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  • Michelle DuVall

What is all this “Intermittent Fasting” About?

Updated: Mar 15, 2022

Just by doing a quick Google search for Intermittent Fasting (IF) this morning, I was able to find pages and pages and article upon article of information. Now, that doesn’t mean that everything I found was valuable. As a future dietitian, I want to make sure that I present and weigh in on science-backed, evidence-based practices. So let’s dig a little bit into intermittent fasting. Is it a magic solution, or perhaps a short-lived fad?

Let’s start by talking about what intermittent fasting is. There are a few different “versions” of Intermittent Fasting:

-Alternate Day Fasting: This is when someone alternates days of fasting with days of eating. On days where someone fasts, they still typically drink water or other non-caloric beverages. They just don’t eat food or drink beverages with calories in them. On the non-fasting days, they typically eat whatever they want to eat; however, some “fasters” will adhere to certain dietary patterns on their non-fasting days (e.g. Mediterranean Diet, Low-Carb, Keto, “Dirty Keto”, Whole30, Paleo, High-Protein, High-Fat, etc.) This plan may or may not result in weight-loss depending on what the person is eating on their non-fasting days. It could result in a calorie deficit each week, or someone could “make-up” the calorie deficit on days where they eat if they go crazy overconsuming on those days.

-Modified Fasting: This plan can vary. Some people may eat their normal intake amount 5 days a week and then “fast”, restricting themselves to ~500 calories per day (or less) 2 days a week (typically referred to as the “5:2 fast”). Others may alternate where every other day is a modified (or low-calorie fast), and the other days are the normal intake amount. Again, the results are mixed on whether or not people lose weight on this type of fast. Safety may also be a concern (more on that later).

-Time-restricted eating (TRE): This plan usually has set “eating” and “non-eating” times or windows. For example, some people may eat for only an 8-hour window during their day and fast for the other 16 hours. It is common for people to set consistent times for their particular pattern (e.g. someone may only eat between 11 a.m. & 7 p.m.). Others may choose a 14/10 timeframe where they fast for 14 hours a day and eat for 10. This method may come more naturally to people who aren’t breakfast eaters or who don’t get hungry until mid-day anyway.

So, where is the science and evidence behind all of this? The jury is still out, unfortunately. There have been some studies that indicate that intermittent fasting might be able to manage weight, help with heart disease, insulin level sensitivity, blood pressure, or even stress management and longevity; however, quite a few of these studies have been done only on animals, or with limited populations in humans. That being said, what can be advised for people? According to one metabolic expert (Dr. Deborah Wexler, Director of the Massachusetts General Hospital Diabetes Center and associate professor at Harvard Medical School), she says, “There is evidence to suggest that the circadian rhythm fasting approach, where meals are restricted to an eight to 10-hour period of the daytime, is effective,” she confirmed, though generally, she recommends that people “use an eating approach that works for them and is sustainable to them.”

People who try intermittent fasting need to be aware of possible side effects as well:

  • Hunger, grouchiness, irritability (AKA “feeling hangry”), mood changes

  • Nutrient deficiencies, especially protein and B vitamins

  • Possible muscle mass decrease

  • Fatigue, trouble concentrating, or changes in sleep patterns

  • Menstrual cycle changes

  • Changes in bowel habits, especially if fiber intake is lowered

  • Becoming obsessed with food, binge-eating on days where you don’t fast, or other unhealthy, negative emotions about food

  • Low blood sugar

Possible reasons NOT to try Intermittent Fasting:

  1. Pregnant and/or breastfeeding women, or if you are having trouble trying to conceive

  2. Children generally should not fast.

  3. People who have uncontrolled diabetes, especially hypoglycemia (low blood glucose)

  4. If you have or are at risk for an eating disorder or eating-disordered behaviors

  5. If you have any chronic health conditions and haven’t discussed fasting with your physician, especially if you are routinely taking any medications

Regardless of whether or not you choose to try intermittent fasting, make sure to discuss with your doctor. There are simply no replacements for eating a well-balanced array of whole, natural foods. Avoid processed foods, added sugars/fats, and refined grains. When you do snack, choose healthy options and be sure to balance what you eat with a good amount of activity (at least 30-45 minutes a day, most days of the week). Maybe, you just avoid eating at bedtime (or after a certain time each night), but you break your fast when you wake up in the morning with a healthy breakfast. Just keep in mind that if something sounds too good to be true, or if the evidence showing its worth isn’t strong or statistically significant, you may just want to wait until there is more information out there before diving into whatever the latest trend is.


Gordon, B. (2019). What is Intermittent Fasting. Retrieved December 14, 2020, from

Monique Tello, M. (2020, February 10). Intermittent fasting: Surprising update. Retrieved December 14, 2020, from

Sutton, E., Beyl, R., Early, K., Cefalu, W., Ravussin, E., & Peterson, C. (2018, May 10). Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Retrieved December 14, 2020, from

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