Intermittent Fasting: What Does the Research Say?

Intermittent Fasting: What Does the Research Say?

By: Kelsey Pukala, dietitian at Nourishing Minds Nutrition

Before I dive in to discussing what the research says about intermittent fasting, I think it’s important to define it.  To do that, I actually Googled “Intermittent Fasting,” which is something I thought I would never EVER do. But here we are.  According to the Harvard School of Public Health, “Intermittent Fasting (IF) is a diet regimen that cycles between brief periods of fasting, with either no food or significant calorie reduction, and periods of unrestricted eating.”

If this sounds like a diet to you, you’re right on!  Even though it’s buried beneath “wellness” and “lifestyle changes,” it’s still an eating pattern with rules and regulations on when to eat and when to stop.  And we already DO know from research that diets don’t work long-term and aren’t sustainable. But, let’s see what the research says about IF.
There are several different types of intermittent fasting and here are the 3 most common:

  1. Alternate-Day Fasting: Alternating between days of no food restriction with days that consist of one meal between 500-600 calories.

  2. Whole-Day Fasting: 1-2 days per week of complete fasting or 500 calories/day with no food restriction the other days (also known as the 5:2 diet)

  3. Time-Restricted Feeding: Following a meal plan each day with a designated time window for fasting (i.e. you can eat within an 8 hour window and then fast for the remaining 16 hours).  In my experience, this seems to be the most common.

Because there are several different types of IF, dissecting the research is difficult.  The definition varies depending on the research. A few other things to note: first, most of the IF studies also involve calorie deficits, second, the majority of studies have been done on rats, men and very specific populations and third, effects of following an IF diet have not been studied long term!  This is similar to the research we see on weight loss… possibly “successful” in the short term, but about 98% of people gain the weight back (plus more) in the long-term.  With IF, we only have short-term research available.

It’s always a good idea to be critical of the studies/articles/blogs you read.  It’s frustrating to see catchy headlines with fear-mongering titles because science and research studies are rarely black-and-white, despite what our culture says.  Everyone is different. There is no perfect diet. Everyone’s body is different and will require different amounts of macro and micronutrients. We all have different genes!  

Diet culture and the media make it difficult to sift through information and find what is true.  If you feel swayed by particular eating patterns, some questions to think through are: what are my intentions in following a particular eating pattern?  Does this align with my values? What does health mean to me? How is my physical, emotional, mental and spiritual health?

I found a very recent overview of IF when looking through PubMed and decided to summarize it for you.  PubMed is a public site, so if you love reading research articles, you can visit and search for various topics and then be overwhelmed with the number of studies that populate!

Intermittent Fasting in Cardiovascular Disorders – An Overview (2019)

  • Periodic fasting led to decreased blood pressure (this data pulled from an observational cohort study (not as reliable as Randomized controlled trials) and performed in a highly controlled environment)

  • No differences observed with cardiovascular diseases

  • Benefit for insulin levels is inconsistent and studied for only 6 months, most studies in rats

  • Potential for weight loss (remember, there is also caloric restriction)

  • Some of other things noted by the authors in subjects who followed IF diet:

    • Bad mood

    • Fatigue

    • Dizziness

    • Use of ketones for energy instead of glucose (our preferred energy source)

    • “Certainly this is not a good diet for patients with reactive hypoglycemia” – the authors

    • Caloric restriction with the simultaneous use of diabetes medications may lead to severe hypoglycemia and even death

    • In the elderly, IF is associated with cardiovascular disease, arrhythmia and stroke

    • Fluctuations in glucose concentration cause instability of the body

    • Higher risk of diabetic ketoacidosis especially when there is not enough insulin due to low food intake during fasting

    • “Excessive restriction of calories” in IF causes dysregulation of hormone management.  Such disturbances may cause menstrual cycle disorders in women and reduced testosterone in men.”

The authors of this review do note that fasting may be dangerous for people with hormonal imbalances.  I would argue that Intermittent Fasting can absolutely LEAD to hormonal imbalances, disordered eating/eating disorders, food obsession, and GI issues.

Even if intermittent fasting could have some benefits for specific people, the question becomes, what are you willing to sacrifice in order to eat in a particular way?  Mental and emotional health should also be considered, as they are parts of health. I never want to discount anyone’s lived experience with following particular eating patterns.  I do think it’s important, however, to ask yourself what your intentions are. If IF (or any other diet) came with a disclaimer that it only potentially “works” long-term in 2% of the population, would you still try it?

As a biased, non-diet dietitian working mostly with women who experience hormone imbalances, digestive issues, disordered eating and eating disorders, I honestly think that IF will only perpetuate ALL of those things.  It’s just another trendy diet with a bunch of rules pulling you further away from tuning in to your body. It will only cause a greater level of distrust between you and your body. AND if you currently have, or previously experienced, an eating disorder or disordered eating, I would not recommend following an IF diet (or any diet).

You will rarely hear about the side effects of IF in the media.  You will only be bombarded with the purported benefits. Try to remember that!

I put IF in the “diet” category because it’s just another way of eating that is external to your body.  On the other hand, eating intuitively can help you tune into what your body needs, and then honoring that without guilt!


Malinowski et al; Intermittent Fasting in Cardiovascular Disorders—An Overview.  Nutrients, Volume 11, Issue 3, 20 March 2019.

Thank you so much for this blog post Kelsey! You can find out more about Kelsey on our about page and reach out to work with her here.

Want to learn more? Listen to Episode 75: Intermittent Fasting With a History of Disordered Eating of the Nourishing Women Podcast

Tell us: What do you think of Intermittent Fasting?

Victoria Myers