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Is intermittent fasting healthy? The science behind the fad

Intermittent fasting has become more popular recently. But, is intermittent fasting healthy? This article features the latest research and expert advice.

Intermittent fasting has become a popular way of eating over the last few years. It’s been championed by various medical doctors, internet gurus and celebrities. However, research on the topic is limited. We’ve looked at the latest research and science and asked some industry experts for their thoughts in a bid to answer the question: Is intermittent fasting healthy?

What is intermittent fasting?

Several types of fasting exist. However, intermittent fasting (IF) and calorie restriction are the most widely used. IF involves cycling between periods of eating and fasting. There are various types of IF and these can be split into three categories:

  • Alternate Day Fasting
  • Whole Day Fasting or Periodic Fasting
  • Time-restricted fasting

Time-restricted fasting (TRF) has received the most attention within fasting literature. You’ve probably heard of some, with the 16/8 method and 5:2 diet both being widely adopted. However, there are others including Eat-Stop-Eat, Alternate Day Fasting and the Warrior Diet.

According to nutritional therapist and exercise physiologist, Ian Craig, IF has been scientifically claimed to improve several health indices. These include insulin sensitivity, weight management, inflammation, oxidative stress, cardiovascular and the detoxification of cellular waste. Fasting is also used for weight loss, chronic disease management and to increase longevity (Horne et., 2008).

Is intermittent fasting here to stay?

We’ve asked some industry experts for their thoughts on intermittent fasting. It’s definitely become more popular over the last few years, but is it just a fad? Or, is it here to stay?

Science and Development of Muscle Hypertrophy author, Brad Schoenfeld gave us his opinion on the topic:

Brad Schoenfeld - intermittent fasting healthy

“It’s not a fad, but it’s not a panacea either…”

Brad Schoenfeld

… at least from a weight loss standpoint. It’s simply a way to manage energy intake. By limiting the “eating window” to a few hours in a day, it basically forces a person to consume less calories. Energy balance is ultimately what drives weight loss; if you take in fewer calories than you expend, you’ll lose weight. Some people find it easier to achieve this goal by IF. Whether there are any health-related benefits to the strategy over and above weight loss remains to be determined.

Advanced Sports Nutrition, 3rd Edition author, Dan Benardot had a different opinion on the topic. As well as, overall thoughts on nutrition trends and fads in general:

Dan Bernadot

The two basic principles I resort to when asked questions about nutrition trends and fads is to ask: “What does the science say?” and “Does it conflict with what we know about human physiology and nutritional biochemistry?”. When asked how I ‘feel’ about trends and fads my response is that I have no ‘feeling’ about it. This is not about how I feel but, rather, how this trend/fad fits within what we ‘know’ about the science. If a trend or fad appears to fit within what we know about human nutrition/physiology/biochemistry, then I have no problem saying so as I have no emotional attachment to it one way or the other. The inverse, of course, is also true. So, if a trend or fad fails to satisfy what we know the science says, then I have no problem saying so.

Skipping meals

When trawling the internet for information on intermittent fasting, one thing that always seems to come up is skipping breakfast. Several articles have suggested that skipping breakfast would allow you to fast for the 16 hours required for the 16/8 method. However, these statements can be misleading.

In an explanation given to us from author Dan Benardot, he mentions there are some studies which have indirectly assessed skipping breakfast. Studies have found that adults who skip breakfast have significantly higher waist circumferences and high Body Mass Index (BMI). Interestingly, it has also been found that those who have skipped breakfast as children and adults, have even higher waist circumferences and BMI. Plus, they are at higher risk of cardiometabolic disorders (Smith et al., 2010; Isacco et al., 2010). Apparently, there’s no adaptation to continuously doing the wrong thing.

Brad Schoenfeld told us:

From the research I’ve seen, there is nothing inherently ‘special’ about any given meal.

Brad Schoenfeld

Brad also added that some people seem to eat less over the rest of the day when they eat breakfast. Plus, some seem to perform better physically and mentally. However, he’s keen to add that these are individual responses that don’t necessarily translate for all.

Should athletes be intermittent fasting?

Author of Nancy Clark’s Sports Nutrition Guidebook, Nancy Clark gave her take on athletes doing intermittent fasting:

Nancy Clark

To date, there is little evidence to support intermittent fasting as a good way to improve an athlete’s health or performance (Lis, 2019). For example, how can you follow the recommendation to eat 20 grammes of protein every 3 to 4 hours during the day to optimise muscle growth if you have long spans of time without eating? (Churchward-Venne, 2012).

Fasting leads to loss of muscle tissue; an unfavourable change for an athlete.  Fear of “running out of energy” could lead to over-consuming calories; the athlete might feel the need to do “last chance eating” (this sometimes escalates into binge-eating).

Some athletes might say that “training on empty” is a good way to force adaptations that encourage fat-burning. This thought can get clouded by the increased risk of injury (if training with low blood glucose), or higher risk of illness (if fasting compromised the immune response). Male athletes who spent time being more than 400 calories in deficit experienced higher cortisol levels and lower testosterone levels than athletes who maintained adequate calorie intake throughout the day (Torstveit et al., 2018). This is counter to the desired hormone profile.

Is intermittent fasting healthy?

What does the latest research say?

A recent article published in the International Journal of Sport Nutrition and Exercise Metabolism looked at the various dietary practices used by track and field athletes. Amid eating challenges, the primary goal for track and field athletes should be to maintain body composition as well as to minimise decrements in performance. Due to the variances in food availability when fasting, there is the potential to overeat at fewer meal times. This can make it difficult for some athletes to maintain stable body composition. Evidence to support any benefit of fasting compared to conventional techniques for improving body composition in track and field athletes is lacking.

Can IF affect muscle growth?

Research has shown that IF can help you lose fat. For more information on fat loss see our post Decreasing Body Fat – The How to Guide.

However, there are worries that it could also cause muscle loss. We asked Brad Schoenfeld for his thoughts on the subject:

There’s not a lot of research on this topic, but there is a good rationale that IF would not be ideal if the goal is to maximise muscle growth. Food is anabolic. Eating somewhat frequently maintains an anabolic state. If you aren’t eating for prolonged periods of the day (~16 hrs), it would not seem to be ideal for maximising hypertrophy

We also asked him if there was an optimum time to workout whilst doing intermittent fasting:

Since muscle is sensitised to protein following an exercise bout, logic would dictate that the feeding window for someone doing IF would be bracketed around the workout. That said, emerging research shows muscle is sensitised long after the exercise bout, so regardless of when you structure the IF window, there would seem to be a negative effect on post-workout anabolism.

Negative effects of IF on athletes

Dan Benardot, author of Advanced Sports Nutrition, 3rd Edition pointed out that IF can also have negative effects on physically active individuals.

The International Olympic Committee has published positions on Relative Energy Deficiency in Sport with supporting evidence that not having sufficient energy available can result in a host of health problems and lowers athletic performance (Mountjoy et al., 2014; Mountjoy et al., 2018). All of this data strongly implies that intermittent fasting should be avoided by athletes. Plus, by contrast, increased eating frequency to sustain a reasonable good energy balanced state is desirable.

What about intermittent fasting for bodybuilders?

Peter Fitschen and Cliff Wilson’s book, Bodybuilding features information on nutrition for bodybuilders. Historically bodybuilders have eaten more frequent smaller meals daily. In fact, case studies on bodybuilders preparing for competition have reported as many as 10 meals a day (Steen, 1991). However, in recent years intermittent fasting has gained popularity. Competitors have actually claimed success using both approaches.

Numerous studies on meal frequency and weight loss haven’t shown a difference in weight loss when daily calories are matched (Cameron et al., 2010). Furthermore, IF hasn’t been shown to be superior to more traditional meal patterns. Therefore, as mentioned in Bodybuilding, it doesn’t appear that the number of meals consumed throughout the day affects weight loss if calorie intake is low enough to induce weight loss. Your meal frequency should be based on your schedule, preferences and what allows you to stay consistent with your macronutrient intake each day. However, it’s worth mentioning that scientific literature about meal frequency mainly reports on studies done for overweight and obese individuals during weight loss interventions.

As a lifetime drug-free bodybuilder himself, we posed the question to Brad Schoenfeld. He said:

Based on what we know from research, my current position is that anabolism is maximised by eating 3-4 daily meals spread out across the day, with fairly equal protein intakes at each meal. There doesn’t seem to be a benefit of increasing feeding frequency beyond 4 daily meals, although there is no detriment either.

Is intermittent fasting healthy for people with diabetes?

For this section, we asked Athlete’s Guide to Diabetes author, Sheri Colberg to help us out. As a leading expert on diabetes and exercise, she’s perfectly placed to give us insight into the effects of IF on those with diabetes:

Sheri Colberg

I have not looked at all the research on IF related to diabetes. I would surmise that the general IF protocol is the same for all types; people with type 1 just have to worry about balancing insulin intake with food, which most with type 2 do not, so that would be the main difference.

Everyone is being bombarded with claims about the superiority of certain diets to lose weight or be a better athlete. In fact, people have tried to use every supplement or technique to get it. These include caffeine, amino acids, sports drinks, carbohydrate loading and even IF. However, few have been scientifically proven to boost physical prowess.

Athletic individuals with diabetes may have special concerns about the effects of various diets and supplements on blood glucose and performance. As one athletic individual with type 1 diabetes puts it, “I practice intermittent fasting, with my first meal each day generally at about 11:00 a.m. and my last food intake around 8:00 p.m. My carb intake varies daily, and I don’t particularly aim to eat low carb as I eat a lot of fruit, sweet and regular potatoes, and beans.” However, active people can eat more than one way and perform effectively in sports and the ‘best’ way might be different for everyone.

The real goal when you have diabetes is to keep your blood glucose in balance. This means you don’t have to stop exercising or deal with unanticipated glucose lows that can be inconvenient if you’re on a run or a long bike ride and still a good distance from your destination.

Is intermittent fasting healthy?

So, is intermittent fasting healthy? The founder of the Centre for Integrative Sports Nutrition, Ian Craig answered this one for us. He posed the question ‘What is the other side of the story?’. Ian has also presented several nutrition-related webinars for us.

Ian Craig

From a general health perspective, assuming we’re eating nutrient-rich food, the food we might be cutting back on during IF, is the same food that provides the micro-nutrition needed to turn over our vital physiological processes. Ian suggests:

Many of us consume a diet that is relatively low in many essential micronutrients, so if we remove one meal per day during an IF regime, we’d better make very good dietary choices at the other sittings!

According to Ian, many athletes eat extremely processed, sugary and inflammatory diets. Therefore, in these cases, it would be expected that the athlete would make substantial health gains with IF. This is one of the reasons for the diet’s success.

Perhaps we could become even healthier simply by eating better quality food?

Ian Craig

As practitioners working with athletes, it’s important to consider both sides of any scientific or media headline. This advice won’t be static either. IF might not be suitable for an athlete on hard training days, but on non-stimulatory days it might work perfectly.

Dan Benardot gave us his summary:

… the science suggests that there are multiple potential health and performance issues associated with intermittent fasting. It would be good for proponents of intermittent fasting to provide scientific evidence that measures changes in body composition and hormone production (insulin, estrogen, cortisol, testosterone etc.) over a sufficiently long time period to adequately assess the impact of this eating pattern.


Although intermittent fasting is obviously a huge trend, it’s important to remember that these trends aren’t a perfect fit for everyone. Essentially, you need to take time and figure out what’s right for you. It’s also worth remembering that the scientific evidence on intermittent fasting is still relatively limited.

Further Reading

Journal Article: Dietary Practices Adopted by Track-and-Field Athletes: Gluten-Free, Low FODMAP, Vegetarian and Fasting

Journal Article: Effects of Ramadan Intermittent Fasting on Sports Performance and Training: Review

For fitness professionals wanting to know more about intermittent fasting, check out FitPro’s CPD course Intermittent Fasting: Fad or Future?

The Truth about Intermittent Fasting Muscle Gain

Header photo by Ella Olsson from Pexels

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Hi, I’m Hannah, marketing assistant and one of the bloggers here at Human Kinetics Europe. I wasn’t blessed with the coordination to play most sports, but that’s not stopped me becoming a great watcher of them. Particularly when it comes to football! So I’m here to bring you all you need to know about exciting new product releases and the latest in sport, fitness and PE.

1 Comment

  1. Nice! I was aware of a fair amount of research on IF prior through my work with my athletes. But this is a great summary and very well balanced which is rare these days 👍🏽

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