Nutrition Hot Topic: Intermittent Fasting

Intermittent fasting (IF), along with many other diets, has been a growing trend and hot topic in the nutrition world. Although fasting itself is not a new phenomenon, IF has taken on many different meanings and forms of restriction - with an ultimate goal of weight loss. As a Registered Dietitian, my role is to look into these hot topics by presenting evidence-based research and a scientific perspective. If I am providing information that is very much my opinion, I will try to make that clear. Below I will get into the research and reality of Intermittent Fasting - and my personal opinion on the topic.

**Disclaimer - if you have a history of an eating disorder or disordered eating, this content may be triggering. Below I am discussing research based off a popular diet trend - with an intention to educate but also warn that diets like these can contribute to EDs. I try to be mindful of the language used but can be hard if still working towards recovery or on your relationship with food!

Bottom Line Up Front:

  1. Most research on IF has been limited and is based on animal studies. Further human research and specific measures are still needed.

  2. Overall research has found weight loss to be associated with IF, but isn’t much different than other weight loss diets (like general caloric restriction). Many animal studies favor changes in insulin sensitivity and heart health as well (mostly in men).

  3. There are sex-based differences that need to be considered. Women biologically react to stress differently and research is needed on IF and it’s effects on women’s hormones and menstrual cycle.

  4. Some studies have ALSO shown increased distraction, fatigue, and feelings of hunger on restricted days. In my opinion, IF will likely lead to more preoccupation with food, more disordered thoughts, and bingeing on the “normal” eating days or eating window.

  5. My personal opinion is this is still a diet - and a restrictive way to ignore your innate body signals of hunger and satiety. It is based on the individual and for some may work, others may definitely not.

  6. I would not recommend IF for: anyone with a history of an eating disorder or disordered eating, those who are pregnant/breastfeeding, and diabetes patients on medications, or anyone that has increased energy needs.

so What is intermittent fasting?

There are several types of the term “Intermittent Fasting” - which can get a bit confusing as they are not always clearly distinguished in the research, yet are very different. Regardless of the type, each diet is aimed to restrict calorie/energy intake for a certain amount of time with the goal of weight loss, increased insulin sensitivity, and greater longevity. Here’s a breakdown of the major types of IF:

  1. Eating window of 6-8 hours with a 16-18 hour fast - majority of this fasting takes place over night. Seems to be the most common

  2. 24 hour fasts - with this method a fast is done for 24 hours, one or two times per week

  3. 5:2 fasting - 5 days of normal eating and 2 days of the week calories are restricted to about 25% of calorie intake (for men only 600 calories and women about 500 calories!!!!!!

Purported benefits: weight loss, increased insulin sensitivity (decreasing fasting insulin levels), increased memory/learning ability, decreasing inflammation, and increased longevity.

breaking down the research

  • Major points to notice:

    • Most studies that show beneficial effects were conducted on rats/animals instead of humans

      • In a 2015 review - results showed the vast majority of fasting research has been on animals, and evidence in humans of health improvements from fasting is preliminary

    • Most studies are conducted on men or on specific subject groups (pre-diabetes or at risk for diabetes, overweight/obese subjects, etc) and we are lacking research on women and the effects on hormones/menstruation with restriction

    • Many studies are short-term and do not look at long-term effects of IF and restriction (or how long these subjects kept up with IF)


    • In a study conducted on men with pre-diabetes, results showed that despite no changes in weight, the group who ate in an 8-hour window (7am-3pm) showed lower insulin levels, improved insulin sensitivity, and lower blood pressure. 

    • In a study that looked at overweight/obese subjects using IF or general caloric restriction, as well as normal weight subjects (categorized by BMI) and the effect of IF —>

      • In overweight/obese subjects, weight loss of 4-5% was found in 6 different studies and all of these studies were relatively small. Research in normal-weight subjects and weight loss or prevention of weight gain was lacking.

      • Results from 2 studies on normal/overweight subjects using alternate day restriction showed sustained hunger with IF and difficulties maintaining daily living activities during the restricted days

      • Results in normal weight women showed increased feelings of hunger, worse mood, heightened irritability, difficulties concentrating, increased fatigue, eating-related thoughts, fear of loss of control and over eating during non-restricted days

    • A 2018 review of both animal and human studies and comparison of different types of IF

      • In general, showed that any intermittent fasting regimen can result in some weight loss.

      • However a study showed that with an extended fasting overnight (and omitting breakfast), participants were hungrier at lunchtime, had higher plasma levels of acetylated ghrelin (a hunger hormone), higher postprandial glucose and insulin levels compared with their levels on the breakfast day, though they did not eat more calories at lunch.


  • Any form of restricting our intake or manipulating our hunger and fullness cues is a “diet.” As mentioned on previous posts, restricting our intake and fighting against our innate signals to eat will backfire and put our body into a starvation mode with a fight or flight response (increased cortisol!).

  • Likely if studies were long-term, we would see weight re-gain and/or increased preoccupation, obsession, and thoughts around food. The more we restrict, the more likely we are to binge on the days or time we allow ourselves to eat!

  • There are sex-based differences in men and women. Particularly with women, fasting or a stress response on the body could cause disruption in our hormones and menstrual cycle.

    • One study showed that restriction initially increases cortisol, a stress hormone. Increased stress in the body is far worse than eating 200 extra calories in a day! Increased stress can also lead to an altered or missing menstrual cycle. Great info on these topics on The Real Life RD’s blog, linked here.

  • The social aspect of IF can be negative. If you and your friends had a late dinner plan, and it is on a fasting day or outside of the eating window, how would this affect you? Would this create more stress/guilt/shame? Health is WAY MORE than just weight and diet - our mental and emotional health is extremely important.

Okay… but what if I am someone who usually isn’t hungry in the morning - what should I be doing?

  • If you are not hungry/used to eating in the morning, and don’t find that this causes you to eat more later in the day, then that is absolutely okay! It is truly the intention of restriction and dieting that can be harmful and cause changes in our psychological and physiological response to hunger.

  • Any style of eating is very individualized. What works for your friend in the office or family member probably won’t work for you. If this works for you and you find no negative changes in your thoughts/feelings around food, energy levels, or eating patterns - I won’t pose any judgment. However, I do recommend working with an RD to make sure you’re still getting adequate nutrition in any form of diet. 

Other good, RD resources on the topic: here, here, and these podcasts here and here!


  1. Horne, B., Muhlestein, J., Anderson, J. (2015). Health effects of intermittent fasting: hormesis or harm? A systematic review.

  2. Harvie, M., & Howell, A. (2017). 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.

  3. Sutton, E., Beyl, R., Early, K., Cefalu, W., et al. (2018). Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes.

  4. Patterson, R., Sears, D. (2018) Metabolic Effects of Intermittent Fasting.