Whether it’s the 5:2 diet, the 16:8 diet or not eating until lunch, it seems like just about everyone is trying intermittent fasting. 

But is intermittent fasting safe for fertility? Or could intermittent fasting harm your chances of falling pregnant? Let’s dive into the facts and research behind intermittent fasting…

Why are people intermittent fasting?

Typically an intermittent fasting diet involves either: 

  • consuming a very low calorie (or energy) intake on certain days (usually around 500 calories, equivalent to what might usually just be one meal, e.g. 2 slices of wholegrain toast with poached eggs and a soy latte, on 2 days of the week); or
  • not eating for long stretches of time each day i.e. having an “eating window” only between 12pm and 8pm, this is most common with 16:8 fasting styles.

Why are people doing this?

Well, some research suggests that fasting in short bursts may have beneficial effects on metabolism, heart health, cognition (brain function) and markers of inflammation (Stockman et al., 2018). It may also result in weight loss, of course!

This isn’t because fasting has any special or magical benefits in and of itself, rather it is creating a reduction in total energy across the day and/or week resulting in weight change.

Keep in mind the scientific research on intermittent fasting is all pretty new, and studies you see quoted in the media can be based on animals, rather than human studies. Also, a number of the studies so far have only involved men – so we know less about how intermittent fasting affects women. This is pretty common in research where the understandings of a particular medicine or dietary pattern are first researched in men before replicating it in women, but that is a story for another blog!

Could Intermittent Fasting be harmful for fertility?

Intermittent fasting may potentially harm a woman’s fertility because of the potential effects of restrictive eating on the menstrual cycle.

To get a regular period, and therefore regular ovulation, women need to consume enough energy (calories) and have adequate body fat. If you: 

  • restrict food intake too much; 
  • exercise excessively; and/or 
  • experience significant weight loss, 

your brain thinks the body is in “danger” and conserves energy by reducing oestrogen and progesterone as the hypothalamic-pituitary-ovarian axis goes into snooze mode (a lot like when you’re taking hormonal contraception). This means ovulation won’t occur, and your periods may become irregular or disappear completely (known as hypothalamic amenorrhea). 

Intermittent fasting requires you to restrict your meals. Studies of athletes, who often follow strict diets, have shown that your monthly cycle can be affected when the body spends too long in “negative energy balance” (i.e. more calories are being burned than being eaten). This can occur even if the body is deprived of energy just for part of a day (Fahrenholtz et al., 2018) as might happen during intermittent fasting.

There has also been one study of rats that specifically looked at the effects of intermittent fasting on their reproductive systems. In the female rats, intermittent fasting suppressed reproductive hormones and cycles, and even slightly reduced the size of their ovaries (Kumar & Kaur, 2013).

So, intermittent fasting may not be safe for fertility because you will not be eating consistently enough to support your reproductive hormones and that all-important ovulation. 

If your period has gone missing, see my tips on getting your period back naturally here and what to eat for hypothalamic amenorrhea here. And be sure to rule out PCOS (polycystic ovary syndrome) or other medical causes with your doctor, more about PCOS available here.

Could Intermittent Fasting ever improve fertility?

Currently, we do not have any evidence that intermittent fasting could be helpful for fertility for the general population. However, there is no evidence that it could be harmful either. Theoretically, it looks as though it may have a negative effect due to the nature of hormone release for ovulation in women. 

However, a group of researchers have made the suggestion that intermittent fasting may help those with PCOS. Their theory is:

  • Insulin resistance is common in women with PCOS.
  • Women with PCOS who are struggling with excess weight sometimes achieve more regular periods when they are able to lose weight. 
  • Some studies show that intermittent fasting may improve insulin resistance and weight loss, by creating an energy deficit or negative energy balance.
  • So, intermittent fasting might help women with PCOS by reducing insulin resistance and/or causing weight loss which can help reinstate more regular periods (Chiofalo et al., 2017).  

In cases of PCOS, improving insulin resistance and achieving a healthy weight can lead to more regular ovulation. However, we do not have any scientific research to show that intermittent fasting is a good way to achieve this!

You can use diet to help manage PCOS and/or insulin resistance in a way that doesn’t involve intermittent fasting. Check out my blog posts here and here on what to eat for PCOS and my article here on eating for insulin resistance!

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So, should you be intermittent fasting while TTC?

It’s not possible to say that intermittent fasting will definitely harm your fertility. However, we do know that restrictive eating can suppress our hormones and prevent regular ovulation, so intermittent fasting is risky and not an eating pattern I recommend especially to women trying to conceive.

Generally, I do not recommend restrictive dieting when you are TTC.

There is also absolutely no evidence that intermittent fasting is helpful for fertility! And if you become pregnant, intermittent fasting is definitely NOT recommended during a time where so many nutrients are required to support new life.

What diet is safe for fertility?

I am a strong supporter of a Mediterranean-style diet for fertility and the science is on my side! This way you can enjoy an abundance of colourful fruits, vegetables, wholegrains, legumes and beans, lashings of extra virgin olive oil, seafood, fermented dairy foods such as yoghurt and cheese in moderation, eggs and less frequent portions of red meat and sweets.

Check out my guide to the Mediterranean Diet for fertility here.

Download our FREE IVF Nutrition Checklist

This article was co-written by nutritionist Monique Cormack. You can find Monique on Instagram @moniquecormacknutrition or connect with her on LinkedIn.

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