Is Intermittent Fasting for You?

by Hong Yiying (Ph.D., CSCS, NROC, CPT)

IF: intermittent fasting, which consists of periods of normal eating and periods of complete fasting

IER: intermittent energy restriction, which reduces energy intake to a certain level during the energy-restricted period.

CER: continuous energy restriction, which is the normal dieting method of equal energy reduction every day.

Intermittent Fasting (IF) is one of the dieting methods that has been gaining popularity over the recent years. It is characterized by a strict food intake window followed by a relatively long period of zero calorie intake. Closely related to IF, Intermittent Energy Restriction (IER) reduces calorie intake on the energy-restrictive days by a certain percentage (e.g. 50%, 70%, etc.). The majority of past research studies on IER/IF found that they work by the principle of calorie restriction, just like most other dieting methods, because of the restricted time window of eating. However, recent research findings revealed additional benefits besides weight loss.

The Methods
There are various methods in IER/IF:
  • Time-restricted feeding: Fasting 12 to 21 hours per day and having a specified eating window to consume food

  • Alternate-day fasting: One day of normal eating followed by one day of no energy intake

  • Alternate-day energy restriction: One day of normal eating followed by one day of reduced energy intake

  • 5:2: Five days of normal eating, two days of reduced intake
    • Note that more research was conducted using the alternate-day or 5:2 method.

The Pros
Excess energy intake, weight gain and subsequent fat tissue accumulation are consistently linked to illness, disability and mortality [1]. Energy restriction has been shown to prevent age-related disease including tumors, cardiovascular disease, diabetes and dementia; retard aging-related functional decline; and increase lifespan [2].

Despite the overwhelming evidence of the benefit of maintaining a healthy body-composition, most dieting methods fail eventually due to poor compliance. The continuous energy restriction (CER) method requires careful daily planning and calculation of calorie intakes, which results in high dropout rate. Many fad diets place extreme restrictions on certain food groups, such as paleo diet limiting dairy products, legumes and grains; and keto diet restricting carbohydrates. Such extremity often causes poor adherence in the long term. Most diet studies (and diet programs) lasted a few months to two years, when obesity is a chronic condition [4].

IER can have a higher adherence rate due to the fact that one only needs to restrict energy intake during certain periods, which makes it more achievable than the standard approach of CER [1]. 

The IF does not work without overall energy restriction. Studies found that when energy restrictions are equal, IF and CER produced similar weight loss and body composition changes [1, 5, 6]. Reduction in fat mass, especially visceral (abdominal), hepatic, and intramuscular fat store in obese individuals has important therapeutic effects. It was found that hepatic and visceral fat stores rapidly mobilize with marked energy restriction [7].

The Cons
Short-term studies of IF/IER raised the possibility of harm among normal weight subjects [1]. Large fluxes of free fatty acids were observed with 1-2 days of IER each week, and were even greater with 1-2 days of IF. These fluxes can lead to skeletal muscle insulin resistance. Single bouts of total fasts lasting 24 to 48 hours in non-obese individuals have been associated with modest increase in hepatic (in men) and intramuscular fat (in women). However, the effect of long-term IF/IER needs more study [1]. Overall, it seems that IER evokes smaller fluctuations in free fatty acids and ketones than IF. IF is linked to short-term impaired glucose tolerance during the resumption of normal feeding [1].

Some studies suggest that IF could have adverse effects on ovulation and fertility in women [8].

There is theoretical concern of erratic eating behaviors induced by IER. Existing data showed that in general, IER can improve eating behaviors and mood amongst subjects with overweight and obesity, but may have potential harmful effects among normal weight individuals with unrestrained eating styles [1]. 

The Uncertainty
In a recent study by Michelle R Jospe et al. published in 2019, it was found that IF (5:2 method) resulted in small and beneficial metabolic changes such as reduction in systolic blood pressure [9]. The same study also found Mediterranean diet resulted in reduced glycated hemoglobin. However, the authors cautioned that the between-group differences in most outcomes were not significant.

The effects on insulin resistance also presented mixed results, and sometimes gender differences [1]. Sutton et al. [10] conducted a study on eight obese men with prediabetes. One group underwent “early time-restricted feeding,” where eating was restricted to an early eight-hour window (7 am to 3 pm); the other group had the meals spread out over 12 hours (7 am and 7 pm). Both groups maintained their weight but after five weeks, the “early time-restricted feeding” group had significantly improved insulin sensitivity and significantly lower blood pressure. However, there have been other studies showing no effect on insulin resistance [1, 11].

Trepanowski et al. [12] in a randomized controlled trial that followed 100 obese individuals for 12 months found “no significant differences in blood pressure, heart rate, fasting glucose, and fasting insulin. At 12 months, although there were no differences in total cholesterol and triglycerides, the alternate-day fasting group showed significantly increased low-density lipoprotein (LDL) cholesterol levels,” suggesting a negative effect in the blood lipid profile. 

Why IER/IF Works
As we can see from above, the main way IER works is still caloric restriction. What makes it a favorable diet option for some lies in:

  1. Caloric intake
    • ​​The overall caloric intake is reduced due to the strict time window of eating, at least this is the way it should be.

  2. Awareness of food and nutrition
    • ​​Creating awareness is actually the first step in any successful diet management. Being aware of the calories you are taking in when under an IER diet helps to curb excessive food consumption.

  3. Long-term adherence
    • ​​The IER has better long-term adherence compared to IF and other more extreme diets. In many of the studies, the weight loss effect were likely a result of longer term adherence [1].​​

Common Mistakes in IER/IF
  • Eat as much as you want
    • ​As mentioned above, for IER/IF to work, overall energy restriction is required for weight loss to happen.

  • Eat whatever you want during the feeding window
    • ​The majority of IER regimens recommend healthy eating and not feasting on non-restricted days. In one study, a alternate day energy restriction diet (-25% energy needs) with high fat (60%kcal as fat) produced equivalent weight loss to that with a low fat (25%kcal as fat) (HF: -6.6 ± 0.5 kg, LF: -4.7 ± 0.6 kg). However,  the high fat diet group had an indicator of increased risk of atherosclerosis and hypertension. The low-fat group showed increased adiponectin and decreased fat mass, waist circumference, leptin, and resistin.

Who Should Not Be Doing IER/IF
IF is not for you if you are:
  • Younger than 23 or older than 75

  • Pregnant or breastfeeding

  • Diagnosed with advance type 1 diabetes

  • Struggling with a chronic sleep disorder

  • Challenged with a history of eating disorders or disordered eating

​Consult your doctors before adopting IER/IF if you are one of the followings:
  • Diagnosed with diabetes or low blood sugar.

  • Having a history of irregular periods

  • Women who are trying to conceive

The Bottomline
  • Not everybody is suitable for IF/IER
    • As seen from the discussion above, the IEF/IF usually has more beneficial effects for obese individuals but may have harmful effects or normal weight individuals, although it remains largely inconclusive as for the latter.

  • It’s personal
    • Some people find IF very challenging while others find it easier to implement compared to other diet regimens. There are also different methods of IER and IF that may appeal to different people. 

  • It’s not a band-aid method; treat it as a long-term commitment
    • Many people treat dieting like a band-aid, thinking by adopting a diet regimen for a short period of time just to reach their weight target is all it takes. It is not. Diet is a lifestyle thing. Short-term dieting results in yo-yo diet patterns, which can be more harmful to health [13]. Choose one that you can adhere to in the long term.

  • ​It’s not just about the eating time-window
    • ​The overall calorie consumption should be reduced. Target a healthy weight loss by reducing no more than 1000 kcal a day.

    • Select an earlier window if it can be sustained. 

    • Proper proportions of nutrients should be consumed. Adhere to the acceptable macronutrient distribution ranges (AMDR) for healthy weight management (45-65% carbohydrate, 10-35% protein, 20-35% fat).

    • Focus on the quality of food. Eating clean can be more important than eating less.

  • ​One step at a time
    • ​You may take a gradual approach by slowly reducing your eating window towards the target, rather than a dramatic switch. For example, if your normal feeding window is 7 am to 10 pm, you can first adjust to 7 am to 9 pm, then 7 am to 7 pm, and so on.

  • ​​​Beyond weight loss
    • ​A good body composition with higher lean body mass and lower visceral fat is the more accurate gauge of health than the number of weight. Losing weight purely by dieting almost always results in loss of lean body mass [14]. Physical activity including muscle strengthening exercises is essential to achieving good health.

  • ​Stay up-to-date

    • ​The research on the effects of IF/IER is still evolving. Evidence is especially lacking on the long term effect. Results on its effect with regard to hormones and metabolic markers are mixing. Stay away from hear-say and use evidence-based method when it comes to health concerns.​

[1] Harvie, M., and Howell, A. (2017). Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects—A Narrative Review of Human and Animal Evidence. Behavioral Sciences 7, 4.

[2] Chung, K.W.; Kim, D.H.; Park, M.H.; Choi, Y.J.; Kim, N.D.; Lee, J.; Yu, B.P.; Chung, H.Y. Recent advances in calorie restriction research on aging. Exp. Gerontol. 2013, 48, 1049–1053.

[3] Anastasiou, C.A.; Karfopoulou, E.; Yannakoulia, M. Weight regaining: From statistics and behaviors to physiology and metabolism. Metabolism 2015, 64, 1395–1407.

[4] Freedhoff, Y. (2016). Weight loss diet studies: we need help not hype. 388, 3.

[5] Seimon, R.V. 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, 153-172.

[6] Harris, L. et al. (2018). Intermittent fasting interventions for treatment of overweight and obesity in adults. JBI Database of Systematic Reviews and Implementation Reports,16, 2, 507-547.

[7] Taylor, R. Banting Memorial lecture 2012: Reversing the twin cycles of type 2 diabetes. Diabet. Med. 2013, 30, 267–275.

[8] Fujiwara, T. and Nakata, R. (2010). Skipping breakfast is associated with reproductive dysfunction in post-adolescent female college students. Appetite,55, 3, 714-717.

[9] Jospe, M.R., Roy, M., Brown, R.C., Haszard, J.J., Meredith-Jones, K., Fangupo, L.J., Osborne, H., Fleming, E.A., and Taylor, R.W. (2020). Intermittent fasting, Paleolithic, or Mediterranean diets in the real world: exploratory secondary analyses of a weight-loss trial that included choice of diet and exercise. The American Journal of Clinical Nutrition 111, 503–514.

[10] Sutton, E.F., Beyl, R., Early, K.S., Cefalu, W.T., Ravussin, E., and Peterson, C.M. (2018). Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metabolism 27, 1212-1221.e3.

[11] Catenacci, V.A.; Pan, Z.; Ostendorf, D.; Brannon, S.; Gozansky, W.S.; Mattson, M.P.; Martin, B.; MacLean, P.S.; Melanson, E.L.; Troy, D.W. A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity. Obesity 2016, 24, 1874–1883.

[12] Trepanowski, J.F. et al. (2017). Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults: A randomized clinical trial. JAMA Internal Medicine, 177, 7, 930-938.

[13] Strohackerâ, K., Carpenterâ, K.C., and McFarlinâ, B.K. Consequences of Weight Cycling: An Increase in Disease Risk? 11.

[14] Willoughby, D., Hewlings, S., and Kalman, D. (2018). Body Composition Changes in Weight Loss: Strategies and Supplementation for Maintaining Lean Body Mass, a Brief Review. Nutrients 10, 1876.
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