Intermittent fasting

It’s 2500 words, despite telling myself to keep it to 1500!

It’ll take 15 minutes or so to read.

A summary before you start…

  • Compared to standard calorie restriction diets, intermittent fasting performs no better when calories are matched
  • Intermittent fasting may provide certain health benefits beyond those seen with weight loss generally. However, more research is needed
  • Intermittent fasting may be beneficial in improving adherence to a calorie restricted diet
  • Getting comfortable with low levels of hunger is (in my opinion) a very powerful tool for weight loss

First of all, allow me to introduce you to Fred. Fred is an imaginary client of mine who I will refer to in this post to illustrate the different strategies I will discuss. Fred is a simple guy. In order to maintain his weight, he has a daily energy requirement of 2500 kcals. This figure is often referred to as Fred’s ‘maintenance calories’. If Fred wanted to use a standard calorie restricted diet to promote weight/fat loss, he would need to eat below 2500 kcals for a sustained period. How much below would be at the discretion of Fred and his trainer; but a fairly typical start point would be 85% of his maintenance requirements, so 2125 kcals. Therefore, a standard calorie restricted diet would see Fred eating 2125 kcals every day and making adjustments when necessary based on factors such as progress, performance, energy, hunger, and so on. Fred has heard that he could make more progress with a fancy new intermittent fasting style diet. So, with the help of Fred, I would like explain just what intermittent fasting is so he (and you) can make an informed decision as to whether it’s something worth exploring further.

Intermittent fasting has become a more widely accepted practice in recent years due to  the popularisation of intermittent fast style diets such as the 5:2 diet (Amazon link), and the warrior diet (Amazon link). Proponents of intermittent fast style diets claim a weight loss and health advantage of their dietary protocols over a standard calorie restricted diet. In this post I want to discuss what is involved in intermittent fasting and what the scientific research says about it.

What is intermittent fasting?

The central theme of intermittent fasting regimens is that individuals cyclically abstain from eating for predetermined periods that exceed the standard overnight fast. Fasting typically refers to complete abstinence from food intake. However, intermittent fasting may involve small amounts of food consumption during fasting periods meaning that strictly speaking, they are actually ‘modified fasts’. For the purpose of this post, intermittent fasting will refer to either. There are three main types of intermittent fast. Alternate day fasting, whole day fasting and time restricted fasting (Seimon et al, 2015).

Alternate day fasting:

Alternate day fasting is the most researched form of intermittent (aside from religious fasting) and involves alternating fasting days with ad libitum (eat what you want) feeding days. Fasting days can involve zero food intake, or be modified to include one meal at lunch time with 25% of the individual’s baseline calorie needs in it. (Varady et al, 2013). Varady’s lab has conducted several studies on alternate day fasting and has a book detailing it as a dietary strategy (Amazon link). Alternate day fasting has been shown to be effective in improving several risk factors associated with cardiovascular disease such as total cholesterol and LDL cholesterol, although these may also be associated with simply losing weight (Seimon et al, 2015).

When undertaking alternate day fasting, Fred would eat whatever he wanted on Sunday, Tuesday, Thursday and Saturday, and ‘fast’ on Monday, Wednesday and Friday by restricting himself to one meal at lunchtime containing 625 kcals, or by eating nothing at all. For the sake of comparing strategies equally, I have imagined a plan for Fred where he has a high calorie target on his feeding days in order to bring his weekly average in line with his standard calorie diet. As you can see, it means he can eat a lot on his feeding days! If all these kcals weren’t consumed, one would assume the weight loss would exceed his standard deficit diet.

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Whole day fasting

Whole day fasting involves abstaining from food for one or two whole days (either spread across the week or on consecutive days). Possibly the most well known whole day fast style protocol is the 5:2, brought to our attention by Michael Mosely of BBC fame, and discussed in this research paper Researchgate link. Like alternate day fasting, some whole day fast protocols recommend a small amount of kcals to be consumed on fasting days (500 kcals and 600 kcals for women and men), but not necessarily in one meal (Brown et al, 2013). If Fred were to take on whole day fasting, he would pick two days to consume 600 kcals, and eat whatever he wanted on the other days. In essence, whole day fasting is very similar to alternate day fasting, with the variation being in the number and frequency of fasting days. Again, I imagined a diet with some calorie control for Fred to highlight the calorie allowance he would have on feeding days.

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Time restricted fasting

Time restricted regimes involve having a daily ‘feeding window’ in which all calories are consumed. Typically, the feeding window will be four to eight hours and allow ad libitum feeding during this time. That leaves the rest of the day as fasted, with the majority of the fasting hours being overnight. Martin Berkhan of lean gains ( is a well known proponent of time restricted, and the Warrior Diet also utilises a time restricted fasting (with only one meal allowed per day). If Fred took it on, he could have a six hour feeding window between 12pm and 8pm, and not eat the rest of the time (This would be referred to as a 16:8 structure). To set up, this is pretty easy as he can simply skip breakfast, and not eat anything after his final meal of the day (assuming it is at 8pm). If he found this daunting, it would be fine for him to start with a 14 hour fasting window and aim to increase it as he gets more accustomed to the feelings that hunger brings. Like previously, to highlight the size of meals Fred could have, I set a calorie target to make all these protocols equal from a calories POV.

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Here’s how all the strategies would compare on paper

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So, do they ‘work’ for weight loss?

Put simply, the research shows that intermittent fasting is no better than continuous calorie restriction if calories are matched (Tinsley et al, 2015). So, if Fred consumed 2125 kcals every day through standard calorie restriction or through intermittent fasting, the science suggests he would see similar results in his weight loss (Seimon et al, 2015). However, there are some nuances that are worth considering:

  • Some research on alternate and whole day fasting reports lean/fat free tissue loss (you don’t really want to lose lean/fat free tissue when dieting!). However, this lean tissue loss was attenuated when a small meal was consumed during the fasting day (as mentioned earlier) (Headland et al, 2016). If this meal was rich in protein (especially the amino acid leucine)(Helms et al, 2014), I am sure all potential lean tissue losses would be minimised.
  • Time restricted fasting has not been studied in any depth, so there aren’t any solid conclusions to be made…yet. One study comparing one meal per day to three did find the one meal per day group lost body fat compared to the three meal per day group (Stote et al, 2007); however, body composition was analysed using methods that aren’t considered the gold standard.
  • There is a possibility that fasting results in lower levels of ad libitum food intake (i.e. Fred would eat less when he had freedom to eat what he wanted) due to a change in the drive to eat. However, this isn’t conclusive across all research (Seimon et al, 2015).
  • It is possible that differences in results between different intermittent fasting set ups (i.e. 1 fast day or 2, consecutive or non consecutive days) could be attributed to the variations of the protocol, but there simply isn’t the research to draw any definitive conclusions. There may be an optimum way of setting up intermittent fasting, but at this moment in time it is not apparent what that is (Seimon et al, 2015).
  • Short fasts of 18 to 24 hours are beneficial in promoting lipolysis (the release and subsequent breakdown of fat from its storage sites) (Tinsley et al, 2015).
  • Resting energy expenditure increased significantly between 14 and 36 hours of fasting, apparently due to an increased stress response (Zauner et al, 2004). However, this time frame may be slightly outside the typical fasting time frame seen in the strategies outlined above; especially when modified fasts are utilised.
  • It is important to note that a reduced meal frequency (eg. two meals per day) will not reduce 24-hour energy expenditure as it is sometimes claimed to by the ‘eat little and often to stoke your metabolic fire’ crew (Tinsley et al, 2015).
  • There is a paucity of data linking intermittent fasting with clinical outcomes such as diabetes, cancer and other chronic diseases such as Alzheimer’s disease (Patterson et al, 2015). The studies from which the claims of an all encompassing health are based are generally animal models. That doesn’t mean these benefits don’t exist, merely they are yet to be shown conclusively in human studies.
  • As with any other weight loss strategy, intermittent fasting will only be successful at driving weight loss if it leaves you in a calorie deficit. Therefore, calories must be accounted for at some point. If you eat more than your maintenance requirements, simply fasting won’t change the fact that your weight loss will be attenuated.
  • However, intermittent fasting may enable someone to stay at calorie deficit for longer, for reasons I will discuss below.

While there is no apparent long term fat burning advantage to be gained from an intermittent fasting protocol when comparing to a calorie matched non-fasting diet, there are some aspects of intermittent fasting that should not be ignored when designing a fat loss strategy. The pivotal yet (unfortunately) often neglected aspect of a successful weight loss strategy is time. It takes time being in a calorie deficit to see an appreciable weight loss. Your body doesn’t just give up its fat stores without a fight! How much time is entirely dependent on how much weight you have to lose. Therefore, the most successful weight loss strategies are the ones that enable you to adhere to them for the time required to see results. Even the most scientifically backed weight loss protocol will not work if you only stick to it for 2 weeks before becoming distracted/bored/impatient. Intermittent fasting could potentially make it easier to adhere to a period of sustained caloric deficit through some mechanisms I will bullet below. I want to make it clear at this point though that I am starting use a little conjecture based on my own experience and intuition; so less will be scientifically validated.

  • Intermittent fasting, especially when set up as time restricted fasting allows for satiating, satisfying large meals to still be consumed. Fred may not be seeing the results he wants at 2125 kcals, so he drops his daily intake to 2000 kcals. Across 5 meals, this would mean 400 kcals for each. For Fred, this may seem pretty paltry and leave him feeling dissatisfied and hungry every time he eats, putting his will power to the test daily. If, however, he fasted for the morning and crammed all his calorie allowance into 2 meals between 12pm and 8pm, he could eat a super sized 1000 kcals at each and feel far more comfortable staying at his new low calorie level for longer. The same applies for the whole day or alternate day fasting. If these strategies allow Fred to maintain his calorie deficit for longer as he enjoys having large meals on his feeding days, you could deem the fasting diets as more successful.
  • A big part of fat loss success is combatting the behaviours that led to one gaining unwanted weight in the first place. These behaviours could potentially be borne from associations between hunger, stress, emotions and general day to day routine. By disrupting these associations, we can potentially learn about the nature of our hunger whether or not is physiological or psychological in origin. Learning to deal with hunger is (in my opinion) and incredibly powerful tool for anyone to have. By being hungry for a few extra hours and realising that you don’t collapse, freak out, or have your brain turn into mush can be quite liberating. Your daily movements won’t have to revolve around your next feed! Yippee…freedom!
  • Following on from the previous point, strange things can happen when you’re hungry. For me, despite thinking I would be a mess before I had tried fasting, being hungry makes me feel alive! My senses sharpen, my performance (to a point) is improved and I feel that fasting is an incredibly beneficial thing to do. Maybe this is some evolutionary trick, where knowing it needs food the body ramps things up in order to make that next kill. Or it might just be my digestive system gets a break and can recover. Who knows…I’m sure research will provide answers in the coming years.
  • It certainly seems beneficial to maintain a decent protein intake during fasting periods. ‘Protein sparing modified fasts’ (ultra low calorie, but high protein diets) have been around for years and are base don the premise that lean tissue should be kept at all costs. This means that you should aim to hit a minimum target for protein on fasting days (~ 1.4-2.2g per kg bodyweight depending on various factors), or have a couple of small protein doses around training if training in fasting windows – for example, Fred could be advised to have 20g an hour before and an hour after training if he trains in the morning during his 16:8 fasting set up.

I’m aware this is already too long, but to finish I want to give some direction of where you can start with your intermittent fasting should you choose to play with it.

  • First of all, remember, calories must be accounted for if you want to lose weight. There is potential that given the freedom to eat what you want but in just one meal (as seen in The Warrior Diet), will automatically leave you in a deficit, but I would still track what you eat in the ‘feasting’ periods to know for sure.
  • Pick a fasting style and give it a go for 4 weeks. Note how you feel and what results you see. I’m personally trialling time restricted fasting at the moment, but will certainly be trying the other methods at some point to see how they leave me feeling. If the protocols described above seem daunting, ease yourself in gradually over a week. For time restricted fasting you may start at a smaller fasting window and start to make it bigger each day, or just try it out on one day to see how you respond. There is no right way, so have a bit of a play around with it. For alternate and whole day modified fasts, you may want to keep the kcals you consume during fasts a bit higher initially and slowly reduce them. It’s up to you. Of course, if you want my assistance setting something up, just get in touch!
  • Read up on it. Then try it for yourself. A good review paper on the subject is HERE, but I don’t think it’s free to read – you may need to ask someone with a uni account to access it for you 😉 Alternate day fasting has been made into a diet by one of the lead researchers on the topic. Her book is here > Amazon link, whole day fasting is the style of fast in the 5:2 diet, book here > Amazon link, and time restricted fasting appears in The Warrior Diet Amazon link.



Brown, J. E., Mosley, M., & Aldred, S. (2013). Intermittent fasting: a dietary intervention for prevention of diabetes and cardiovascular disease? The British Journal of Diabetes & Vascular Disease, 13(2), 68–72. Researchgate link

Headland, M., Clifton, P. M., Carter, S., & Keogh, J. B. (2016). Weight-Loss Outcomes: A Systematic Review and Meta-Analysis of Intermittent Energy Restriction Trials Lasting a Minimum of 6 Months. Nutrients, 8(6). Pubmed link

Helms, E. R., Aragon, A. A., & Fitschen, P. J. (2014). Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. Journal of the International Society of Sports Nutrition, 11, 20. Pubmed link/

Patterson, R. E., Laughlin, G. A., LaCroix, A. Z., Hartman, S. J., Natarajan, L., Senger, C. M., … Gallo, L. C. (2015). Intermittent Fasting and Human Metabolic Health. Journal of the Academy of Nutrition and Dietetics, 115(8), 1203–1212. Pubmed link

Seimon, R. V., Roekenes, J. A., Zibellini, J., Zhu, B., Gibson, A. A., Hills, A. P., … Sainsbury, A. (2015). Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials. Molecular and Cellular Endocrinology, 418 Pt 2, 153–172. Pubmed link

Stote, K. S., Baer, D. J., Spears, K., Paul, D. R., Harris, G. K., Rumpler, W. V., … Mattson, M. P. (2007). A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults. The American Journal of Clinical Nutrition, 85(4), 981–988. Pubmed link

Tinsley, G. M., & La Bounty, P. M. (2015). Effects of intermittent fasting on body composition and clinical health markers in humans. Nutrition Reviews, 73(10), 661–674. Pubmed link

Varady, K. A., Bhutani, S., Klempel, M. C., Kroeger, C. M., Trepanowski, J. F., Haus, J. M., … Calvo, Y. (2013). Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial. Nutrition Journal, 12(1), 146. Pubmed link

Zauner, C., Schneeweiss, B., Kranz, A., Madl, C., Ratheiser, K., Kramer, L., … Lenz, K. (2000). Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. The American Journal of Clinical Nutrition, 71(6), 1511–1515. Pubmed link