Healthy fuelling vs intermittent fasting.
Updated: Oct 22
How facts are communicated and how knowledge is presented obviously has a big impact on opinions about what is best. In the nutrition and lifestyle space consensus is less common than you might assume and many niches within this area, including the topic for this blog here: about fuelling and fasting have been represented in a variety of ways, leading to a confusing picture. To a large extent this is because the science we have has changed, but economic and cultural factors have also played a role. From my own point of view, I've always felt that eating when feeling hungry is the important thing, and of course not eating too much and generally speaking eating enough of the right things.
Simple; right? ...
Well, perhaps not, and so let us consider what is backed by current credible science in relation to healthy eating.
What to eat
In my work as a personal trainer I like to refer myself, and my clients back to the PFR* plan which I wrote (to represent the importance of *sufficient protein; more fibre and reducing sugar for a healthy diet), with the intention to clarify what a healthy diet was, in the light of important new evidence from the work of Professor Tim Spector and his team. It is hard to underplay how important this work was, and so I will take a quick detour to signpost this. Please do skip to the paragraph just below beginning 'everyone wanted to know more about their microbiome' if you are less interested in this.
In the PREDICT 1 Study there were papers written and published in prestigious peer reviewed journals that were of huge importance to our understanding of diet and healthy human nutrition. The first paper in 2020 was titled "Human postprandial responses to food and potential for precision nutrition," and focused on how individuals' blood sugar and fat levels spike differently after eating identical meals.
The second paper which was published in early 2021 was then also very famous and made the word 'microbiome' part of our everyday vocabulary with the title "Microbiome connections with host metabolism and habitual diet from 1,098 deeply phenotyped individuals," in this paper, Professor Spector and Dr Sarah Berry's team isolated the 15 "good" and 15 "bad" gut microbes and mapped them to human health. This was huge as it showed how good choices when it comes to nutrition make a really big difference to our health in terms of metabolic health as well as immunity, the second part of which captured the public imagination in a way that was probably unexpected as a result of the timing being whilst we were all in the midst of the Covid-19 lock down's.
Democratisation of nutritional health insights
Everyone wanted to know more about their own microbiome as a result of the PREDICT 1 Study, and so to discover more about their metabolic health Tim Spector et al also developed a programme called ZOE, which allowed ordinary people to find out more, and to be able to do just that in exchange for a modest fee and of course a sample. This enabled the collection of large amounts of data as well as a step change in how individuals could interact with their own health.
The picture for women's health
Around the same time there was also a spotlight thrown on women's health, as Davina McCall and other popular figures called for more consideration around Menopause. There was a growing awareness of the effect of our hormones on everything, as well as how we needed to eat in order to best navigate hormonal changes. The idea of moderating optimal inputs in relation to exercise as well as nutrition by varying the level of effort that was optimal over the course of a woman's monthly cycle was also introduced and popularised. This was called cycle synching. This could be described as being pragmatic about what kind of training worked best at each phase of the menstrual cycle rather than being at odds with biology.
Push for women to fast
It seems this idea was then extrapolated (particularly on social media) from not just exercise and WHAT to eat, but also, WHEN to eat and specifically creating a longer period of not eating otherwise known as intermittent fasting or sometimes just IF. To some degree this made sense as the connection between fasting and increasing the diversity of the healthy gut bacteria was evidence based. However the idea of fasting for women was always more controversial than for men because of our hormonal cycles as well as the additional challenge presented by peri to menopause with massive decreases in oestrogen and progesterone having multiple knock on effects during this time, including muscle loss and redistribution of fat from predominantly subcutaneous to visceral and abdominal storage. These two factors in particular made fasting for a lot of midlife women something that was very appealing, as many felt under pressure to find an answer to "fix themselves" as they saw it.
My experience with fasting and Ketosis
In the midst of all this noise, in my late forties I was not necessarily experiencing too much difficulty with metabolic changes (yet), but was aware I wanted to keep them at bay. So, naturally I was very drawn to the idea which at first seemed logical, that a little more fat could be lost, without food or glucose for fuel in my body. Fasting to some extent made sense. I say to some extent, because for me personally, as I tried this protocol of not eating every morning until lunchtime, I would find myself extremely hungry by 11 am and sometimes embarrassingly low on energy, I was also in client facing sessions trying to join in, or at least demonstrate the heavy lifting not just metaphorically, but literally. This was difficult.
Around the same time my husband had been experimenting with a Ketogenic diet ( with some success) and so I thought I would support him by mostly joining in ( yes, there may be a pattern!). Although admittedly I was eating more carbohydrates and not doing the morning testing regimen as he was. I was still tempted to self experiment even though I was already familiar with the concept of something called metabolic flexibility. This was something I first learnt about when quite a few years prior to this I had used a device called a Lumen, which was able to measure the level of C02 in the breath and make an approximation of whether fat or glucose was being used as the primary fuel source for running the body. The idea being that metabolic health could be improved by aiming to be in the 'green' every morning (rather than amber or red), which indicated according to the measurement on the Lumen, burning fat for fuel rather than glucose. However as I discovered, being in the green after a pasta dinner the previous evening or even just eating too late, made being in the green totally impossible. A similar discovery will have been made by anybody who has worn a CGM (or continuous glucose monitor).
What two main sources we use to create energy
It obviously depends on what we eat as well as our activity levels. It is healthy to be burning fat at least some of the time, either as a result of an overnight fast, or a workout, which results in the glucose stored in the body to be used up, and as a result the switch is made on a cellular level to burning fat for energy in its place. This is pretty useful as clearly we need to be able to keep our bodies running and of course it is not always possible to eat at regular intervals, and so we have evolved this truly ingenius back up method of powering ourselves with fat from our bodies, rather than glucose derived from dietary carbohydrates.
However whilst better metabolic flexibility is a good thing, and it does hope improve the diversity of our microbiome which in turn improves insulin sensitivity and also body composition over time. These things are are complicated by the context of the midlife female. They are also complicated by individual context. Inevitably one persons life and activity levels, stress and body composition are all unique, and create an impact accordingly which is essentially what the first PREDICT 1 study found.
So, we have discussed the context that gave rise to a popular fascination with fasting and specifically in relation to the midlife female. I have mentioned in the title a concept of healthy fuelling, and as we continue we must look at how this phrase has come about and question is it really at odds with the idea of IF?
But first, What is fuelling?
This is a concept that has been popularised Dr Stacy Sims a highly regarded scientist who has published multiple papers (also midlife female) and has branched out to being a media superstar as she appears as a frequent guest on podcasts from the Diary of a CEO - Rich Roll. For some time she has offered advice to women around strength training and most recently has received lots of publicity as a result of her countering what she see's as the popular misunderstanding that breakfast is bad, specifically for women. This really amounts to a rebranding of breakfast as 'fuelling'. Which goes against the trend for intermittent fasting we have seen explode in recent times. The reason behind her passionate and determined campaign is compelling.
Why fuelling is so key?
The problem amongst women in particular, is that not eating breakfast to lengthen the fasting window also spikes cortisol, especially if combined with caffeine and activity. The trouble with this, is that if cortisol is elevated it makes the likelihood of building muscle from a workout (which is also key for metabolic health) much lower.
What Dr Stacy Sims shares is that by simply eating breakfast, not necessarily in a huge amount, but something. The body then has fuel, it can turn into energy that helps elevate a morning workout from lower levels of effort, to work that is more likely to create muscle both because the work is more intense and also cortisol is supressed rather than spiked, which it would be if it were in a fasted state, as a result of the specifically female vulnerability when calorically restricted to stress.
Another problem with fasted workouts for menopausal women, is that a decrease in oestrogen also creates more sensitivity to stress, and so by doing a morning workout without fuel the cortisol spike can doubly significant which is really counter productive.
Can fasting can still be beneficial?
The answer to this, is that it certainly can, even for midlife females for example specifically, when they are not being active. For example, going on holiday is quite stressful, but once the packing is done and travelling is just sitting, a bit of a longer break from eating is likely not a bad thing. However context is key.
Men who need to lose weight can usually successfully take on fasted training and get both the benefits of the workout and caloric deficit because of the different response to stress they have.
Fasting can also be beneficial for some illnesses and disorders, under medical supervision where the cellular clearance it allows to take place can be extremely helpful. However that is beyond the scope of this article.
To re-cap and conclude: Healthy fuelling vs intermittent fasting:
The concept of metabolic flexibility is key to maintaining a healthy metabolism and ultimately to be in good shape physically as well as feel good. The idea that fasting was a silver bullet for weight loss specifically for women in midlife was flawed, but the idea that the microbiome is important is definitely a reality.
So we now know that healthy fuelling is critical which means keeping an eye of the quality of what we consume as well as the timing is essential. Eating at the right time in order to fuel a workout is important. Too long before (more than 4 hours) is generally too long, but less than 2 hours and this is generally not long enough to allow digestion to take place (which is why something very small is fine for an earlier morning workout). The fasted window however remains worth maintaining for everyone. Ironically especially midlife women as the diversity of the good bacteria in the microbiome is more easily damaged as stress and hormonal changes make their impact more readily.
So, the solution to exactly how to fuel and fast, will be different for everyone. However, if you are interested I get my 12-14 hr fasted window when I can, by eating dinner earlier and making it smaller and ideally lunch quite a bit more substantial. Then sleep improves, recovery improves and control over eating and cravings etc also improves. Obviously the PFR is also something I try and maintain. Creatine helps.
If you are interested to learn more, please do take a look at the references below or request at copy of the PFR plan directly by e-mail.
Please Note: This article is a personal perspective, for information only. I am a Personal Trainer. I do offer nutritional advice only, but I also do my research; but, so should you.
References:
Kate Brooks 2023, PFR plan - Headline recomendations for eating healthily.
The Metabolic Findings (2020): Berry, S. E., et al. "Human postprandial responses to food and potential for precision nutrition." Nature Medicine, June 2020. Read the full study on Nature Medicine.
The Microbiome Findings (2021): Asnicar, F., et al. "Microbiome connections with host metabolism and habitual diet from 1,098 deeply phenotyped individuals." Nature Medicine, January 2021. Read the full study on Nature Medicine.
June 2025, Why Fasting doesn't work for active women -https://www.drstacysims.com/newsletters/articles/posts/fasting-for-active-women-risks
Zoe.com, Chmielewska, 2024, “Can intermittent fasting improve your gut health”?
This a really well-informed and well-written post Kate that debunks most of the myths and misconceptions around this complex topic. Love the fact that you also summarise your research at the end with actionable strategies.