jar of dill pickles.

Our guide to electrolytes while fasting covers amounts by fast length. Take potassium or magnesium supplements only on your doctor's advice, and ask before adding salt if you have high blood pressure, kidney disease or heart disease. Water-only fasts beyond 72 hours belong under medical supervision. FastTrack's timer shows how many hours into the fast you are, so you can see when you cross the 24-hour line.

6. Drink more for heat and workouts, without overdoing it

Heat and exercise raise your needs quickly. On a 90°F afternoon of yard work or a fasted morning run, drink extra during and after. For sessions over an hour, weigh yourself before and after in the same clothes; each pound lost is roughly 16 to 24 ounces to drink over the next few hours.

The scale also catches the opposite mistake. At the 2002 Boston Marathon, blood tests on 488 finishers found that 13 percent had hyponatremia, a low blood sodium level, and 0.6 percent had a critical one. Runners who gained weight during the race, meaning they drank more than they sweated, were at higher risk, as were those who took over four hours. Drinking more than 3 liters during the race was also linked to it in a simpler analysis.

A long fast adds to that risk, since sodium may already be low. Never drink very large amounts quickly to quiet hunger or to "flush" your system; outside heat and hard exercise, a glass or so an hour is plenty. Nausea, puffy hands, confusion or vomiting after a lot of water calls for medical help, and more water makes it worse.

7. Know when to stop and who should ask a doctor first

Get a plan from your doctor for both fasting and fluids before you start if you have any of these:

• Heart failure or kidney disease, since many people with these conditions have a daily fluid limit that overrides anything here
• A diuretic (water pill) or other blood pressure medicine
• Diabetes treated with insulin or tablets that lower blood sugar
• Pregnancy or breastfeeding, a history of an eating disorder, or an age under 18

Whatever your health, stop the fast and eat if dizziness does not pass after water and a pinch of salt. If you faint, feel your heart racing or skipping, or become confused, end the fast and get medical help right away.

Water on a fast, in short

Drink about what you normally would, plus a glass for each meal you skip, spread from waking to bedtime. Pale urine and ordinary thirst tell you when you have it right. On fasts longer than a day, add salt before you add more water.