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A few years back, you would have been forcibly ejected from the fitness space if you so much as hinted that Intermittent Fasting was a viable dietary protocol.

At this time, we were given to understand that eating 6 meals per day was the only option if you were serious about improving muscle mass and/or stoking the metabolic fire.

Eating every 2 hours (ideally chicken, broccoli, and rice, or some iteration...) was a non-negotiable.

Fast-forward to today, and the pendulum has swung back quite aggressively.

Intermittent fasting has gained traction as a valid dietary strategy, which is entirely at odds with the ‘little and often’ approach of days gone by.

So - which is it?

Is a diet that offers superior weight loss, better muscle retention, improved health benefits, and reduction in disordered eating, too good to be true?

Or should we be concerned that fasting for long periods of time might negatively affect muscle mass, cause unmanageable hunger, and blunt metabolism?

You’ll hear ‘Intermittent Fasting’ used as an umbrella term, covering fasting strategies from 5:2 fasting, alternate-day fasting, and the most popular: Time Restricted Feeding.

Proponents of Intermittent Fasting will claim superior weight loss, improved cardiovascular health, decreased cancer risk, neuroprotective effects, improved glucose metabolism and lengthened life span as benefits of fasting.

Which admittedly sounds very attractive!

It’s not entirely understood why these advantages might occur, but one prevalent theory is that periodic food deprivation serves as a preconditioning stress, improving one’s capacity for resistance to stressors in the future.

Before signing on the dotted line, though, it’s important to note that these benefits are mainly observed in research conducted on rats,¹ and largely resulting from alternate day fasting strategies as opposed to Time Restricted Feeding.

Time Restricted Feeding is by far the most common form of Intermittent Fasting, widely popularised within the fitness industry.

This form of Intermittent Fasting covers anything for a 16-20 hour fasting window with a 4-8 hour eating window.

The most prevalent form of Time Restricted Feeding is a 16-hour fasting window with an 8-hour eating window.

Practically, this usually means simply skipping breakfast, consuming all of your daily calories between the hours of 12:00 - 20:00, or thereabouts.

You can, of course, have your eating window fall wherever you’d like should you choose to adopt Time Restricted Feeding - the breakfast skipping simply seems to offer the most practical timeline.

Understandably, the primary concerns with shorter eating windows followed by long fasting windows have to do with muscle retention, metabolism, and dietary adherence.

A recent meta-analysis (a study of studies) explored body composition changes for subjects who followed intermittent fasting vs continuous caloric restriction in subjects engaging in resistance training.²

From the studies including young/active subjects participating in regular, rigorous resistance training, we can conclude that 16:8 Time Restricted Feeding supports both strength and muscle gains, provided daily protein intake is adequate.

Results actually indicated that not only did intermittent fasting result in greater weight loss and fat loss - crucially these strategies did so without negatively impacting muscle mass.

With reference to muscle mass, it seems that so long as you're able to consume 3-4 boluses of protein during the eating window, your muscle and strength gains are safe.

This tallies with previous literature, which indicates that the most advantageous outcomes for muscle mass occur when we combine resistance training with protein spread evenly across ~4 meals throughout the day.³

If protein intake drops below the optimal 1.2g per lb (1.6g/kg) of body weight,⁴ however, muscle mass is at risk.5

With reference to superior fat loss outcomes for fasting subjects...I think it's safe to conclude that the fasting subjects in this meta-analysis were able to indirectly reduce their calorie intake by limiting their eating window, resulting in a greater weight/fat loss than the subjects following a conventional (non-fasting) dietary protocol.

This is perhaps a surprising finding; a common concern regarding fasting is increased hunger resulting from a period of abstaining from food.

Research has actually indicated that fasting can have an appetite blunting effect.⁶

Anecdotally, there’s usually a period of adjustment. You typically get hungry when you’re used to eating, so you may need to allow time to adapt to the new eating window before deciding whether or not it’s a good fit.

Intermittent Fasting should only be considered as an alternative to a continuous caloric restriction if it does not present issues with food focus.

Although fasting has been shown to improve disordered eating,⁷ suitability should be assessed on a case-by-case basis. If you find yourself with increased food focus during the fasting window, it is perhaps not the approach for you.

These are the three caveats to take into account if you’re considering trialing Intermittent Fasting:

1. It is advisable to train in a fed state, IE during your eating window, not your fasting window if possible. This allows for adequate energy for and recovery from your resistance training session.

2. You should set protein intake at 0.8 - 1.2g protein per lb bodyweight (~1.6g per kg bodyweight) to allow for muscle recovery and development.

3. Like any dietary strategy, Intermittent Fasting should only be undertaken if it allows you to better adhere to your calorie deficit. If breakfast is your favourite meal of the day, breakfast-skipping probably isn’t a long-term solution!

Assuming it’s an approach that you do get on with, though, it’s certainly a more-than-safe option for a resistance-trained individual.

This strategy offers lifters the opportunity to consume 3-4 boluses of protein throughout their eating window, and to train in an adequately fed state.

According to the current body of research, we can let go of the notion that we must eat every 2 hours in order to achieve progress in body composition.

On the contrary, it seems as though Intermittent Fasting can offer superior body composition outcomes when compared to continuous caloric restriction. 

About the author Charlotte Tulloch

Lottie is a UK-based online coach who works with clients all over the world and is passionate about sharing evidence-based content with the view to making the fitness space less of a minefield. Lottie specialises in strength training, improving body composition, and cultivating a balanced approach to exercise and nutrition.

Instagram @LottieLifts

Citations

¹Wan et al (2003), Ahmet et al, (2005), Goodrick et al (1990)
²Ashtary-Larky et al (2021)
³Areta et al (2013)
⁴Tinsley et al (2017)
⁵Tinsley et al (2019)
⁶G. Duncan (1963)
⁷Hoddy et al (2015)