Is Intermittent Fasting Healthy?

Single black olive and parsley on the middle of a plate, knife and fork.

We’ve always been told to eat three square meals a day. Or that going too long without eating will slow your metabolism. But what if it turned out that fasting for part of a day  or even a whole day might actually have health benefits AND help you lose weight? Intermittent fasting has been a hot topic lately so I decided to do a little research. There are several different kinds of intermittent fasting. Read on to learn more about it, as well as any potential benefits and the pros and cons.

First of all, fasting isn’t new. People have fasted for religious reasons for centuries. Examples include Ramadan and Yom Kippur. But the fasting craze really took off in 2013 with the launch of the book The Fast Diet by Michael Mosley and Mimi Spencer. This book touts the benefits of restricting energy intake severely for 2 days a week (500-600 calories) and eating normally the other 5 days.  This is known as 5:2 fast. There are several other types of intermittent fasting. See below.

Different types of intermittent fasts

Complete alternate-day fasting These regimens involve alternating fasting days (no energy-containing foods or beverages consumed) with eating days (foods and beverages consumed ad libitum)
Modified fasting regimensThese regimens allow for the consumption of 20% to 25% of energy needs on scheduled fasting days. This is the basis for the popular 5:2 diet, which involves severe energy restriction for 2 nonconsecutive days a week and ad libitum eating the other 5 days. For example, two days a week you limit yourself to less than 500 calories if you are a female and less than 600 calories if you are a male. The other days you eat as you normally would
Time-restricted feedingThese protocols allow individuals to consume ad libitum energy intake within specific windows, which induces fasting periods on a routine basis. For example, the fast may occur for 12-20 hours a day. So you might only eat between 8 am and 8 pm or between noon-4 pm.
Religious fasting A wide variety of fasting regimens are undertaken for religious or spiritual purposes


Are there health benefits?
You know how your doctor tells you to fast overnight before having a morning blood test? E
ven this short fast overnight fast can have a favorable impact on your body.  Lab values associated with chronic disease such as blood sugar, insulin, triglycerides tend to be lower after an overnight mousefast versus after a meal. So what could happen in the body with longer or more regular fasting? Could intermittent fasting help decrease risk of cancer, heart disease and diabetes – as well as aid in weight loss? More research is needed … but the answer is possibly. The problem is that most of the research has been done on animals and very little done on humans. But I did find a great review article on fasting and humans  in the Journal of the Academy of Nutrition and Dietetics: Intermittent Fasting and Human Metabolic Health , August 2015. The article looked at health outcomes in animals and humans including effects on weight and metabolic parameters associated with type 2 diabetes, cardiovascular disease and cancer. But again, keep in mind that the following health outcomes are based on very few studies in humans.

Health benefits of various types of intermittent fasts: 
– Alternate day fasting:
In animals: associated with weight loss, decreased glucose, insulin, cholesterol and triglycerides, beneficial effect on cancer risk factors.
In humans: modest weight loss and some improvements in metabolic parameters. However, self-reported hunger on fasting days was considerable and didn’t decrease over time. This is obviously a problem!

– Modified fasting regimens:
In animals:
associated with weight loss, decreased visceral fat, reduced fat cell size,

In humans: associated with weight loss, modest and mixed effects on glucoregulatory markers (insulin and glucose), lipid levels and inflammatory markers. However, there is little evidence to suggest that this type of fast is superior to standard energy restriction when it comes to weight loss.

– Time-restricted feeding:time to eat
In animals: associated with reductions in weight, cholesterol, triglycerides, glucose, insulin, interleukin 6, and improvement in insulin sensitivity. Animals whose fast occurred in the evening versus the day had increased health benefits.
In humans: not many studies done. One study was associated with weight loss, decreased glucose and cholesterol.  As with animals, limited research suggests increased health benefits when the eating occurs during the day versus eating. So the time you fast plays a role. This knowledge could curb night time snacking!

Fascinating research
So it sounds like there may be some health benefits to intermittent fasting. But this is just the tip of the iceberg. There’s exciting research being done in the area of intermittent fasting, the time we eat, our circadian biology (our sleep-wake cycle) and health. Hint: eating your calories during the late evening hours and into the night is not a great idea!

Another fascinating area of research is how intermittent fasting may affect our gut microbiota. We are learning more about how our gut microbiome plays
 a huge role in our overall health as well as our weight. Too complicated to get into in this post, but VERY cool stuff.


Is intermittent fasting right for you?
At this point, I’m not routinely recommending intermittent fasting for my clients as there isn’t enough research on it. If it does prove to be beneficial, we would need to know what technique works best (i.e. how often and how long to fast). However, I have had some clients who do the 5:2 as well as time restricted fasts and report positive results. If you are thinking about it, here are some pros and cons:Sleeping on the Job
– People with certain medical conditions should not fast. Examples, people with hypoglycemia or  diabetes on insulin

– If you don’t feel well when you go too long without eating, this is not for you! Some people need to eat snacks every few hours to feel their best
 – If you have an eating disorder (or disordered eating ), this is not the best plan for you
– Beware of it bringing on the “gorge then fast” mentality. Gorging is not healthy physically or mentally
– If you are an avid exerciser most days of the week, you will likely find that having 500 calorie days won’t sustain your workouts

– It is likely that giving the body  rest from eating (aside from the time we sleep) has some health benefits. So if you wanted to “dip your toes in the water”, I would suggest starting with a time-restricted fast.  Maybe start with a 12 or 14 hr fast.

– It can be a way to curtail eating after a certain time and help you lose weight. Most of us snack into the wee hours of the night. Going on a 12 hr fast could help you close the kitchen at 8 pm (if you’ve had breakfast at 8 am) and cut out excess calories at night
Man tosses his crisps in shock– If nothing else, putting yourself of a time-restricted fast of even 14 hours (9 am – 7 pm) could  prevent heartburn and help you sleep better.
– But don’t get any ideas of fasting during the day and gorging all night! Most of the studies showed that eating the majority of your calories into the night may have a 
negative effect on your circadian biology and lead to potential health problems.
– The 5:2 fast may help with weight control for those people who want something simple and don’t want to restrict themselves daily. But pay close attention to how you feel on those 500-600 calorie days and see if it leads to overeating on the other days (many studies done revealed this did not lead to a significant increase in calories on the other days). You don’t want to trigger a gorge/starve mentality. Now keep in mind that the standard weight loss diet of d
aily calorie restriction also leads to weight loss and improvements in metabolic parameters – so you don’t need to fast to get those benefits.  However intermittent fasting may require less thought for some people. 

Bottom line,
more research needs to be done in the area of intermittent fasting before it is recommended as a tool to improve your health. We need to learn what type/length of intermittent fasting is best … and if the benefits actually pan out in humans. While intermittent fasting isn’t for everyone, if you decide to give it a try, make the most nutritious food choices possible. And pay attention to your body – if you don’t feel well, go back to your usual diet or at least modify what type of fast you choose. But for now, my first recommendation for health, is a nutritionally balanced diet!  

Have any of you tried any kind of an intermittent fast? What are your thoughts on it? 


Today’s Dietitian: Intermittent Fasting: The Key to Long-Term Weight Loss?

Scientific American:  How Intermittent Fasting Might Help You Live a Longer and Healthier Life

Canadian Medical Association Journal: Intermittent fasting: the science of going without

Journal of the Academy of Nutrition and Dietetics: Intermittent Fasting and Human Metabolic Health 









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I especially love problem-solving, whether it’s helping women defeat issues plaguing them for years, helping a busy executive find practical ways to get heart healthy, or providing tips to help you reverse diabetes. That’s why I’m on a constant quest to expand my knowledge by staying on top of the latest research.

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