The Science Behind Intermittent Fasting: are the claims true?

Crystal, my other RD half over at Real Life Women’s Health is back today with a post for you on a frequently requested topic – intermittent fasting. Also, if you don’t already, you can follow her on instagram for a bunch of other good stuff. Enjoy!

In the early days of studying nutrition I was a guinea pig of sorts, always dabbling in the newest & “best” way to eat (spoiler alert: there isn’t one). I was convinced this was all in the name of health, so when I came across PhD’s, dietitians, and influencers promoting the supposed health benefits of Intermittent Fasting (IF), I figured I’d give it a try.  The driving force behind my decision was yes, partly health, but there was also an underlying hope that I could somehow prolong my lifespan and maybe even cheat death (<< I didn’t recognize this at the time…obviously this is impossible) and of course I was also hoping that it would give me the “perfect” body and therefore, according to diet culture, my problems would go away.  

Intermittent fasting was a distraction for me. I was stressed out. My experience basically involved waiting as long as I could to eat, so I was not “allowed” to eat if it hadn’t been 14-16 hours since my last meal. Saying this now sounds ridiculous, but I also think it served its purpose. Now I can caution people that it’s often a rabbit hole to disordered eating. IF wreaks havoc on hormones, especially for women, and potentially even our mental health! 

For me, doing IF led to increased urges for foods high in sugar, feeling out of control when I was around high sugar foods, irritability, painful acne, and disordered eating. I was lucky enough not to lose my period, but many women who try IF report a loss of menses – a clear sign from the body that something has gone awry. Also, please consider this a public apology to anyone that I suggested IF to, I was wrong.  I also want to add that with any history of disordered eating or eating disorder, any type of diet or restrictive eating like fasting is highly problematic and can lead to a relapse. Dieting is the #1 cause of the onset of an eating disorder.  So, even if it had been proven that without a doubt intermittent fasting would improve health (it most certainly hasn’t), I still wouldn’t recommend it because it’s not worth slipping back into an eating disorder or risking the development of a new ED. Eating disorders can happen to anyone.

So first let’s get a good understanding of what intermittent fasting means.

There are different types of IF:

1. Eat in a “window”, typically 8 hours and then fasting for the other 16
2. 5:2 which means you eat “normally or ad libitum(as you wish)” 5 days out of the week and fast for two days where you eat 500-600 calories per day
3. A 24 hour fast once or twice a week – meaning no intake of food at all on these days

There are other variations but I think these are the top 3 people are most familiar with.

If it sounds like IF is just another diet, your insight is good. But it’s cloaked in wellness and other “non diet” messages such as, you will “still eat a normal amount of calories, there’s not restriction!”. Like all diets, it has rules of when you can and cannot eat which means in the long run, it’s just not sustainable. What happens when you stop following this “plan”?

Let’s break down the research:

1. The majority of studies to date that show benefits of IF have been done in rats, men, and very specific populations (i.e. people with Type 2 diabetes) under highly controlled environments.  While there have been some studies in women (small sample sizes), the focus is always on weight loss and doesn’t take into consideration changes in hormones or mental health
2. Purported benefits have not been studied long term (we see the same thing when we study the weight loss/diet literature)
3. Negative effects have also not been studied, hormones are of particular interest
4. The definition of IF varies widely between studies
5. The majority of IF studies also include caloric restriction

I think it’s important to remember as consumers that most journalists/writers for health magazines, blogs, etc. have not been trained in how to read and analyze research. Articles can also often be written based off one person’s experience. One sentence is taken from a study and out of context and that’s how we end up with these headlines saying that IF (or some other diet) is the new and improved magical way to avoid all health problems and look ageless forever.  It’s important to dig deeper. But most people don’t have the time to read every article or know how to analyze research. So these articles prey on people’s vulnerabilities. It’s completely understandable that the majority of us don’t sit around analyzing research and instead rely on these types of sources for nutrition knowledge.

However, I would take everything with a grain of salt and know that no one diet is perfect for everyone – can you imagine everyone eating the exact same foods all of the time? It’s just not realistic – we all have different genes which means we all have different needs. One person might tolerate a higher carbohydrate diet while another does better with more fat or protein. Eating intuitively helps guide you towards what your body needs. Another factor IF overlooks or neglects to mention is macro and micro nutrients; meeting these baseline needs can be difficult on a calorie restricted and/or fasting diet.

I understand wanting to make changes that could improve one’s health. I’m a dietitian, I deeply value health. And personally I’ve been there, our culture is OBSESSED with being “healthy” – we have an unhealthy obsession with health.  However, I think it’s also important that when we start to wonder if a diet would do “XYZ” for us and see others trying something out and “working” for them, that we check back in with ourselves and take some time to figure out what we really need.  Are we stressed? Upset? Going through a change or transition? There is usually something else happening in our lives that is the driving force behind wanting to control food.

Questions to ask when you hear about a particular eating pattern, diet, or “lifestyle change” that promises health benefits/weight loss:

1. What purpose does this serve me?
2. What does health mean to me?
3. Is this helping me live a better life?
4. Does this help me live in line with my values?
5. What else is going on in my life right now – stress/change/emotionally?

Now you might be asking, “But what if I naturally don’t eat for 12 hours? Isn’t that a fast?” I think the bottom line is looking at the whole picture and the intention, if that’s your normal eating pattern and you are properly nourished/not ignoring hunger and don’t have thoughts about food being good or bad or let food choices determine your worth then there probably isn’t anything problematic happening.  It’s when we purposefully inflict these things that it gets sticky. Waiting more than 12 hours to eat after your last meal typically isn’t intuitive for the majority of people, especially women.

Here’s a summary of some of the research:

Alternate Day Fasting (ADF) and Chronic Disease Prevention, 2007:

• Effects seen vary between animals and humans (example: animal studies show a decrease in blood pressure where human trials show no effect)
• To date, the effect of ADF on cancer has been tested only in animal models
• In terms of the effect of ADF on the risk of type 2 diabetes, the results to date from human trials have been inconsistent

Energy balance and reproductive dysfunction, 2013:

• 3 month old rats underwent periodic fasting, they were deprived of food every other day for the day and fed ad libitum on the alternate day for 12 weeks
• Significant decrease in mean plasma luteinizing hormone (LH) and testosterone pulse frequency after fasting for 48 hours
• This regimen adversely affected reproduction in the rats by disrupting the reproductive cycle in female rats as well as altering serum concentration of estradiol, testosterone, and LH in both male and female animals. These results are supported by other animal studies showing that food restriction or inadequate nutrition adversely affect the hypothalamic–pituitary-gonadal (HPG axis) and reproduction (more on that here).

IF review, 2015:

• The claims that IF extends longevity are solely extrapolated from studies on animals
• The vast majority of fasting research has been in animals, and evidence in humans of health improvements from fasting is preliminary
• For fasting to be more than a weight-loss fad, greater scientific rigor is needed from interventional trials than is found in the literature
• Excessive fasting could lead to malnutrition, eating disorders, susceptibility to infectious diseases, or moderate damage to organs. In a study of rats, alternate day fasting was found to result in increased left atrial diameter, myocardial fibrosis, and reduced cardiac reserve

Potential benefits and harms of IF, 2017:

• Two studies in normal/overweight subjects reported sustained hunger with IF, and difficulties maintaining daily living activities during restricted days of an ADER (alternate days of energy restriction) regimen.
• The short term studies of IF report mixed results when it comes to insulin sensitivity and raise the possibility of different body responses to IF depending on gender –> interesting
• Energy restriction in “normal” weight women has been shown to increase feelings of hunger, worsen mood, heighten irritability, make it difficult to concentrate, increase fatigue, and lead to fear of loss of control and overeating during non-restricted days 

Side note: From an evolutionary perspective, the “high” feeling or euphoric state some people describe from fasting is likely an adaptive response so that if we were trying to survive as a species we would be motivated to continue to seek out food.

Yes, I am focusing on the adverse outcomes because almost every article written about IF addresses the positive benefits without bringing any of this up. I also recognize, as a non-diet dietitian that I am biased. However, any claimed “benefits” of IF have not been proven long term. Again, even if we knew without a doubt that IF was beneficial to our health – at what cost are we willing to fast and ignore hunger? All too often we ignore mental health in the name of weight loss/changing our bodies in hopes of living a better life. But is it then really life enhancing?

Proponents of IF can be very convincing. But really, it’s another trend promoting disordered eating under the guise of wellness. Bottom line, each version of IF involves ignoring hunger cues – if you’re hungry at 10 AM but the IF schedule says you can’t eat until 12, that’s a rule. Ignoring your bodies internal clock and signals never lead to a good place in the long run. Diets are sexy and glamorous. Listening to your hunger/fullness cues and tuning into your body’s needs isn’t exactly exciting, glamorous or sexy to most people. Rules and restrictions like this give us something to focus on, it’s another distraction/coping mechanism in this highly chaotic world. And while the majority of people will claim it’s for their “health” (I did), it’s really much more than that. Because we KNOW that you can do SO many good things to nourish your mind, body, and soul that have nothing to do with dieting or food rules. Health is so much more than food.

We typically only hear about the benefits of intermittent fasting, we rarely hear anything about potential adverse outcomes. For example, this 2016 meta-analysis showed acutely elevated plasma cortisol levels following fasting. Cortisol is a stress hormone and while our levels fluctuate throughout the day, having increased levels means that the body is under too much stress. And stress is far worse for our health than eating outside of a 12 hour window. And and for us females, stress is a big contributor to irregular or missing menstrual cycles (more on that here).

In conclusion, we don’t have enough evidence to use/recommend fasting for health, weight loss, longevity, or anything else, in any population.

If you want to learn more, Meg and Victoria at Nourishing Minds Nutrition did a great podcast episode on Intermittent Fasting (Episode 38), check it out!

Have a lovely weekend!

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References:

Benjamin D Horne, Joseph B Muhlestein, Jeffrey L Anderson; Health effects of intermittent fasting: hormesis or harm? A systematic review, The American Journal of Clinical Nutrition, Volume 102, Issue 2, 1 August 2015, Pages 464–470

Harvie M, Howell A. Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects—A Narrative Review of Human and Animal Evidence. Sainsbury A, Luz F, eds. Behavioral Sciences. 2017;7(1):4.

Krista A Varady, Marc K Hellerstein; Alternate-day fasting and chronic disease prevention: a review of human and animal trials, The American Journal of Clinical Nutrition, Volume 86, Issue 1, 1 July 2007, Pages 7–13

Kumar S, Kaur G. Intermittent Fasting Dietary Restriction Regimen Negatively Influences Reproduction in Young Rats: A Study of Hypothalamo-Hypophysial-Gonadal Axis. Mezey E, ed. PLoS ONE. 2013;8(1):e52416. doi:10.1371/journal.pone.0052416.