Intermittent fasting while breastfeeding: what is known and what to do instead

1. Count what breastfeeding asks of your body

Making milk has a real cost. A 1997 review of energy needs during lactation put the extra need of exclusive breastfeeding at about 670 calories a day. If you allow for gradual weight loss, the extra food needed is about 500 calories. The same review put the extra protein at about 20 grams a day and found little evidence that your metabolism slows down to save energy.

In kitchen terms, 500 calories is roughly a peanut butter sandwich and a banana. Twenty grams of protein is about a cup of plain Greek yogurt. These figures are averages, so your own need moves with how much milk your baby takes and how active you are.

A young baby also feeds about 8 to 12 times in 24 hours, night included. An 8-hour eating window would have to carry all that extra food while your hands are often full.

2. See how little is known about intermittent fasting while breastfeeding

Every answer you read on this topic, ours included, is borrowed from nearby evidence. Two kinds of study come closest:

• Religious fasts. Mothers who fasted for their faith were measured during daytime fasts, sometimes without water, or around fasts of more than 24 hours.
• Calorie cuts. Mothers ate less for a short stretch, which tells you more about smaller meals than about long gaps between them.

Neither matches a 16-hour gap repeated daily for months. The three fasting studies in the next step add up to fewer than 100 mothers, and the oldest dates from 1984. Where the summaries give the baby's age, babies were around one to six months old, so the first weeks with a newborn stay untested in the research we read.

3. Read what short fasts did to breast milk

Gambia, 1984. Researchers studied 10 lactating women during Ramadan, with no water from 5 am to 7:30 pm and high water losses through skin and breath. Between 7 am and 7 pm they lost 7.6% of their body water, significantly more than 10 women who were not nursing. Blood markers of dehydration rose more too, though evening values stayed in the normal range. Changes in milk lactose, sodium and potassium pointed to what the authors called "a marked disturbance of milk synthesis." The mothers appeared to compensate by drinking heavily overnight.

Israel, 2009. A preliminary study sampled milk from 48 healthy mothers of babies aged 1 to 6 months around religious fasts of more than 24 hours. Right after the fast, sodium, calcium and protein were higher while phosphorus and lactose were lower, and triglycerides, the main milk fat, did not change. A day later most values were back to baseline, with protein still raised and lactose still lower.

Turkey, 2006. A study of 21 mothers of babies aged 2 to 5 months compared milk during Ramadan with milk two weeks after it. The major nutrients in the milk held steady and the authors reported no effect on the babies' growth. Zinc, magnesium and potassium in the milk dropped, and the mothers' energy and most nutrient intakes fell below the levels recommended for lactation. The authors concluded it would be prudent to excuse nursing mothers from the fast.

The Israeli and Turkish summaries report what was in the milk and say nothing about how much milk the mothers made. None of the three describes following babies for months. A fair reading: a fast can shift milk chemistry for a while, and nobody knows what repeated fasts would mean for a baby over months of feeding.

4. Keep a calorie cut and a fast apart

The best data on milk volume comes from dieting trials. At the University of California, 67 exclusively breastfeeding women about 12 weeks after birth were randomly assigned to 11 days of a 35% energy deficit, with or without exercise, or to a control group. Changes in milk volume, milk composition and baby weight did not differ significantly between the groups. Among women who only dieted, though, milk energy output rose in those with more body fat and fell in the leaner ones.

Some mothers read results like these as permission. Eleven days is short, the authors called for longer studies, and the trial says little about 16 hours without food repeated for months. The 1997 review also judged a moderate shortfall unlikely to affect lactation, and it limited that view to women with adequate fat reserves. The same plan can land differently on two mothers, which is why this decision belongs in a talk with someone who knows you and your baby.

5. See why a structured fast is generally not advised

Our guide to intermittent fasting for women says it is not recommended while you are breastfeeding unless your doctor advises it. Some everyday problems sit behind that advice:

• Demand does not stop at night. Your baby feeds on their own schedule, and a fixed window asks you to go without food for hours while you keep making milk.
• Fluids slip. Many people drink less during fasting hours without meaning to, and the Gambian data show how much water a nursing body can lose.
• Tiredness stacks up. Broken sleep plus a long gap between meals can leave you light-headed, and you may be carrying a baby on the stairs.
• Early supply is still settling. Frequent feeding in the first weeks helps build your supply, which makes it a poor time to add another stress.

Your situation	Structured fast?	What to do
Newborn or first months of feeding	Not advised	Eat when hungry, night feeds included, and ask your midwife before any change
Low supply now or in the past	Not advised	See a lactation consultant and keep every meal and snack
Your baby's weight gain is being watched	Not advised	Follow your pediatrician's feeding plan
Diabetes on medication, an eating disorder history, or underweight	Not advised	Speak to your doctor before changing when you eat
Older baby on solids, growing well, fewer feeds	Untested, ask first	Bring the options in step 6 to your doctor and keep checking the signs in step 7
Weaning soon or already weaned	Ask about timing	Wait until you have stopped, then talk to your doctor before you start

6. Ask about gentler options

If you want more structure, take these to your doctor, midwife or lactation consultant. They are ordinary eating habits, and any fast beyond them is a medical decision.

• A normal overnight gap of about 12 hours. Dinner at 7 pm and breakfast at 7 am is how many people already eat. If you wake hungry at a 3 am feed, eat. Keep a banana or crackers with peanut butter by the chair.
• Regular meals. For example: oatmeal with walnuts and berries at 7:30, a turkey and avocado sandwich with an apple at noon, plain yogurt around 3:30, salmon with rice and broccoli at 6:30. Move it around your baby.
• Food quality. Reach for eggs, fish, beans, yogurt and whole grains when you can. Beans, pumpkin seeds and potatoes bring the minerals that dipped in the Turkish milk samples. That helps your own intake; the study did not test whether it changes the milk.
• Fluids through the day. Keep a water bottle where you feed and drink when you are thirsty. Our page on water intake while fasting is written for people who fast, and its urine check works for you as well: pale straw is fine, dark yellow means you are behind.

7. Stop and get help if you see these signs

Keep watching even on a gentle routine. If you have changed anything and one of these signs appears, go back to your usual eating straight away.

Sign	Who to call	What to do now
Supply dropping: pumped amounts falling for several days, or your baby unsettled after most feeds	Lactation consultant	Return to regular meals and fluids, and keep feeding on demand
Fewer than six wet diapers in 24 hours after the first week, or dark urine	Pediatrician, same day	Keep feeding and keep counting diapers
Weight gain slowing at baby checkups	Pediatrician	Drop all restriction until you have a feeding plan
Baby unusually sleepy, hard to wake or too weak to feed	Pediatrician or emergency care, right away	Get help before the next feed
You feel dizzy or shaky	Your doctor if it happens again	Put the baby down safely, sit, eat and drink
You faint or nearly faint	Your doctor, same day	Stop all restriction and keep another adult with you and the baby

Skip any fast and speak to your doctor first if you take medication for diabetes, since missed meals can push blood sugar too low. The same goes if you are pregnant again, have a history of an eating disorder, or are under 18. Our page on intermittent fasting side effects lists other red flags, such as heart palpitations.

Your next step

Right now, a structured fast would be a bet on evidence that barely exists. Regular meals and an ordinary overnight gap are a better starting point, after a talk with your doctor, midwife or lactation consultant. Fasting can wait until your baby no longer relies on your milk.