Intermittent fasting; an update of the evidence

Throughout the last decade, intermittent fasting (IF) has become possibly the most popularised approach to reducing weight. Studies have shown IF to effectively promote weight loss in animal models, and appear to have promising results in humans also.
Here, we will provide a review of IF and an update of the current evidence.

What is intermittent fasting?

IF originated from religious traditions, such as Ramadan fasting. Muslims fast during the daytime (approximately 15 hours between sunrise and sunset) for a month during the Ramadan period every year. Ramadan fasting has been reported to improve human health (Azadbakht,. 2014).

IF involves limiting food intake for a prescribed period of time, often in alternation with ones regular diet (Patterson and Sears, 2017, Hull, 2019). Not all approaches to IF are the same. Some are highly restricting while others are more reasonable and therefore sustainable. Some approaches emphasise the importance of healthy foods while others simply prioritise remaining under energy targets.

One approach to IF is alternate-day fasting (ADF), whereby individuals consume 25% of energy requirements on a ‘fast day’, followed by a ‘feed day’ where a normal diet is consumed.  Variants of ADF include consuming no food on fast days, extending fast periods up to 36 hours, and reducing feeding periods to 12 hours (Hull, 2019, Ganesan et al., 2018, Patterson and Sears, 2017).
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Another method is modified fasting, involving fasting or extreme energy restriction for 24 hours at a time. An example of this is the popular 5:2 diet, whereby individuals select 2 non-consecutive days of the week to limit their food intake to 25% of their energy needs (around 500 kcals). Their normal diet is consumed for the remaining 5 days (Hull, 2019, Patterson and Sears, 2017).

A third form of IF is time-restricted feeding; which involves reducing the ‘feeding window’ to certain hours of the day and fasting outside of that time e.g., feeding window limited to 8 hours between 10am - 6pm and fasting from 6pm - 10am each day (Hull, 2019, Ganesan et al., 2018).

What are the perceived health benefits?
Other than promoting weight loss, IF is thought to improve various health markers that are indicative of chronic disease risk including cardiovascular disease, diabetes, and some cancers. The effects include increasing the mobilisation of fatty acids and reducing visceral fat, increasing lean muscle retention, improving insulin resistance and glucose metabolism, and improving lipid profiles (Patterson and Sears, 2017, Mattson et al., 2017, Hull, 2019). The mechanisms by which these metabolic changes occur are via influencing the gut microbiome, our circadian biology, and adjusting lifestyle behaviours like sleep and dietary habits (Patterson and Sears, 2017, Hull, 2019).

What is the evidence saying?

Current evidence indicates that IF is as effective, yet no more effective, than an energy-restricted diet for weight loss (Hull, 2019, Trepanowski et al., 2017, Patterson and Sears, 2017).

Some of the most comprehensive data on IF to date is from a 2017 study looking at the effects of ADF vs. calorie restriction for weight loss over a one-year period. Alternate day fasters were to consume 25% energy requirements on fast days, and 125% energy requirements on non-fasting days while the calorie restricted group were limited to 75% of energy requirements every day. The results showed no superior health benefits with ADF. Average weight loss was similar between the two groups, and there were no significant differences in health biomarkers such as blood pressure, triglycerides, blood glucose, insulin levels, and insulin resistance. Diet adherence levels were similar between groups and in fact, the ADF group had a slightly higher dropout rate than the calorie-restricted group (37% vs. 29%), suggesting that ADF may be less sustainable than calorie restriction in the long run (Trepanowski et al., 2017).
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A systematic review into the effectiveness of IF published in 2018 also found that ADF was effective in reducing weight, regardless of individuals BMI. Additionally, there were significant reductions in some biomarkers including LDL cholesterol and triglycerides.  Results for other biomarkers were largely inconsistent. Authors concluded that while IF appeared to be an effective weight loss method in the short term, more research is needed to into diet sustainability and the efficacy of maintaining weight loss long term (Ganesan et al., 2018).

Advantages of IF

A reported advantage of IF is that many individuals find it easy to adhere to as fasting periods are relatively short are followed by a ‘normal’ eating period soon after. Individuals trying to lose weight that struggle with the smaller meals associated with reduced calorie diets for example, may prefer IF as restricted periods are only brief (Hull, 2019).  Furthermore, IF appears to be a safe weight loss method, with no evidence of causing harm in adults (Patterson and Sears, 2017).

Disadvantages

IF results vary greatly and for many reasons. Fasting days are restricting (often limited to 500 kcals) and as such long term dietary adherence is challenging for many (Trepanowski et al., 2017).

Another flaw is that individuals often compensate for their missed meals by eating more than they normally would on their ‘feed days’. This does not result in a calorie-deficit and as such, does not result in weight loss (Hull, 2019).
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Perhaps the crucial disadvantage however, is that aiming to remain below energy targets as in 5:2 or alternate day fasting allows individuals to ignore the quality of their diet as the focus is on sticking to below 500 kcals, regardless of foods consumed.

Summing up…

Intermittent fasting can be an effective weight loss method in adults however; any unique health benefits are still debated (Hull, 2019). IF won’t be the preferred weight loss method for everyone. As the efficacy of IF and calorie-restriction are not significantly different, it’s important that the approach used is what works best for the individual. The key is to still make healthy meal choices, and to ensure consistency of dietary habits throughout the program.

Keep in mind also that Well College supports a non-diet based approach where it is possible. A non-diet approach tends to result in positive pyscho-social outcomes for the individual and at the very least a healthy maintenance of weight. It’s also recommended that you look into the obesity paradox to familiarise yourself with some of the research challenging our very western perspective on overweightness.
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From a science perspective, there is too little evidence-based support for IF and its efficacy on humans for it yet to be used as a public health intervention. As always, more research is necessary. (Patterson and Sears, 2017, Hull, 2019).

Author

Iydi Willis. BFoodSciHNutr; MIntlPubHealth; CertifiedCP; RPubNutr
Head of Health Programs & Community Health Initiatives - Well College Global