My Take on Intermittent Fasting
Updated: Mar 29
Earlier this week I saw a great article where 9 registered dietitians chimed in about what they think of the current most popular dieting fad - intermittent fasting. This is definitely the question I get asked more frequently than anything else these days. What do I think of intermittent fasting?!
First of all, like everything in nutrition, it isn't black and white. Intermittent fasting isn't good for all people or bad for all people. This middle ground is one where a lot of true answers lie. Our inability as humans to exist and embrace this grey area is one I'm truly fascinated by and one I'll be writing about soon. (Is this the Tao? Hoping for comments on this below.)
What is intermittent fasting (or IF)?
The practice of not eating or drinking anything for various periods of time has been around in ancient and religious practices for thousands of years, so isn't exactly a new fad. It is now being used as a dieting and health technique based on limited, but interesting research. There are several protocols, but generally there is a window for eating and a window for fasting. The most popular protocols are:
Alternate day fasting - such as eating every other day
Time-restricted feeding - such as having a 8 or 10 hour eating window and a 16 or 14 hour overnight fast
Modified fasting regimens - such as the 5:2 diet - which involves eating a very little amount (maybe 400 calories or so) on 2 days and eating as your normally would the other 5
And what do I think of intermittent fasting?
There is definitely compelling research about the benefits that IF can have on digestion and metabolism, but the human studies are short-term and have small population sizes. Studies like this are not enough to prove statistical significance or provide me enough information to recommend these strategies wholeheartedly. We need more long-term human studies and good evidence. That said, I think it's possible it can be beneficial for some people - the right people.
Alternate day fasting studies have laughably short study times (like two days) and do show some improvement in metabolic markers, but who cares? Two days?! Plus, they also show extreme hunger and low mood - no thank you.
Population studies do show that consuming the majority of one's calories early in the day and having a longer rest-and-digest period overnight is associated with reduced risk of several chronic diseases and improved weight regulation. One major reason IF may be beneficial is because of the influence our circadian rhythm can have on hormones and metabolism. And that makes sense - having a big meal or midnight snack right before bed and then only sleeping 6 hours will not give your body adequate time to rest, digest, and detoxify. I talk to a lot of overly busy people who's eating and sleep patterns sound similar to this.
One important thing to consider, is that diet quality still matters - a lot. The benefits one would get from IF are much stronger if focusing on whole foods and limited processed foods.
Even though doing IF correctly may have benefits for some, it can create more problems for others. There are countless potential problems that could occur if intermittent fasting is wrong for the person doing it. We see that prolonged periods without food can cause nutrient deficiencies, fatigue, obsession about food, disordered eating, decreased immunity, slowed metabolism, and more. Very interesting is the evidence that IF is generally harmful for women, which is scarcely mentioned in the blogs or reviews on the subject. Stefanie Ruper, a women's health researcher and advocate, did a great review of the literature and wrote about it here - highly recommend checking that out. Pregnant and breastfeeding mothers should not do intermittent fasting at all. And just generally long periods of time without eating is harder on women's bodies than mens.
I've talked to clients who have tried IF and it has wreaked havoc on their life. They try like hell not to eat (or to eat very little) all day, only to be famished and stressed and uncomfortable by the time dinner comes. So every night, they eat a portion much larger than they would normally eat - and in an out-of-control fashion, becoming more out of tune with their fullness meter, and without taking time to enjoy the meal. IF is definitely not working for this person and is doing them harm. If this is you, you should be eating more throughout the day (ie breakfast and lunch) to reduce the risk of eating until uncomfortably full at night, and the out of control feelings and behavior that come from being extremely hungry all day long.
Who intermittent fasting is definitely not okay for
Importantly, IF is not okay for those who are recovering from an eating disorder or who have a history of disordered eating. Finding a peaceful relationship to food requires getting rid of all the rules. If you are prone to obsessing about your food choices and scrutinizing nutrition labels out of fear you'll ingest something that's "bad" or "not natural", partaking in a very restrictive and rule-based eating pattern could trigger another set of rules and worries for you. If this is you, what's best is less rules, not more. You need to learn how to listen and trust your body first. It takes time, but it can be done.
So all in all, I think a light and flexible version of IF could fit into some people's lifestyles. A 12-14 hour window between dinner and breakfast could be a pattern of eating that you follow with flexibility. Meaning - if it works for you and your schedule and your life and your family's life - to eat dinner at 6pm and then not have any breakfast until 8am, then great. But if you have to wake up at 5am and get the kids out the door and are ravenously hungry - then you need to eat something. And if your schedule doesn't allow you to eat dinner until 8pm and you have 8am meetings that last 2 hours, this doesn't really suit you either, at least not on those days. And if you find that you're overeating at meals because you know you're about to go a long time without food - stop. This is a signal that IF is not right for you.
Research shows that by ignoring hunger signals and not allowing yourself to eat when you're hungry, you could be eroding your trust in your body. So be honest with yourself about what your goals and values are. If following a strict time schedule sounds like a drag that would interfere with your quality of life, your friendships, or you peace of mind, then I encourage you to forget it. If you want to make positive changes to your eating habits, start by tuning into your hunger and fullness levels and see how that feels.
As always, let me know what you think!