Getting the Facts Straight // Intermittent Fasting

This post is adapted from one that I wrote for Glow Gathering sometime last year on the topic of intermittent fasting, which is a topic that I have been receiving many questions about in the past couple of months. I hope that the information that I share her will inform you, get you thinking, and help you assess your intentions before jumping onto the intermittent fasting bandwagon.

What is Intermittent Fasting?

There has been increased interest in a concept known as Intermittent Fasting (IF) in both popular media and the scientific world over the years as a potential strategy for weight-management and improving metabolic health. There are two main variants of IF, intermittent energy restriction (IER) and time-restricted feeding (TRF), which involve periods of complete or partial abstinence from food (fasting) alternated with periods of free, habitual food intake (feeding). Fasting has taken place in one form or another for thousands of years, having an important role to play in the Christian faith and other religions, and even as a consequence of natural disasters and lack of food during times of war.

The Physiology Behind Intermittent Fasting

IF results in overall chronic calorie restriction with periods of no food intake that can ultimately lead to reduced body fat and weight loss over time. Research has shown that humans do not compensate for the energy deficit of fast days, and thus over time less calories are consumed when one follows an IF eating pattern. From a metabolic perspective, IF allows us to extend what is known as the fasted metabolic state, during which the body relies on fat as a principal fuel source. To better understand this let’s take a look at the different metabolic states that we switch between after eating a meal:

  1. Postprandial state

This period lasts from when you eat up until approximately 4 hours after the meal. This is the time when your body digests, absorbs, stores, and utilises the food that you’ve eaten. Insulin is secreted in the presence of carbohydrates, driving the storage of glucose as glycogen in the liver and muscles (which provide limited reserves) and as fat in adipose tissue.

2. Post-absorptive state

This period follows the postprandial phase and lasts for about 6 hours after a meal. During this time, your body starts to break down stored carbohydrate (glycogen) into glucose, releasing it into circulation as glucose for body cells to use. The body also begins to utilise some fat from adipose tissue as fuel.

3. Fasted state

This phase only really kicks in after 10 or more hours after a meal. It is during the fasted state that the body relies on the breakdown of fat stored in adipose tissue as its main fuel for the proper functioning of our cells and tissues.


Considering a typical daily eating pattern, where we eat breakfast, lunch, dinner, and snacks in between, most of us rarely experience being in the true fasted state for more than 4 hours each day. Thus, IF provides the body with a bit more time to utilise its fat stores and limits the time it has to store excess glucose in adipose tissue.


What Does the Evidence Show?

Alterations in meal timing and the timespan between meals has been shown to beneficially influence glucose and lipid metabolism independent of weight-loss. Compared to traditional energy restriction (i.e. pretty much all diets), variants of IF have the potential to make it easier to achieve a calorie deficit over time. Complete and partial fasting approaches have demonstrated short-term weight loss and positive metabolic health benefits in both animal and human studies, and it has also been shown to be effective for weight-maintenance for periods of up to 1 year. Rodent studies have shown that IF has the potential to protect against heart disease, type 2 diabetes, cancer, and Alzheimer’s disease, however it’s worth noting that results from animal studies cannot be automatically applied to humans.

Although the evidence supporting the potential benefits of variants of IF on short-to-medium term weight loss, weight maintenance, and metabolic health, there is still a lot that we have to learn and understand about IF in humans, particularly with regards to its health effects and overall sustainability in the long-term. Most studies conducted up until this point have been performed in animals, or have been conducted in humans for short periods of time without very long-term follow-ups.


Some of The Pros and Cons of Intermittent Fasting


  • IF has the potential to be an effective weight-loss and weight-maintenance strategy

  • It may also provide health benefits that include reducing inflammation, improving insulin sensitivity and blood lipids, reducing cardiovascular risk, and as an effective weight-loss strategy

  • Different variants of IF are relatively simple to understand and follow, as they dictate when to eat rather than what to eat

  • Fasting for a couple of days a week or for a few more hours a day than normal is more achievable for some people than other diets and eating plans


  • Using IF as a long-term weight-loss strategy does make it a diet, and just like many other diets out there, research has shown that in the long-term, they just don’t work (you can give this paper and this blogpost a read if you want to learn more about what we know about this)

  • Although it is not likely to be harmful for most individuals, the long-term effectiveness and effects of IF are largely unknown at this point

  • Reducing how much you eat is likely to result in hunger, irritability, poor concentration, tiredness, fatigue, and dizziness. From a practical perspective, following an IF regimen could affect work and other daily tasks.

  • Although some evidence has shown that individuals do not fully compensate for fasting days on days of free eating, it can be argued that there is a likely risk of IF promoting a restrict/binge mentality and an unhealthy relationship with food

  • IF is not for everyone, and it’s a good idea to consult with a qualified healthcare provider before trying it out. It should be avoided during pregnancy and breastfeeding, during stressful periods, and for anyone on long-term medical conditions (e.g. diabetics)

A note on intermittent fasting and fertility

A 2013 study by Kumar & Kaur conducted in rats (10 male, 10 female) found that IF impacted negatively on their HPO-axis and fertility. Half of these rats were allowed to eat whenever they wanted to, whilst the other half ate food on every second day, for a period of 12 weeks (according to this blogpost that is the equivalent of 10 years in a human’s life, although I haven’t fact checked this haha). At the end of the study period, the female rats had lost 19% of their body weight, had lower blood glucose levels, and their ovaries had shrunk. Not only this, but hormone levels in these female rats were completely out of whack:

  • LH decreased significantly

  • Oestradiol (a form of oestrogen, which regulates GnRH) was four times as high as normal

  • Leptin (which controls appetite through inhibiting hunger) was six times less than normal

Now it’s always good to remember that animal studies cannot be directly translated to humans, but it is worthwhile exploring the results of this study because based on what we know about the HPO-axis, the relationship between hormones and appetite, and women’s sensitivity to environmental factors, it is plausible that IF could have a similar impact on women’s hormones and fertility. A woman’s reproductive and metabolic functions are closely connected, which means that when our bodies are under ‘threat’ they will go into protection mode. Starvation/restriction/not eating for an extended period of time will trigger the female body to preserve and protect her by:

  • Holding onto fat reserves (energy in case she needs to survive a family)

  • Altering production of ghrelin and leptin (to encourage her to eat what she can whilst it is still around)

  • Slowing down non-essential functions including reproduction (to help her preserve energy for more important things like staying alive)

Who shouldn’t try Intermittent fasting?

This is a tricky question, because my own bias leads me to want to say that no one should try this if your primary intention is to manipulate and control what your body looks like. But let’s take a look at some population groups who should most definitely not engage in this way of eating, especially on their own without consulting a qualified healthcare professional:

  • Anyone experiencing diagnosed hormone issues - PCOS, fibroids, endometriosis, etc.

  • Anyone who is suffering from an irregular or non-existent period (this is NOT normal, and needs to be addressed)

  • Anyone who is trying to conceive or is currently pregnant

  • Anyone with a history of eating disorders

  • Anyone who is suffering from insomnia or has trouble waking up in the morning and functioning properly

My Point of View

IF has potential metabolic benefits, but there is a lot we still need to learn and understand before it can be used for therapeutic interventions. However, I think that it is wise to first assess your intentions and the things that are important to you before giving something like this a try. For example, starting the day off with a cup of coffee and a warm bowl of oats and sharing an enjoyable meal with my husband after a long and busy day is an important part of my daily routine. Although life doesn’t revolve around food, the daily ritual of sharing moments around meals is important for my heart, mental health, and overall wellbeing. Figuring out what works for your body and for your body clock is also important. I know that although there are times where it isn’t always practical, my digestive system treats me better when I allow for a good 12 hours between dinner and breakfast the next day. Something else to be aware of is that IF also has the potential to be detrimental to female hormones and fertility, although further research is needed to better understand this area. I am an advocate for finding balance, learning to eat intuitively and developing a healthy, guilt-free relationship with food, and believe that although IF may have benefits, just like any ‘diet’ it has the potential to promote unhealthy restrict/binge cycles that are not constructive in the long-term.

Ask yourself:

  • Why is this something I want to do? (If your first thought is to lose weight / change your body, I would encourage you to rather first address some of these underlying body dissatisfaction / body image issues)

  • Is this really sustainable in the long-term (over the next 1, 2, 5, 10 years)? (If not, remember that diet/weight cycling is more detrimental to our health than you’d probably think)

  • Does following this approach give me freedom when it comes to food, or does it leave me feeling anxious, guilty, and upset? (If the latter, this probably isn’t the healthiest thing to do for the sake of your mental and emotional health)


[1] Antoni R, Johnston KL, Collins AL, Robertson D. The Effects of Intermittent Energy Restriction on Indices of Cardiometabolic Health. Research in Endocrinology. 2014:1-24. Available from:

[2] Antoni R, Johnston KL, Collins AL, Robertson D. The Effects of Intermittent Energy Restriction on Glucose and Lipid Metabolism. Proceedings of The Nutrition Society. 2017 Jan. Available from:

[3] Antoni R, Johnston KL, Collins AL, Robertson D. The Effects of Intermittent Energy Restriction on Postprandial Substrate Metabolism. Proceedings of The Nutrition Society. 2016 Jan. Available from:

[4] Harvie M, Wright C, Pegington M et al. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. Br J Nutr. 2013;110:1534-47.

[5] Wegman M, Guo M, Bennion D et al. Practicality of intermittent fasting in humans and its effect on oxidative stress and genes related to aging and metabolism. Rejuvenation Res. 2015:18;162-72.

[6] Mann T, Tomiyama AJ, Westling E, Lew A, Samuels B, Chatman J. Diets Are Not the Answer. American Psychologist. 2007 Apr;62(3):220-33. Available from:

[7] Chris Kresser [Internet]. 2015 Dec 3 [cited 2018 Feb 3]. Could You Benefit From Intermittent Fasting? Available from:

[8] Helen Kollias [Internet]. [cited 2019 Jan 30]. Intermittent Fasting for women: Important information you need to know. Available from:

[9] Kumar S, Kaur G. Intermittent Fasting Dietary Restriction Regimen Negatively Influences Reproduction in Young Rats: A Study of Hypothalamo-Hypophysial-Gonadal Axis. PLoS ONE. 2013;8(1):e52416. Available from: