Fasting for weight loss
Weight loss and the treatment of obesity
In the great majority of cases you can control your bodyweight with lifestyle and behaviour – in particular diet. Although pharmacological and surgical treatments are becoming increasingly prevalent, lifestyle modifications (physical exercise and dietary interventions) are still the most common and effective methods in the treatment of overweight and obese individuals.
You can’t discount the numbers
In the most basic terms weight loss/gain follows a very simple formula.
Calories in vs Calories out
If you consume more calories than you use - you will gain weight
If you consume fewer calories than you use - you will lose weight
In terms of dietary intervention, continuous energy restriction (CER) diets – where patients embark on extended periods of caloric deficit - are the most common and researched. However their efficacy, especially in the long term, is disputed.
(A note for clarification – a continuous energy restriction (CER) diet is what you would normally think of as a standard diet, where you simply try and consume fewer calories. Don’t let the jargon confuse you.)
Intermittent Fasting (IF)
Intermittent fasting is any eating pattern that intentionally cycles between periods of eating and fasting. Although it has gained huge popularity in the world of commercial nutrition it is not new culturally and has been practiced throughout human history for reasons of culture, religion and necessity.
Proponents claim benefits include weight loss, improved hormonal profile, increased energy levels, improved blood glucose regulation, reduced inflammation, increased stimulation of cellular autophagy and even increased lifespan.
I will write future posts discussing the relevant research for some of the other claims. Now I want to focus on the efficacy on using intermittent fasting for weight loss.
Types of intermittent Fasting
One issue with the current literature is the lack of standardisation of the various terminologies.
Time Restricted Eating (TRE)
This is where a subject will have a certain timed ‘eating window’ with which to consume all their daily calories. For example, the 16/8 protocol calls for a daily eating window of 8 hours (12-8pm for instance) with fasting for the remaining 16.
We all utilise this to some extent when we sleep (assuming we’re not getting up in the night to have some snacks). TRE simply extends this window to an intentional, pre-determined amount of time.
It should be noted that some proponents disagree with TRE as a valid version of intermittent fasting – claiming that it is simply an appropriate eating pattern (eating during the day and not at night) that humans have only recently veered away from.
Intermittent Energy Restriction (IER)
This is where different days of the week will subscribe differing calorie intakes.
For example:
5:2 – where 5 days of ‘normal’ eating – i.e. regular calorie intake - are contrasted with 2 days of low-calorie intake (normally 500cal or 25% of usual intake)
Alternative day fasting (ADF) – similar to 5:2 but with low calorie intake day frequency increased to every other day.
24 hour fast – a more extreme version of 5:2 where no calories are consumed at all for 1 or 2 days per week.
For the purposes of this post I define intermittent fasting as any dietary practice that intermits intentional, extended periods of fasting with periods of eating. However, I note that more research is needed to determine whether or not the different protocols have varying levels of efficacy.
Mechanistic overview of intermittent fasting as a strategy for weight loss
There are two reasons why fasting may have benefit over CER diets as a weight loss strategy.
1 – Adherence and ease
There is evidence that IF diets are easier to follow than standard continuous energy restriction diets and consequently those following them experience higher levels of dietary adherence.
One study (on the 5:2 protocol) noted that the fact that subjects did not need to count calories each day was a strong factor that positively affected dietary adherence. This gave subjects a sort of ‘willpower break’ that improved adherence on days where calorie intake needed to be monitored.The study further noted that subjects displayed high levels of satiety throughout the study duration and displayed no compensatory eating on non-restricted days.
A further review noticed even greater benefit. Observing an important ‘carry over effect’ of reduced energy consumption by approximately 20% on non-restricted days. This is important as people often expect that they might ‘compensate’ for the low-calorie days by simply overeating on the non-restricted days. The research suggests otherwise.
These results are not universal however, with one study noting that a majority of participants rated the fasting days as making daily activities more difficult with 70% of participants claiming that this diet was more difficult to follow than previous (CER style) diets.
2 – Physiological Benefits
There may be some beneficial metabolic effects found through IF diets beyond simple weight loss. These mainly focus around reduced insulin levels and the consequent increase in lipolysis (the use of body fat for fuel).
One study revealed that fasting was better at promoting consistent insulin levels and preventing hyperinsulinemia (higher than average blood insulin concentration) than both a standard diet and a low carbohydrate diet. However, it was further stated that the lack of carbohydrate accounted for at least 71% of the reduction in insulin levels – suggesting that the majority of benefit was from the dietary content change, not the fasting itself.
These findings reiterated an earlier study that found IF was more effective at improving insulin sensitivity than CER diets. The trial concluded that IF is a suitable and safe alternative to CER for weight loss.
A further trial compared IF to a standard CER diet and found weight losses to be equated but noted that IF was most effective at reducing post prandial lipaemia (the presence of an abnormally high concentration of emulsified fat in the blood post eating).
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Evidence of Efficacy
Recent studies point to IF diets being both safe and effective in inducing weight loss and treating obesity.
A 2018 systematic review noted that IF was effective for weight loss among normal weight, overweight and obese people in the short term. This echoed an earlier review that concluded IF diets were not harmful physically or mentally in healthy, normal weight, overweight, or obese adults and could be used as a valid weight loss strategy.
More effective than CER diets?
There may not yet be sufficient evidence to suggest IF diets are superior than CER diets overall.
A 2017 review of 7 randomised control trials found IF diets to be equivalent to CER diets for total weight loss in six of the studies and superior for reducing body fat in one. These results were reiterated in a later review that found - in 9 out of the 11 studies examined - IF diets produced roughly equivalent weight and body fat loss compared to CER diets.
A later (2019) review found that intermittent fasting diets do not seem to illicit greater weight loss than standard calorie restriction diets but further noted that there is need to discover whether fasting had favourable effects on body composition and/or metabolic markers.
Clearly, the general consensus is that both IF and CER diets illicit a comparable effect in promoting weight-loss and metabolic improvements and can both be used for maintaining weight in healthy weight individuals and in the treatment of obesity/overweight.
What should I do?
IF diets can be a safe and useful tool for people who wish to lose weight. Their main potential benefits (when compared to standard diets) include:
1 – potential for increased dietary adherence
2 – increased ability to regulate insulin levels
3 – increased use of body fat as an energy source
There is, however, not yet sufficient evidence to suggest that IF diets confer more benefit than standard CER diets – if adhered to properly. If you are someone wanting to lose body weight and are struggling to adhere to a standard calorie restricted diet then fasting can be a safe and effective alternative to utilise as a weight loss tool.
On a personal Note
I love intermittent fasting and use it daily. I find my energy levels are more consistent and, counterintuitively, I feel less hungry when I fast - making me less likely to overeat.
I use the 16/8 protocol (described above) mainly because I enjoy the freedom it allows. I simply wake up in the morning, drink some water and get on with my day. I often workout in a fasted state and have noticed no negative impact on my strength or endurance in doing so.
Feed the life you want to lead,
Duncan