One thing I notice, as a health professional, is that that there are a lot of different opinions about what it means to be healthy. These change over time according to old wives’ tales, favorite magazines, even a little inspiration from Google. Currently, a big diet trend that I see being advertised and spoken about a lot is Intermittent fasting (IF)… But what is it really and does it actually work to help improve health and aid in weight loss?

So, what is IF?

IF is defined as eating according to a specific schedule or pattern that alternates between periods of normal eating and periods of severe caloric restriction (also known as fasting). The main idea behind this calorie restriction is, like all diets, to reduce the calorie intake on those specific fasting days; in order to lose weight. However, unlike most diets, this way of eating only focuses on when to eat and does not look at what to eat. The fasting is also not specifically defined; some IF diets refer to no food intakes at all, while others specify only liquid foods, and a few refer to eating only 500 kCal per day.

Now, let’s talk about the types of IF:

There are a few variations of the IF diet:

1. 12 Hour fasting. This diet pattern, and arguably the more common one, allows food to be eaten over a period of 12 hours, while someone is awake; and then moves to a 12 hour fasting pattern, mainly before and during sleep.

2. Meal skipping. This fasting pattern allows occasional skipping of either the breakfast or a dinner meal.

3. Modified fasting regimens:

a. 5:2 – This diet pattern refers to eating “normally” over 5 days and then restricting food intake to only 500 kCal on the other 2 days of the week.

b. The Alternate Day fasting – Fasting every alternate day

c. Eat Stop Eat Diet – Fasting for 24 hours – 1 or 2 days during a week

d. Time Restricted Eating (16:8) – Fast for 16 hours and only eating during an 8 hour window, usually between 12:00 and 20:00.

e. The Warrior diet – arguably the most extreme version of the IF pattern. It includes eating very small amounts of food during a 20 hour fasting period and then only one large meal at night.

Okay… So, why would people want to do this?

The main reason why most people attempt such restrictive eating routines is the proposed weight loss benefit. The other claims are that it allows for increased fat breakdown, improves brain function, decreases blood sugar and reduces inflammation – to name a few. But are these claims even true or are there just words being used to generate a buzz?

Now, let’s see what the Science has to say about IF:

There aren’t many published and validated research studies available on the IF diet, however, there are a few that are available (skip past this next bit if your just looking for the summary 🙂

– Animal studies – the animal studies available seem to support the hypothesis that restrictive feeding cycles improves the metabolism, decreases obesity risk and improves health conditions in rodents.

– The studies available on humans are limited to a few observational studies around religious fasting – e.g. the Ramadan fast and intervention (trial based) studies with small sample sizes and short trial periods:

Study Summaries

Health effect Pros Cons
Effect on weight “Significant” weight loss – but according to 1 or 2 day fasting periods and small sample sizes 1 Showed weight regain 2 weeks post fast 1
Effect on Cholesterol Decrease in LDL cholesterol (part of the unhealthy cholesterol)2 AND One study showed improved triglycerides (unhealthy cholesterol) and improved HDL (healthy cholesterol) BUT also increased LDL (unhealthy cholesterol) 2,4 Mixed results in cholesterol profiles 2,4
Effect on Blood Glucose/Sugar Decrease in blood sugar on fasting day2 Detrimental effect on blood sugar and insulin the day after fasting 4

Effect on hunger/ mood and food intakeA study showed the potential in eating more frequently, decreasing evening snacking and fasting for longer nightly intervals which may decrease inflammation 3,4Participants complained of distraction, poorer moods and lower perceived work performance 6


The general conclusions:

– Weight loss was similar for people in the IF diet group and people in the regular (non fasting) calorie restricted eating group 5

– Intermittent energy restriction (IF diet) resulted in short term weight loss and changes in body composition which were seen to be more effective than no intervention at all/ not dietiting at all 1

– No differences in blood pressure, fasting blood sugar, fasting insulin, insulin resistance

The conclusion: weight loss was not superior compared to daily healthy, balanced, regular eating and more studies are needed to verify any of the claims made by IF studies

So, in other words…?

Fasting decreases the number of hours available to eat and therefore reduces overall food intake; which is how people could lose some weight. However, the studies show that the overall weight loss benefit was not greater than that resulting from a healthy and balanced meal plan. While this type of dieting is not generally harmful, it is also not necessarily safe for everyone; as studies have commented that participants with certain medical conditions showed less favourable health outcomes. And so, it is for this reason that IF is not recommended for those who have diabetes, insulin resistance, high cholesterol or cardiovascular disease and other medical conditions; including those who are pregnant or breastfeeding.

What are my thoughts on IF?

I think the 12 hour eating pattern, followed by a 12 hour fasting over the night routine could potentially be a healthy way of following this diet trend. However, more studies, done over a longer duration and that follow reliable protocols, are needed in order to advocate for any long term health benefits/ pitfalls or complications of the IF diet.

For now, if one is considering this diet pattern in order to lose weight, I think the most important question to ask yourself is ’why’? Is IF going to be yet another diet added to your long list of tried and tested diets? Is to feel like you can lose weight by eating without restriction (only to fast and restrict food and to “compensate” or “fix” food intake later)? Are you looking for a quick-fix weight loss? Or is it because everyone is trying and talking about it, so it’s worth an attempt?

If so, your relationship with food should be questioned. A healthy relationship with food is so important! And is often the main component in successful weight loss. This is where a dietician can make all the difference with healthy dietary recommendations. It’s always worth a try?!

The main conclusion is that this type of diet is not superior, nor is it revolutionary, and therefore I would not advocate for it over a balanced eating pattern focused around healthy food and good exercise. Remember everyone is different, so just because something is trendy doesn’t mean that it will result in a healthy weight which is sustainable long term.


1 Sadeghirad B, Motaghipisheh S, Kolahdooz F, Zahedi MJ, Haghdoost AA. 2014. Islamic fasting and weight loss: a systematic review and meta-analysis. Public Health Nutr. 17:396–406

2 Kul S, Savas¸ E, O¨ ztu¨ rk ZA, Karadag˘ G. 2014. Does Ramadan fasting alter body weight and blood lipids and fasting blood glucose in a healthy population? A meta-analysis. J. Relig. Health 53:929–42

3 Marinac CR, Sears DD, Natarajan L,Gallo LC, Breen CI, Patterson RE. 2015. Frequency and circadian timing of eating may influence biomarkers of inflammation and insulin resistance associated with breast cancer risk. PLOS ONE 10:e0136240

4 Patterson, R and Sears, D. 2017. Metabolic Effects of Intermittent Fasting. Annu. Rev. Nutr. 2017.37:371-393.

5 Trepanowski JF1, Kroeger CM2, Barnosky A et al. 2017. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med 77(7):930-938.

6 Appleton KM, Baker S. 2015. Distraction, not hunger, is associated with lowermood and lower perceived work performance on fast compared to non-fast days during intermittent fasting. J. Health Psychol. 20:702–11

7 Harris L, Hamilton S, Azevedo LB et al. 2018. Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database System Rev Implement Rep. 507-547

8 Heilbronn LK, Smith SR, Martin CK, Anton SD, Ravussin E. 2005. Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. Am. J. Clin. Nutr. 81:69–73

9 Research based on ADSA presentation from Dietician K Hofmann

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