Ask the RD: Is Intermittent Fasting Good or Bad?


Intermittent fasting, or IF, is a timed approach to eating that has gained popularity in the last few years as a weight-loss strategy. There are many different types of IF with various periods of fasting and non-fasting, including the 16/8 or 5:2 methods. The former involves eating for 8 hours during the day — for example, 10 a.m.–6 p.m., and fasting the other 16 hours. The latter focuses on “normal” eating five days out of the week and eating minimally — 500–600 calories or less — on the other two days. IF is also the subject of research studies on various types of cancer, metabolic disease and diabetes due in part to the effect fasting may have on improving insulin sensitivity and facilitating the body’s usage of stored fat for energy.

Here, a look at the science behind IF, what the research says in terms of weight loss and how to tell if it’s right for you.


The concept of time-restricted feeding, like the 16/8 method mentioned above, began due in part to the intricate relationship between circadian rhythms and the feeding/fasting cycle. Circadian rhythms are daily 24-hour rhythms that regulate metabolism, physiology and behaviors not only under light or dark conditions, but are also related to food intake. When they are disrupted, individuals typically have an increased risk of metabolic disease and diabetes. Research on how fasting (and how much fasting) can promote a more robust circadian rhythm is still very much developing, but some animal studies suggest the benefits may also include improved sleep and longevity.


Although there are some animal studies showing possible weight-loss benefits of IF and caloric restriction, the studies have not been replicated on humans. While there is no shortage of anecdotal stories and individual opinions, there is little scientific evidence-based research supporting claims that IF, and what type of IF, is beneficial for long-term weight loss or general health to date.


If you’re struggling with your relationship with food, adapting any restrictive diet can compromise mental and physical health. Intermittent fasting may often involve ignoring hunger cues in favor of watching the clock and dealing with feelings of prolonged hunger or semi-starvation. This can trigger certain individuals to become fixated on food and to overeat once food is reintroduced at the “appropriate” time.

Certain medical conditions and behaviors are also contraindicated with an eating pattern like IF, and you should always check with your doctor first before trying it. IF should not be attempted if you:

  • Have poorly controlled diabetes and/or take medication for diabetes
  • Have a history of disordered eating or eating disorders
  • Are pregnant or breastfeeding
  • Are trying to conceive
  • Are an endurance athlete
  • Are taking medication that relies on food for absorption


When implementing any dietary change into your lifestyle, the most important thing to ask yourself is “Can I do this for the rest of my life?” If you’re trying IF or any diet as a means to an end to change your body size or number on the scale and are struggling to get through each day, chances are it’s not going to last very long (and neither are any perceived benefits). That’s because diets with overly strict rules tend to have detrimental long-term effects on health such as weight cycling, cardiovascular risk and disordered eating.

The best diet for any individual is one they can sustain long term, that helps them feel good in their bodies and allows for a variety of nourishing, satisfying foods without restriction. If IF in some form fits that bill for you, wonderful. If it doesn’t, which is the case for many people, that’s OK, too. There are countless ways to eat that can fit within individual lifestyles, schedules and dietary preferences. If you need guidance for where to start, work with a registered dietitian.

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