Intermittent fasting (IF) best for weight loss and body composition?

Is IF superior for weight loss, fat loss and improving body composition?

Table of contents:

• IF types
• What does the research show?
• Practical recommendations for using IF to lose weight
• Practical recommendations for using IF when optimising body composition
• Take aways

IF types

IF, also known as time restricted feeding (TRF), alternates periods of feeding with periods of no food intake (fasting). Here are a few examples of different types:

Feeding window: you eat in a specific window and fast in a specific window.

Example: 16:8 – you fast during 8pm-12pm and you eat during 12pm-6pm.

Random meal skipping: you randomly skip a meal.

Eat-stop-eat: includes fasting or severely restricting calories on certain days.

Example: 5:2 diet, you severely restrict calories on two days of the week.

Alternate day fasting: there are different forms of this.

Example: 24 hours – eat for 24 hours, fast for 24 hours.

What does the research show?

Weight loss:

A large-scale review of nine randomised clinical trials lasting 12-52 weeks involving 782 overweight individuals, compared IF energy restriction with standard caloric energy restriction and found no significant differences for weight loss (1). This is supported by multiple other systematic studies (2,3).

This does not mean that IF may not be an effective tool for weight loss (4). How effective IF may be will depend upon a variety of factors – the individual, the specific IF variant studied, how well it suited the individual and whether they could adhere to it.

For example, skipping breakfast may be beneficial for some individuals to lose weight (5), but not for others (6). For individuals with more weight to lose, IF may be more beneficial.

Weight loss requires creating a meaningful calorie deficit and IF may be a tool for this. The ability to effectively restrict calorie intake with IF could be explained mechanistically by increased levels of ketones, associated with increased levels of fullness (7).

However it is likely that IF results in overall reduced energy intake because there is less time to consume food. This is shown in an alternate fasting day study where the IF group, although prescribed to eat at maintenance calories, consumed upon average 239 less calories per day, because they had 3 days within the week where they were fasted (8).

There is some evidence that IF may result in individuals spontaneously reducing their calories even on unrestricted days (9). However, there is also evidence that people eat more than prescribed on fasting days and less on feeding days, unconsciously adopting a more continuous caloric restriction model.

Other benefits of IF may include not feeling like you are “dieting” (10) and providing very simple guidelines that people can easily follow and therefore stick to.

Additionally, once the schedule of feeding and fasting at certain time has been practised, hunger and fullness signals are likely to fall in line with these times and therefore make it easier for the dieter to adhere to.

Verdict: IF is as effective as continuous caloric restriction for reducing bodyweight when the calorie deficit is matched.

Body composition (lose body fat and retain or gain lean muscle mass):

IF is associated with a significantly greater reduction in lean body mass than continuous caloric restriction (1). Studies that show this reduction in lean body mass (11,12) included very different protocols – one was a 5:2 protocol of severely restricting calories for two days, with five days of habitual eating (11), the other had two-week blocks of energy restriction alternated with two-weeks of energy balance (12).

The fact that these studies were so different, but there were similar lean mass losses could indicate that there is something inherent to IF causing this. These findings are supported by other studies showing a trend for a larger reduction in fat free mass during caloric restriction from IF versus continuous caloric restriction (8).

However, the proportion of lean mass lost during an energy deficit depends upon various factors: the degree of energy deficit imposed (13), type of exercise undertaken (14), and protein intake (15). Therefore whether the difference in lean body mass we see associated with IF is due to one of these factors rather than IF itself, requires further research.

Additionally, it is important to note that the difference in lean body mass found, whilst statistically significant, is actually relatively small <1kg (1) and could be explained by a reduction in glycogen and body water.

Verdict: IF may result in greater losses of lean body mass than continuous caloric restriction.

Practical recommendations for using IF to lose weight:

Plan your calorie deficit over the week: as how well you consistently adhere to this over the week determines the weight loss. If you currently have consistent and regular meals, then you could create a calorie deficit from omitting a daily snack/meal component. If you track your calories, then you can also do it this way.

Either way, plan and know how you will create the calorie deficit.

Choose a fasting window that suits you: so that you can consistently stick to it. Reflect upon your current daily schedule and non-negotiables i.e any mealtime or work/life commitment that influences your eating schedule. Once you have determined your preferred window of feeding and fasting, trial it. You can then review it and introduce more flexibility later if desired.

Focus on whole-food balanced meals: to create sustainable and healthy eating habits and avoid over consuming in the feeding window. Meet your protein and fibre intake and ensure that the calorie deficit is not too aggressive.

Choose your training time wisely: regular exercise can contribute to a meaningful caloric deficit and weight loss maintenance. Consider the time within the feeding and fasting window to make the most gains from your exercise session.

Practical recommendations for using IF when optimising body composition:

Shoot for a higher protein intake: in general, an intake of 1.6-2.4 g/kg bodyweight is optimal for fat loss and muscle gains. However, even increasing protein intake to 2.4-2.8 g/kg protein/bodyweight may be beneficial (16)– associated with better lean muscle mass retention and feeling full (17).

If you are already especially lean and train a lot, then a protein intake of up to 3g /kg may even work for you. However, if you are overweight, go for the lower end of 1.6 g/kg protein/bodyweight.

Distribute protein evenly throughout your feeding window: for example, with a daily intake of 2.4 g/kg, consume four meals of 0.6 g/kg protein per meal within your feeding window. Try to distribute your protein into as many meals as you think is reasonable.

More isn’t necessarily better: if you are greatly overweight, then you will likely have more flexibility with your fasting capacities. However, if you are already at a healthy weight/lean (wanting to optimise body composition), or wanting to build lean muscle, then be mindful of the fasting window.

IF may provide practical benefits for many and more fasting time may be enticing. However, the longer the fasting period, the more of a catabolic state you will be in, which over time may be detrimental to you – depending upon who you are.

Also, if you are prioritising optimising body composition, then you will likely be training regularly and therefore need the appropriate nutrient availability to respond to that stimulus.

Practise resistance training: resistance training stimulates muscle protein synthesis so that you can build muscle mass. Essentially, the resistance exercise damages the muscle protein fibre and a signal is then sent for ingested proteins to be turned into new muscle tissue as a repair mechanism.

Take aways

Prioritise creating an appropriate calorie deficit for weight loss: which IF may be a means to do so. However, IF may increase the likelihood of overconsuming food in a feeding window and therefore be ineffective for restricting calories.

Prioritise choosing the method of creating a calorie deficit that you can most consistently adhere to versus what you perceive to be most popular.

Ensure that you meet your daily protein intake: as IF may be a barrier to you more easily reaching this. Anywhere between 1.6-2.4 g/kg is likely to be benefit you reaching your goals – but listen to your body and if you do better on the lower/higher end, then stick with that, and vice versa. We are all different.

Plan, schedule, commit: whether you include IF or not, planning and scheduling your meals will help you to appropriately meet your energy and protein needs and fuel yourself with good quality food that you feel great on and enjoy – integral for any long-term sustainable change.

And obviously, you won’t meet your goals without consistent commitment!

Train appropriately: resistance training is essential for losing fat and optimising body composition and will be super helpful for weight loss. Determine how much cardio you need for overall health and to contribute to a potential calorie deficit. Don’t forget that cardio training is not an exchange for daily activity.

Sleep enough: sleep is critical for managing hunger levels, energy intake and stress. Your muscles need to repair and you need to recover – mind and body. If you are in a calorie deficit and fasting, sleep becomes increasingly important to effectively manage these different factors.

Related articles

Fasted cardio for weight loss?

Intermittent fasting and metabolic health

Nutrient timing – what’s the deal and does it matter when you eat?

What are your thoughts and experiences with IF for weight loss and body composition?

References

1. Roman et al. (2018). Effects of intermittent versus continuous dieting on weight and body composition in obese and overweight people: a systematic review and meta-analysis of randomised controlled trials.
2. Harris et al. (2018). Short-term intermittent energy restriction interventions for weight management: a systematic review and meta-analysis.
3. Harris et al. (2018). Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis.
4. Tinsley et al. (2015). Effects of intermittent fasting on body composition and clinical health markers in humans.
5. Geliebter et al. (2014). Skipping breakfast leads to weight loss but also elevated cholesterol compared with consuming daily breakfasts of oat porridge or frosted cornflakes in overweight individuals: a randomised controlled trial.
6. Betts et al. (2014). The causal role of breakfast in energy balance and health: a randomised controlled trial in lean adults.
7. Boden et al. (2005). Effect of a low-carbohydrate diet on appetite, blood glucose levels and insulin resistance in obese patients with type 2- diabetes.
8. Hutchinson et al. (2019). Effects of intermittent versus continuous energy intake on insulin sensitivity and metabolic risk in overweight women.
9. Harvey et al. (2018). Intermittent energy restriction for weight loss: spontaneous reduction of energy intake on unrestricted days.
10. Donnelly et al. (2018). ‘For me it’s about not feeling like I’m on a diet’: a thematic analysis of women’s experiences of an intermittent energy restricted diet to reduce breast cancer risk.
11. Carter et al. (2016). The effects of intermittent compared to continuous energy restriction on glycaemic control in type 2 diabetes; a pragmatic pilot trial.
12. Byrne et al. (2018). Intermittent energy restriction improves weight loss in obese men: the MATADOR study.
13. Chaston et al. (2007). Changes in fat-free mass during significant weight loss: a systematic review.
14. Ballor et al. (1988). Resistance training during caloric restriction enhances lean body weight maintenance.
15. Kim et al. (2016). Effects of dietary protein intake on body composition changes after weight loss in older adults: a systemtatic review and meta-analysis.
16. Mettler et al. (2010). Increased protein intake reduces lean body mass loss during weight loss in athetles.
17. Aragon et al. (2017). International society of sports nutrition position stand: diets and body composition.