Updated: Feb 26
I am a fan of intermittent fasting.
You may be surprised to hear this. After all, I am a Registered Dietitian, and as a licensed health professional, I don't do fad diets. I recommend what's good for you and what's evidence-based.
But, guess what? I don't consider intermittent fasting a fad diet. There are some truly astonishing health benefits to intermittent fasting (IF), and there is actual scientific evidence to back it up (see below). After all, intermittent fasting may be much closer to the way our ancestors ate, back before we had food available to us round-the-clock.
Think about it: Do you really think we had the luxury of eating three big meals a day, and then some snacks in between, while we were still out, hunting and gathering our food on a daily? Not likely. More likely is that there were more hours in a day when our stomachs were empty. And it was likely those hours when we needed the energy to hunt for food.
In essence, our body may just be able to function a little better when it doesn't have to deal with the digestion of food round the clock.
But what about us breastfeeding moms?
After all, you're likely here because you're breastfeeding, or pregnant and plan on breastfeeding. You've probably heard of intermittent fasting and read about its benefits, and would like to know if you can eat this way and still nurse your baby. After all, humans were breastfeeding back in caveman days, too, right?
What is Intermittent Fasting?
Intermittent fasting is essentially the practice of not eating for extended periods of time. Instead of eating your meals and snacks spread out throughout the day, you shorten your eating window to only 8, 6 or 4 hours. Or you eat nothing or very little a few times per week, and eat normal the other days.
There are a few popular versions of intermittent fasting:
Alternate-Day Fasting: Fasting for 24 hours every other day
Eat-Stop-Eat: Fasting for 24 hours once or twice per week
The 16/8 Method: Fasting for 16 hours each day; for example, from 4pm until 8am the next day
The 14/10 Method: Fasting for 14 hours each day; for example, from 6pm until 8am
The 5:2 diet: eating just 500 to 600 calories on 2 days of the week and eating normal during the other 5
The Warrior Diet: Eating one large meal at night and very little (for example, a hard boiled egg or some raw fruits and vegetables) for the rest of the day.
What are the Benefits of Intermittent Fasting (for the Average Person)?
The idea of intermittent fasting is to give your digestive system, liver and other organs involved in food processing some rest - time during which the body doesn't have to deal with the digestion, break down and metabolism of food and can concentrate on repairing cells and eliminating waste. It is also a time during which your insulin and leptin receptors get a break and (as studies have shown), re-sensitize to the effects of food (1, 2, 3)
Intermittent fasting may have some incredible benefits for the average person:
It may slow or prevent the degeneration of brain cells and improve brain function (8)
It may have anti-aging effects (4)
If you think about it, it makes good sense: Back in caveman days, we didn't have food available to us all day, every day. Hunting and gathering of food was hard, and likely not always successful. Very likely, our bodies are adapted to (or maybe even meant to?) go without food once in a while.
But what about breastfeeding? How does intermittent fasting affect us breastfeeding moms and our milk?
The Research is Scarce on Intermittent Fasting While Breastfeeding
There is some (but not much) research on intermittent fasting while breastfeeding. Most of it was conducted on breastfeeding women during religious fasts such as Ramadan (monthlong fast from sunrise to sunset) and Yom Kippur (a 25 hour fast during this religious holiday). Not all of these research studies are of great quality, however. Sample size is usually not very big (20 to 30 women), most did not measure all aspects of breast milk (for example, some only measured 'major' nutrients such as protein and lactose, but failed to measure micronutrients such as vitamins and minerals) and some did not take into account the fact that breast milk composition changes over time and throughout the day (so, taking breast milk samples at different times of the day, for example, will always result in a different composition).
However, these are the things we know so far about intermittent fasting while breastfeeding:
1. Fasting Affects Female Hormones
Fasting can affect female reproductive hormones such luteinizing hormone (LH), follicle stimulating hormone (FSH) and estrogen and can potentially decrease your fertility (13, 14). Women who fast for extended periods of time often see their menstrual cycle changing and some even stop getting their period. Fortunately, this negative effect on female hormones is usually only seen if fasting is done for too long or too excessively (15, 16).
Of course, if you are breastfeeding, you may not be ovulating yet anyway. Hence, you likely won't notice if there are any changes to your menstrual cycle. It may even sound like a great thing for us postpartum women to not have to worry about getting pregnant for a while (although this can backfire!). But, of course, you always want to be careful when something affects your hormones or when your menstrual cycle changes as a result of a diet change, because it tells you your body is not liking the change.
If you've been practicing intermittent fasting and you're having trouble getting pregnant, rememberer that fertility is usually one of the first things to go when the body thinks food is scarce. You may simply not be eating enough!
2. While Breastfeeding, you Reach the Fasting State Faster
Women reach the fasting state faster than men, breastfeeding or not. That's because we generally have less glycogen stored in our liver and muscles.
Let me explain: Before the body reaches the fasted state, it uses up the glucose (and other nutrients) from the food we last ate. Once that is gone, it starts using up glycogen from our muscles and liver. The more glycogen we have in stock (larger liver/more muscles), the longer the body can use this source of energy before it goes into the fasted state. Once you are fasting, insulin levels are low and your body starts to burn fat for energy. So, the more glycogen you have, the longer it takes to reach the fasting state.
Women who are breastfeeding reach the fasting state even faster because the nutrients are not only used to power its own metabolism, but to produce breast milk as well. Hence, glucose and glycogen is used up faster while breastfeeding.
It's essentially as if you were exercising, which has a similar effect. Exercising also uses up glucose and glycogen and you reach the fasted state faster.
While breastfeeding, you may already be fasting after only 4 to 8 hours!
Especially in the first few months of breastfeeding, you are likely hungry **all the time**, even waking up at night sometimes with your tummy rumbling! Have you experienced this? This is a sign your glucose and glycogen is all used up and you are already fasting!
It's not a good idea to ignore these hunger signals for too long, especially in the first few months of breastfeeding. If you fast for too long, your body will go into 'starvation mode'. In starvation mode, your protein stores (muscles) will start to break down, your metabolism will decrease and (yes) your milk supply will also decrease.
Most concerning: if you starve yourself for too long while breastfeeding, your body can go into a metabolic state called ketoacidosis. --> See point 5 below.
It's important to know, while breastfeeding, your body will reach the fasting state faster than if you were not breastfeeding. That's because breastfeeding uses up the glucose, glycogen and other nutrients faster in order to produce breast milk. Hence, it may only take 4 to 8 hours to reach the fasting state in the first months of breastfeeding! Once your baby starts sleeping through the night and your fasting window goes to 8+ hours, you are probably already intermittent fasting!
3. Fasting can decrease milk supply, but only if...
As mentioned above, studies have shown, fasting for longer than 24 hours can decrease your milk supply (17). We also know that your milk supply can also decrease if you eat less than 1800 calories per day (18) or your calorie deficit is too high (500 calories or more deficit).
However, it looks like short-term fasting such as the 16:8 or 14:10 method may not necessarily decrease your milk supply, if you get enough calories in during your eating window (19).
So, fasting can decrease your milk supply, but only if you fast for 20 hours or more or you don't eat enough calories during your 'eating window'.
This, of course, only goes for us mamas who already have a well established milk supply. During the first 3 months or so, your breast milk supply is still very susceptible to any form of 'dieting', so fasting within the first 3 months postpartum will also likely decrease your milk supply and is definitely NOT recommended. For those mamas who are 6+ months postpartum with a well-established milk supply and a baby that latches well, you may have found that your breast milk supply is pretty resistant to dieting and mindful fasting may be possible.
4. Fasting Affects Breast Milk Composition, but it may be Preventable by Eating a Nutrient-Dense Diet
Not a lot of studies have yet been published on the effects of intermittent fasting on the composition of breast milk. However, to date we know that short-term fasting (such as 16:8 or 14:10) probably does not change the macronutrient composition of breast milk (20). The fat, protein and carbohydrate content of your breast milk is usually not affected, unless you fast for 20 hours or more.
However, even short-term fasting may decrease the content of some micronutrients (vitamins and minerals) in your breast milk (21). This decrease may be related to the fact that you are eating less food overall during your eating window, since many micronutrients in breast milk are dependent on mom's intake.
This decrease in micronutrients may be preventable by eating an especially nutrient-dense diet during your 'eating' window and taking a high quality multivitamin.
5. Fasting while Breastfeeding CAN be Dangerous
I know you don't want to hear it, but I have to say it to give you the 'full' picture. Fasting can be dangerous for a breastfeeding mom. While it's rare, there have been a cases of ketoacidosis while breastfeeding (22, 23, 24), which is essentially when the body goes into starvation mode and produces excessively high levels of ketones (this is different from ketosis, where only small amounts of ketones are produced).
Ketoacidosis may start with suddenly having to go pee a lot, followed by some belly pain and nausea or vomiting. You may notice your breathing getting more rapid and you may act confused. This is definitely the time to grab some fast-acting sugar such as a piece of fruit, candy or sugary soda and quickly head to the emergency room, because ketoacidosis can be life threatening.
To ease your mind a bit, the cases of ketoacidosis are rare and usually only seen in breastfeeding women who eat nothing for 24 hour or more or are on a very low carbohydrate (keto) diet for several days or weeks (25). (Sidenote: I wrote about the keto diet while breastfeeding here)
However, I absolutely recommend against fasting for 20 hours or more while breastfeeding because it simply is a risk you shouldn't take.
It is important to understand, there is still some research needed in the field of (intermittent) fasting while breastfeeding!
However, let's recap some of what we found out so far about (intermittent) fasting while breastfeeding:
Short term fasts (up to16 hours) do not seem to have an effect on the main (macro) nutrients in breast milk: fat, protein and lactose (source)
However, short term fasts may decrease some micronutrients in breast milk such as zinc, magnesium and potassium (source).
The reason some micronutrients decrease during short-term fasting is probably because we eat less overall while fasting (source), so you could counteract that by eating a nutrient-dense diet during your eating window and taking a high quality postnatal multivitamin.
Fasting for 24 hours or longer can decrease breast milk supply and changes breast milk composition more significantly (source)
Long-term fasting (24 hours+) can be dangerous because it can lead to Ketoacidosis, which can be life-threatening (source)
My Conclusion: You can do intermittent fasting while breastfeeding under certain circumstances
Intermittent fasting can be done safely, even while breastfeeding. But it is critical to know what is safe and how many hours of fasting is max!
Intermittent fasting is NOT safe for everyone...
IF is NOT safe for a breastfeeding mother who is less than 6 months postpartum
IF is NOT safe for a breastfeeding mother who has a history of diabetes
It is NOT safe to fast for more than 20 hours per day.
Not every method of Intermittent fasting is safe while breastfeeding!
Alternate Day Fasting as well as the Eat-Stop-Eat Method is NOT safe while breastfeeding, since it requires you to fast for more than 20 hours. I would definitely stay away from it, even if your breastfeeding child is older.
The 5:2 diet, where you eat 500 to 600 calories on two days per week is a diet that will likely decrease your milk supply over time, since eating less than 1500 calories per day has been shown to decrease milk supply. I therefore don't recommend it either.
The Warrior diet involves eating very little for 20 hours, and then 'overeating' during your 4 hour eating window. This is also likely not safe while breastfeeding.
Is there a method of intermittent fasting that is safe while breastfeeding?
I believe Time Restricted Eating (TRE) such as the 16/8 method or the 14/10 method, is the ONLY safe way to do intermittent fasting while breastfeeding, granted you are mindful of your maximum fasting windows per months postpartum (see below) and eat enough nutrient-dense food during your eating window.
Since your milk supply CAN be affected by calorie restriction, it is important to make sure you are eating enough calories during your eating window (see breastfeeding calorie calculator here). Be aware that this can be a bigger challenge while intermittent fasting, since your eating window is shorter! If your milk supply is declining while intermittent fasting, look at how many calories you are eating during your eating window - you may find that you have to eat more!
It is also crucial to eat a nutrient-dense diet during your eating window (and also a good idea to take a high-quality multivitamin), since micronutrients in breast milk can decline with intermittent fasting. To counter this effect, you should also make sure your baby is eating a nutrient-dense solid foods diet and you are fasting at a time when your baby does not solely rely on your breast milk for nutrition anymore.
In any case, your breast milk will still be a super nutrient-dense source of nutrition for your baby, even if you are intermittent fasting, so keep on boobin'!
My recommendations for maximum Intermittent Fasting windows per months postpartum:
Months 0 to 6 postpartum: Fasting is NOT safe. Eat whenever you feel hungry, even at night.
Months 6 to 9 postpartum: You may be able to fast for a maximum of 10 hours per day
Months 9 - 12 postpartum: You may be able to fast for a maximum of 12 hours per day
1 year to 1.5 years postpartum: You may be able to fast for a maximum of 14 hours per day
1.5 years+ postpartum: You may be able to fast for a maximum of 16 hours per day
Important Notes (please read):
It is important to understand that these are MY personal conclusions based on the literature research I've done on the topic of fasting and intermittent fasting while breastfeeding, as well as my personal experience breastfeeding my two kids until the ages of 3 and 4.
My conclusions are not 'official' guidelines by any health organization and certainly do not represent any form of medical advice. Please consult your doctor or other trusted health professional for that purpose. You may find that most doctors and health professionals will strictly advice against intermittent fasting while breastfeeding. I believe that is because the 'classic' breastfeeding mom is usually one that feeds a baby less than 6 months of age.
I also recommend against any form of fasting for breastfeeding moms less than 6 months postpartum. However, for a mom who breastfeeds an older child (and I know extended breastfeeding is becoming more and more normal!), it may very well be possible to benefit from intermittent fasting while continuing to breastfeed their child!
If you like my articles, consider sharing it with fellow breastfeeding moms or supporting me by downloading my nutrient-dense meal plan for breastfeeding moms, which can be found here!