Understanding Intermittent Fasting

Intermittent Fasting (IF) is a method that has become popularised for weight loss.   The three broad methodologies are:

Alternate Day Fasting (ADF)
Involves alternating fasting days (no energy-containing foods or beverages consumed) with eating days (foods and beverages consumed ad libitum)

Time-Restricted Feeding (TRF)
Allows ad libitum energy intake within specific time frames within a 24hr period, say, an 8hr period between 10am and 6pm.

Modified Fasting Regimens (MFR)
Allows consumption of 20–25% of energy needs on scheduled fasting days; the basis for the popular 5:2 diet, which involves severe energy restriction for 2 non-consecutive days per week and ad libitum eating for the other 5 days

 

Alternate-Day Fasting

Alternate-day fasting involves alternating fasting days, during which no calories are consumed, and feeding days, during which foods and beverages are consumed ad libitum. 

  • Of four studies (1-4), two of three studies reported significant weight loss (1, 3, 4), although one of those studies (4) lasted only one day which is far from robust.

  • In the 22-day study (3) of alternate-day fasting, participants experienced a mean 2.5% weight loss.

  • Three of the studies (2-4) found a significant decrease in at least one glucoregulatory marker, however, the study that included overweight and obese participants did not and, in fact, reported a detrimental effect of 1-day total fasting on postprandial glucose and insulin the following day (3).

  • A consequence of ADF are issues of lower mood and perceived work performance (5) hunger (3) and mental status, as well as post-fast energy intake, are important outcomes to consider with extended fasting during waking hours.

The sparse data on ADF suggest that this regimen can result in modest weight loss and lead to improvements in some metabolic parameters.  However, reports of extreme hunger while fasting and adverse mood effects suggest are hardly desirable.

 

Time Restricted Feeding

There is limited data on human studies in TRF:  Two studies found significant reductions in weight.  A study (6) in 29 normal weight men (2 weeks per study condition) prescribed a night-time fasting interval of ≥11 hours, which resulted in a significant weight change difference between the intervention (−0.4kg) and control (+0.6kg) conditions, equivalent to a 2.1% weight loss.

Another crossover study reported a 4.1% weight loss effect of consuming a single meal in the afternoon each day for 8 weeks without calorie restriction compared with an isocaloric diet consumed as three meals per day (7, 8).  The one meal per day condition was also associated with reductions in fasting glucose and improvements in cholesterol.

Although self-reported hunger was higher in the morning for those consuming one meal per day, this fasting regimen was considered acceptable because there were no mean changes in measurements of tension, depression, anger, vigour, fatigue, or confusion.

 

Modified Fasting Regimens

Modified fasting regimens generally specify that energy consumption is limited to 20–25% of energy needs on regularly scheduled fasting days.  In these studies, the term fasting is used to describe periods of severely limited energy intake rather than no energy intake.  This type of regimen, also called intermittent energy restriction, is the basis for the popular 5:2 diet, which is one example and involves energy restriction for 2 non-consecutive days per week and unrestricted (ad libitum) eating during the other 5 days of the week (9).

  • Of nine studies (10-18) of different regimens, 7 of them (78%) reported statistically significant weight loss, which ranged from 3.2% in comparison with a control group (10) during a 12-week period to 8.0% in another during an 8-week period (15).

  • Three of six studies found significant decreases in fasting insulin, and one found reductions in fasting glucose.

  • Three of eight studies found significant improvements in lipids and / or cholesterol.

  • In only one case did the fasting regimen result in significantly more weight loss (mean loss 9.6%) than a standard weight-loss diet (mean loss 5.5%) (18).

  • The 12-week, controlled weight-loss trial found that the modified fasting regimen combined with an exercise protocol produced significantly superior weight loss results (mean loss 6.5%) compared with fasting alone (mean loss 3.2%) or exercise alone (mean loss 1.1%) (10).

  • Half of these studies assessed some aspect of mood or other behavioural side effects in response to the fasting regimen (12, 13, 15, 17).  In general, these studies reported that a small proportion (generally <15%) of participants reported negative side effects, such as feeling cold, irritable, low energy, or hunger.  There were also improvements in mood, including reductions in tension, anger, and fatigue, and increases in self-confidence and positive mood.

 

Summary

  • Alternate-day fasting (ADF) appeared to result in weight loss, as well as reductions in glucose and insulin concentrations, in the three studies evaluating this regimen. However, this fasting regimen may not be practical because it leads to intense hunger on fasting days.

  • Data from related human studies are limited regarding the positive impacts of Time Restricted Feeding (TRF) on weight or metabolic health.

  • Modified Fasting Regimes result in reduced weight, with reductions ranging from 3.2%, in comparison with a control group (10) during a 12-week period, to 8.0%, in a one-arm trial during an 8-week period (15).

  • Overall, evidence suggests that intermittent fasting regimens are not harmful physically or mentally (i.e., in terms of mood) in healthy, normal weight, overweight, or obese adults.

  • It appears that almost any intermittent fasting regimen can result in some or even significant weight loss.

 

Intermittent Fasting or a Calorie Controlled diet?

Reviews and meta-analyses (19-23) unanimously report that the overall metabolic benefits of IF are not superior to those of a traditional calorie-controlled diet.

So why bother?

All diets work on the basic science of reducing calorie intake, the difference between some is the composition (e.g. carbs, proteins, fats and certain foods which are allowed or not allowed), whilst the difference with Intermittent Fasting is the scheduling of the restricted calorie intake.

IF, especially MFR, allows for ‘days off’ which means that long term adherence to IF could be more likely than a 24/7 diet every day of the week (24-27).  Not only can this be punishing and less enjoyable, we tend to beat ourselves up if we mess up on a calorie-controlled diet or cave into our cravings one day.

Intermittent Fasting is more manageable as the effort required on the restricted intake days, is tempered by the knowledge that it won’t last; we only have to be ‘good’ for a set number of days until we are allowed to cheat, completely guilt free.

It is also more manageable, meaning we can schedule our cheat days around social events – who wants to be dieting at a party?   Or we can plan our restricted intake around work on Monday to Friday, but completely relax and let go, without conscience, at the weekend when we are with friends and family.

Weight loss is much more about restricting calories in, rather than the effort of calories out (See Myth 2: Calories in Vs Calories Out) so the difference with IF is really down to the practicality and comparative ease of adherence compared to typical calorie-restricted diets.

In short, be good most of the time but have a cheat day or take the weekend off.   It will be more manageable, more enjoyable and – as a consequence – more sustainable. And that is the goal of any healthy diet regime.

References

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  2. Halberg N, Henriksen M, Soderhamn N, Stallknecht B, Ploug T, et al.  (2005)  “Effect of intermittent fasting and refeeding on insulin action in healthy men”.  Journal of Applied Physiology.  99:2128–36

  3. 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”.  The American Journal of Clinical Nutrition.  81:69–73

  4. Horne BD, Muhlestein JB, Lappe DL, May HT, Carlquist JF, et al.  (2013)  “Randomized cross-over trial of short-term water-only fasting: metabolic and cardiovascular consequences”. Nutrition, Metabolism & Cardiovascular Diseases.  23:1050–57

  5. 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”.  Journal of Health Psychology.  20:702–11

  6. LeCheminant JD, Christenson E, Bailey BW, Tucker LA.  (2013)  “Restricting night-time eating reduces daily energy intake in healthy young men: a short-term cross-over study”.  British Journal of  Nutrition.  110:2108–13

  7. Carlson O, Martin B, Stote KS, Golden E, Maudsley S, et al.  (2007).  “Impact of reduced meal frequency without caloric restriction on glucose regulation in healthy, normal-weight middle-aged men and women”.  Metabolism.  56:1729–34

  8. Stote KS, Baer DJ, Spears K, Paul DR, Harris GK, et al.  (2007)  “A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults”.  The American Journal of Clinical Nutrition.  85:981–88

  9. Mosley M, Spencer M.  (2013)  The FastDiet: Lose Weight, Stay Healthy, and Live Longer with the Simple Secret of Intermittent Fasting. New York: Atria

  10. Bhutani S, Klempel MC, Kroeger CM, Trepanowski JF, Varady KA.  (2013)  “Alternate day fasting and endurance exercise combine to reduce body weight and favorably alter plasma lipids in obese humans”.  Obesity.  21:1370–79

  11. Eshghinia S, Mohammadzadeh F.  (2013)  “The effects of modified alternate-day fasting diet on weight loss and CAD risk factors in overweight and obese women”.  Journal of Diabetes & Metabolic Disorders.  12:4

  12. Harvie MN, Pegington M, Mattson MP, Frystyk J, Dillon B, et al.  (2011)  The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. International Journal of Obesity. 35:714–27

  13. Harvie MN, Wright C, Pegington M, McMullan D, Mitchell E, et al. (2013)  The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. British Journal of Nutrition.  110:1534–47

  14. Hoddy KK, Gibbons C, Kroeger CM, Trepanowski JF, Barnosky A, et al.  (2016)  “Changes in hunger and fullness in relation to gut peptides before and after 8 weeks of alternate day fasting”. Clinical Nutrition.   35:1380–85

  15. Johnson JB, Summer W, Cutler RG, Martin B, Hyun DH, et al.  (2007)  “Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma”.  Free Radical Biology & Medicine.  42:665–74

  16. 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”.  The American Journal of Clinical Nutrition.  90:1138–43

  17. Varady KA, Bhutani S, Klempel MC, Kroeger CM, Trepanowski JF, et al.  (2013)  “Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial”.  Nutrition Journal.  12:146

  18. Williams KV, MullenML, Kelley DE, Wing RR.  (1998).  “The effect of short periods of caloric restriction on weight loss and glycemic control in type 2 diabetes”.  Diabetes Care.  21:2–8

  19. Barnosky AR, Hoddy KK, Unterman TG, Varady KA.  (2014)  “Intermittent fasting versus daily calorie restriction for type 2 diabetes prevention: a review of human findings”.  Translational Research.  164:302–11

  20. Harvie MN, Howell T.  (2016)  “Could intermittent energy restriction and intermittent fasting reduce rates of cancer in obese, overweight, and normal-weight subjects? A summary of evidence. Advances in Nutrition  7:690–705

  21. Headland M, Clifton PM, Carter S, Keogh JB.  (2016)  “Weight-loss outcomes: a systematic review and meta-analysis of intermittent energy restriction trials lasting a minimum of 6 months”.  Nutrients.  8:354

  22. Seimon RV, Roekenes JA, Zibellini J, Zhu B, Gibson AA, et al.  (2015)  “Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials”.  Molecular and Cellular Endocrinology.  418(Pt. 2):153–72

  23. Varady KA.  (2011).  “Intermittent versus daily calorie restriction: Which diet regimen is more effective for weight loss?”  Obesity Reviews.  12:e593–601

  24. Tiffany A. Dong, Pratik B. Sandesara, Devinder S. Dhindsa, Anurag Mehta, Laura C. Arneson, Allen L. Dollar, Pam R. Taub, Laurence S. Sperling.  (2020)  “Intermittent Fasting: A Heart Healthy Dietary Pattern?”  The American Journal of Medicine.  133(8):901-907

  25. Enríquez Guerrero A.  San Mauro Martín I.  Garicano Vilar E. et al.  (2021)  “Effectiveness of an intermittent fasting diet versus continuous energy restriction on anthropometric measurements, body composition and lipid profile in overweight and obese adults: a meta-analysis”.  European Journal of Clinical Nutrition.  75:1024–1039

  26. Patikorn C.  Roubal K. Veettil SK. et al.  (2021)  “Intermittent Fasting and Obesity-Related Health Outcomes: An Umbrella Review of Meta-analyses of Randomized Clinical Trials”. JAMA Network Open.  https://jamanetwork.com/journals/jamanetworkopen/article-abstract/2787246

  27. Keenan S.  Cooke MB. Belski R.  (2020)  "The Effects of Intermittent Fasting Combined with Resistance Training on Lean Body Mass: A Systematic Review of Human Studies".  Nutrients. 12(8): 2349.