This is a very hot topic right now, and a controversial one to some people. So I thought I would get my research hat on and take a look at the pros and cons of intermittent fasting.
Some call it a natural way of eating, some call it a fad, but what is it really? The first thing we need to do is have a look at what it actually involves.
Intermittent fasting is an eating pattern (not a diet) which involves long periods of fasting (not eating) with short periods of feasting (eating). This could take the form of fasting for 16 hours and feasting for 8, or fasting for 20 hours and feasting for 4, or even fasting for a full 24 hours now and then. There are also popular methods such as the 5/2 diet which is 5 days of feasting per week, with 2 days consuming less than 500 calories. All of these protocols are just variations on intermittent fasting.
What are people saying?
I've heard people claiming that intermittent fasting is a miracle way to burn body fat and gain muscle at the same time, it can extend your life, it improves hormone balance, increases energy levels, and all kinds of other things. I've also heard people on the other side of the fence saying it's unhealthy, skipping meals causes problems with your metabolism, you can't gain muscle with this way of eating, and much more. There are also some great articles out there giving a more balanced view, and I thought I would add in my own 2 pennies worth.
Does it burn more fat?
This is a two-part answer. Firstly the answer is no, intermittent fasting does not inherently burn fat more effectively than any other diet when calorie intake is matched (Varady, 2011; Harvie et al, 2011). HOWEVER, some people can find this method of eating more sustainable and for those people it can be more effective because they can stick to it longer term.
For example, if like me you prefer to eat larger meals or maybe don't find it easy to fit in eating regular small meals throughout the day then IF could be ideal. However, if you are in a calorie deficit you will lose weight and if you are in a calorie surplus you will gain weight, no matter what the eating pattern looks like.
Will it make me live longer?
Studies have shown that intermittent fasting can extend lifespan! Oh wait, that was in rats not humans, but the media as always have happily left that part out because it sounds better and sells more papers/gets more clicks. Standard.
According to Longo & Mattson (2014) "in rodents intermittent or periodic fasting protects against diabetes, cancers, heart disease and neurodegeneration, while in humans it helps reduce obesity, hypertension, asthma and rheumatoid arthritis" and this is borne out by other data discussed in CMAJ (Collier, 2013). There is huge potential to extend lifespan and improve quality of life but much more research needs to be done on humans.
Can I gain muscle with intermittent fasting?
Yes, if you are in a calorie surplus and training in the correct manner, the same as any other eating pattern. However, some people struggle to consume the extra calories they need when they restrict the hours in which they eat. Again, it all depends on the individual.
My advice if you go down this route is put your workouts during the feasting window. This way you can use the energy you consume to fuel the workout and lift more, stimulating more muscle growth, and then to refuel and recover afterwards.
Lots of studies have shown that exercising in a fasted state can reduce or even reverse muscle growth (Aragon & Schoenfeld, 2013; Schoenfeld, 2011; Lemon & Mullin, 1980; Loy et al, 1986; Schabort et al, 1999; Johnson & Leck, 2010). It's also been suggested that if muscle gain is important to you, do not fast longer than 16 hours. So for example pick the 16/8 method over 20/4.
Does it improve insulin levels?
Some people claim that extended periods of fasting are beneficial for insulin levels in the body, but studies show that the improvement with IF is no greater than with calorie restriction (Barnosky et al, 2014). In general, just eat a healthy balanced diet and reduce your calorie intake!
Will I have more energy?
Personally I have found that my energy and focus is improved in the morning before I first eat at around 12:30/1pm, but this is anecdotal evidence. Some people I know personally have found that they have much more energy if they eat small regular meals throughout the day. So the key here is to have a play around and find out what works best for you. We are all different!
Will it screw my metabolism?
Your metabolism will always adjust to the way you eat, and this is a sign that your body is functioning well. It's a survival response! If your body is in a calorie deficit your body will adjust to burn less calories, simple as that. However, there is no evidence that intermittent fasting slows your metabolism any more than calorie restriction.
Basically, it looks pretty damn good on paper, but can you stick to it? Some people will be able to sustain it and they will thrive, for other people this is a terrible idea. My advice is to pick a fasting protocol for 4 weeks and stick to it like glue, then you'll get a good indication of whether it is the right path for you. As always, mindset and habits are key. For support check out my coaching group here.
For me, the 16/8 plan works and I can sustain it. How about you? Comment with your experiences!
- Aragon AA, and Schoenfeld BJ. 2013. "Nutrient Timing Revisited: Is there a post-exercise anabolic window?" Journal of the International Society of Sports Nutrition 10 (1) (January): 5 https://doi.org/10.1186/1550-2783-10-5
- Barnosky AR, Hoddy KK, Unterman TG, Varady KA. 2013. "Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings" Translational Resarch 164 (4) (October): 302-311 http://www.sciencedirect.com/science/article/pii/S193152441400200X
- Collier R. 2013. "Intermittent Fasting: The science of going without" Canadian Medical Association Journal 185 (9) (June): 363-364 https://dx.doi.org/10.1503%2Fcmaj.109-4451
- Harvie MN, Pegington M, Mattson MP, Frystyk J, Dillon B, Evans G, Cuzick J et al. 2011. "The Effects of Intermittent of Continuous Energy Restriction on Weight Loss and Metabolic Disease Risk Markers: A Randomised Trial in Young Overweight Women." International Journal of Obesity 35 (5) (May): 714-727 http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=3017674&tool=pmcentrez&rendertype=abstract
- Johnson S, and Leck K. 2010. "The Effects of Dietary Fasting on Physical Balance among Healthy Young Women." Nutrition Journal 9 (January): 18 http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2864190&tool=pmcentrez&rendertype=abstract
- Lemon PW, and Mullin JP. 1980. "Effect of Initial Muscle Glycogen Levels on Protein Catabolism During Exercise." Journal of Applied Physiology 48 (4) (April): 624-629 http://www.ncbi.nlm.nih.gov/pubmed/7380688
- Longo VD, and Mattson MP. 2014. "Fasting: Molecular Mechanisms and Clinical Applications" Cell Metabolism 19 (2) (January): 181-192 http://linkinghub.elsevier.com/retrieve/pii/S1550413113005032
- Loy SF, Conlee RK, Winder WW, Nelson AG, Arnall DA, Fisher AG. 1986. "Effects of 24-Hour Fast on Cycling Endurance Time at Two Different Intensities." Journal of Applied Physiology 61 (2): 654-659 https://www.ncbi.nlm.nih.gov/pubmed/3745057
- Schabort EJ, Bosch AN, Weltan SM, Noakes TD. 1999. "The Effect of a Preexercise Meal on Time to Fatigue during Prolonged Cycling Exercise." Medicine and Science in Sports and Exercise 31 (3) (March): 464-471 http://www.ncbi.nlm.nih.gov/pubmed/10188753
- Schoenfeld BJ. 2011. "Does Cardio After an Overnight Fast Maximise Fat Loss?" Strength and Conditioning Journal 33 (1) (February): 23-25 http://content.wkhealth.com/linkback/openurl?sid=WKPTLP:landingpage&an=00126548-201102000-00003
- Varady KA, Bhutani S, Church EC, Klempel MC. 2009. "Short-Term Modified Alternate-Day Fasting: A Novel Dietary Strategy for Weight Loss and Cardioprotection in Obese Adults." American Journal of Clinical Nutrition 90 (5): 1138-1143 https://www.ncbi.nlm.nih.gov/pubmed/19793855