walking you through intermittent fasting – what it is, what the research says and my take on it as a registered dietitian
Intermittent Fasting is a trendy topic right now. Numerous books have been published on it and every day it seems like a new celebrity or prominent influencer comes out singing its praises.
But before you hop on the bandwagon, it’s important to understand what exactly intermittent fasting is and what the research says. Below, I explore all of these questions and provide my take on the trend.
What Is Intermittent Fasting?
Intermittent Fasting is pretty much exactly what it sounds like: the abstinence from food and/or drink for a certain period of time.
There are many different forms of fasting in today’s world. Some programs recommend eating healthfully when you do eat, others say you can eat whatever you want. Some programs feature 24 hour periods free from food while others only recommend 3 days per month or simply extending your overnight fast.
The “5:2 diet” is one intermittent fasting diet that you may have heard of as it has gained popularity recently. In this diet, individuals eat normally (restriction-free, for the most part) for 5 days of the week, and on 2 non-consecutive days, consume only 500 (for women) or 600 (for men) calories per day.
While there are many media outlets that have reported remarkable findings associated with intermittent fasting, it’s not as straight-forward as they would have you believe.
First, in many ways, the research on the topic is not clear cut. Like I explained above, there are many different definitions for “fasting” and you’ll find the same variations in the literature. This in and of itself makes it hard to come out with a definitive finding.
Second, it’s worth noting that in the body of research that exists, a large number of the studies are on rats and mice which, while they may give a good indication, are not always able to be directly translated to humans.
That said, there have been studies in humans that look to test the efficacy of fasting which have shown favorable results (increased loss of abdominal fat, improved insulin resistance, reductions in cholesterols and blood pressure) among specific populations (namely obese men and women, and obese women at-risk for breast cancer) during the trial periods.
The American Heart Association (AHA) published a scientific statement after reviewing the research on meal timing and frequency concluding that while evidence exists to show that both alternate-day fasting and periodic fasting may be effective for short-term weight loss, there isn’t enough evidence to understand its long term effectiveness. It also found that a weight loss of 6% or more may be required for fasting to be effective for lowering blood pressure.
Bottom Line: Modified fasting regimens appear to promote weight loss and may even improve metabolic health in specific populations. That said, there is still a lot of research needed in order to determine populations for whom intermittent fasting is optimal, ideal fasting regimen (including length of fasting interval, number of fasting days per week and amount of calorie restriction needed on fasting days), ideal intake for non-fast days in order to see an impact and long-term effects.
Please note: I’ve chosen not to go into all the specifics of the studies (or this would be really long, and I think most people are only interested in an overview), but I’m happy to provide specific studies offline to any interested and have listed references below.
It’s easy to see the appeal of diets like this. Unlike other diets where you’re counting macros, calories, grams of carbohydrates or ketones daily, this type of diet frees you from that for all but 2 days per week. It seems like a cinch compared to everything else! But remember, there are still 2 whole days of your week where you have to go back to obsessively counting calories, fighting off natural hunger cues and staving off cravings.
As with any diet, when there are days that you are restricted, it often sets you up for wanting more on the days you’re ‘allowed’ to eat. The logic being “I really wanted XYZ while I was fasting, so I’m going to add that to my non-fasting day on top of everything else I eat.” Just think, if you simply listened to your body and internal cues during your fasting days, rather than restricting, you’d likely be taking in the same amount of calories for the week anyway.
Fasting days can also set you up for an unhealthy obsession with the foods you’re not ‘allowed’ to eat. The more fasting days you experience, the more your cravings will intensify as your body works to get the fuel it needs and expects. This can lead to an unhealthy food obsession, where all you can think about are the foods you’re not able to eat on those fasting days. An effect that can have long-lasting and harmful results.
Since our brain relies on the fuel we take in to function, it’s possible that you may experience a good degree of mental fog during fasting days which can be especially difficult if a fasting day takes place during a day you’re at work or in class.
Fasting days can also have a negative impact on your physical wellness. You may experience headaches, dizziness as well as a drop in blood sugar levels.
Finally, it’s possible that fasting days may lead to social isolation – it’s very hard to stick to eating 500 calories a day (which equates to about 4 or 5 pieces of bread for the entire day) when everyone around you is enjoying tacos with extra guac and margaritas.
Bottom Line: Programs that require you to fast for hours or days, move you away from intuitive eating. This only harms your ability to listen to and trust in your internal hunger cues, but also threatens your peaceful relationship with food. While individuals may see short-term weight loss, it’s likely not a physically and mentally sustainable, long-term approach (as with most restrictive diets). Finally, while there has been a link found between intermittent fasting and weight loss, there is very little in the literature to date to show its better for weight loss than simply eating a healthful diet. At this point, a great deal more research needs to be done on the topic before recommending IF as an approach for either weight loss or health.
If you’re interested in my take on other popular diets, see my Thoughts on Popular Diets post.
- Today’s Dietitian: Fasting Regimens for Weight Loss By Densie Webb, PhD, RD
- Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46-58.
- Harvie M, Wright C, Pegington M, et al. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. Br J Nutr. 2013;110(8):1534-1547.
- Varady KA, Bhutani S, Church EC, Klempel MC. Short-term modified alternate-day fasting: a novel dietary strategy forweight loss and cardioprotection in obese adults. Am J Clin Nutr. 2009;90(5):1138-1143.
- St-Onge MP, Ard J, Baskin ML, et al. Meal timing and frequency: implications for cardiovascular disease prevention: a scientific statement from the American Heart Association. Circulation. 2017;135(9):e96-e121.