What do Kourtney Kardashian, Halle Berry, and Moby all have in common? Well, besides their eight-digit net worth, they’re all promoters of intermittent fasting.
Fasting for health reasons has gained a lot of attention recently. But is the hype merited? Is life in the fast lane right for you? Let’s break it down …
History of Fasting
While it may be trendy now, fasting has been practiced for thousands of years and is deeply rooted in religion. The word itself comes from the Anglo-Saxon word fasten, or “to hold oneself” from food.1 The sacrificial act of abstinence is still commonly practiced today during holy observations like Ramadan, Lent, and Yom Kippur.
The Physiology of Fasting
Mammals need food to survive and make babies. Period. Therefore, our bodies have evolved to adapt to hard times when food isn’t readily accessible.2 After 12 hours without food, we start a process called “lipolysis” and pull energy from fat tissue.3 Humans can live off fat stores for a few weeks, followed by protein breakdown and eventual organ failure.4 Unfortunately, there lies the reason why that McFlurry gets stored as fat in our butt, belly, and liver. Fat acts our reserve energy source, if we should ever need it.
Popular Fasting Approaches
Continuous Calorie Restriction: This approach is different from most forms of fasting. Instead of fasting or “restraining from” food all together for a given period of time, calorie restriction reduces our overall intake of food without restricting to the point of malnutrition. This approach mimics most reduced-calorie diets.
Alternate Day Fasting: This one on, one off method allows you to eat as you please for one day, followed by a day of fasting (no calories or very few calories).
Modified Fasting: Instead of fasting completely, this modified approach limits food intake to less than 25% of your estimated needs. A popular form of this diet is the 5:2 method. The two on, five off approach (and its #1 New York Times bestselling book) sparked a lot of today’s interest in intermittent fasting.5 Basically, you do a modified fast for two days of your choice (<500-600 kcal per day) and then eat normally for the other five days.
Time Restricted Fasting: This fasting approach confines the hours you can eat, rather than the amount you eat. For example, a common approach is limiting your food intake to 8 hours on from 12pm to 8pm, and 16 hours off from 8pm to 12pm the next day. Another popular method is only eating during the daylight hours (if you’re from New England or Alaska, you may want to rethink this approach during winter months).
Possible Health Benefits
Fasting is gaining a lot of attention in the research world. However, many studies have relied on rodents and not human subjects. While animal models are an instrumental part of research, there are disadvantages from methodological and pathophysiological perspectives.11,12 Therefore, I’ll focus on the evidence from human studies.
By cutting back on how much we eat or when we eat, weight loss is expected thanks to, well, math (caloric deficit). The majority of research backs this up too. Each type of fasting approach has been linked to improved weight loss efforts.2,3, 5-10, 14, 17,18 This includes folks of all body sizes too.8
Worried about negative effects on your hunger or metabolism? A few studies found that time-restricted fasting actually reduced appetite and had no effect on metabolic rates.13,15,16 However, caloric restriction has historically been linked to metabolic adaption (slowed metabolism).21,22 So keep an eye out for more research on the specific effects of fasting.
Which approach is best? Well, it’s not exactly clear. There isn’t a ton of research directly comparing fasting approaches, however current literature shows that both intermittent fasting and caloric restriction have similar weight and fat loss effects.2,7,9,10,18 Intermittent fasting might be better at preserving lean body mass (muscle), however more research is needed to draw conclusions.10
Bottom Line: When it comes to weight loss and fat loss, intermittent fasting works … but it’s nothing magical. Its results are comparable to typical calorie-reduction diets. However, fasting may have an edge as far as muscle mass retention.
Various forms of fasting have been linked to protective effects against heart disease, diabetes, and possibly cancer.
Heart healthy effects:
- Decreased LDL (bad) cholesterol and triglycerides8,9,17
- Decreased blood pressure13,14
- Decreased oxidative stress13
- Improved glycemic control and insulin resistance3,13,17,18
- Improved adipokine concentrations (fat tissue-related hormones)3,8,15,20
- Only animal models have linked fasting and calorie restriction to reduced rates of cancer 19,20
- Indirect protective effects include decreased inflammatory markers (CRP, tumor necrosis factor-α, interleukin-1β)8,15,20 and oxidative stress13
But are these preventive effects simply due to the weight loss? Possibly. Weight loss is associated with the same observed improvements in cardio, metabolic, and inflammatory markers. However, a recent small study found that time-restricted fasting improved blood pressure, insulin sensitivity, oxidative stress, and appetite, without weight loss.13 Perhaps fasting has unique protective effects beyond weight management. Keep an eye out for more research.
Bottom Line: Intermittent fasting can have protective effects against chronic disease risk factors. Whether or not it’s related to weight loss is yet to be determined.
Cognitive Health and Aging
Caloric restriction has been linked to brain benefits via mechanisms like adaptive stress resistance, anti-inflammatory and regeneration markers, neuron synapsis, and neurogenesis (basically things that keep our brains healthy, sharp, and resilient).23-25,28 Unfortunately, the majority of this research comes from animal studies … womp womp. However, there is a growing body of research testing intermittent fasting and caloric restriction on human cognitive performance and age-related disease. The results are both promising and disappointing, so keep an eye out for more research.26-29
Bottom Line: The effects of fasting and caloric restriction may support healthy aging and cognitive health. However, more research is needed to understand the exact mechanisms and draw conclusions for humans.
Sleep and Circadian Rhythm
This is another grey area that’s based mostly on animal research. However, theories propose that daytime eating (and nighttime fasting) are a natural way to align with our sleep-wake cycle. Research links this alignment to improved hormone function, GI movements, gene expression, and metabolic phases.17,30 Additionally, human studies do show that late-night eating and night shift work are both associated with disrupted circadian rhythm and increased risk of cardio and metabolic disease.17
Bottom Line: Fasting during evening hours may support a natural “reset” in our body. However, more research on human subjects is needed to understand how.
My thoughts on Intermittent Fasting
- It makes things black and white (good for folks who do better with routine and structure)
- It discourages late-night (and usually mindless) eating
- For weight management, it may feel more doable than a typical reduced-calorie diet
- It’s linked to reduced risk of chronic disease, healthy aging, and improved circadian rhythm (directly or indirectly TBD)
What makes me hesitate:
- Potential consequences of fasting on eating behavior (overeating, compensating, non-balanced meals, disordered eating)
- Potential effects of meal skipping (skipping breakfast has been linked to increased risk of diabetes and heart disease)31,32
- Potential impacts on metabolism
- People who are pregnant, manage diabetes or a chronic health condition, or have a history of an eating disorder should consult their Doctor/Dietitian
Where you can start:
Intermittent fasting doesn’t have to be a rigorous lifestyle change. Just giving your body a little R&R from digesting foods can be a good thing. If you’re interested in trying it out, below are a few tips for time-restricted fasting:
- Eat an early dinner
- Make sure your dinner is balanced (includes protein, quality carbs, and healthy fats)
- Rethink evening snacks – are they due to hunger or habit? If hunger, then adjust your dinner to be more satiating
- Aim to eat a healthy and balanced diet during your non-fasting periods
- Finishing dinner by 7pm and eating breakfast after 7am can be a doable and natural way to fast for 12 hours
The research on intermittent fasting is fascinating! But I think more studies are needed to make any conclusions — specifically, long-term studies on humans, randomized controlled trials, and studies on clinical outcomes (disease incidence, not just disease risk factors).
If intermittent fasting works for you, rock on! If even the thought of fasting frightens you, no worries! As I’ve said before, choose the diet that best fits into your lifestyle, your nutritional needs, and your individual health goals.
Side note: If you’re interested in all things fasting, be sure to follow the work of Dr. Krista Varady. She is considered queen in this field of research and has published over 50 studies related to fasting.
- Sabaté, J. (2004). Religion, diet and research. British Journal of Nutrition, 92(2), 199-201. doi:10.1079/BJN20041229
- Mattson, M. P., Longo, V. D., & Harvie, M. (2017). Impact of intermittent fasting on health and disease processes. Ageing Res Rev, 39, 46-58. doi:10.1016/j.arr.2016.10.005
- Cho, Y., Hong, N., Kim, K. W., Cho, S. J., Lee, M., Lee, Y. H., . . . Lee, B. W. (2019). The Effectiveness of Intermittent Fasting to Reduce Body Mass Index and Glucose Metabolism: A Systematic Review and Meta-Analysis. J Clin Med, 8(10). doi:10.3390/jcm8101645
- Berg JM, Tymoczko JL, Stryer L. (2002). Biochemistry. 5th edition. New York: W H Freeman. Section 30.3, Food Intake and Starvation Induce Metabolic Changes. Available from: https://www.ncbi.nlm.nih.gov/books/NBK22414/
- Orenstein, B. (2014). Intermittent Fasting: The Key to Long-Term Weight Loss?Today’s Dietitian, 26(12), 40. Accessed from https://www.todaysdietitian.com/newarchives/120914p40.shtml
- Patterson, R. E., & Sears, D. D. (2017). Metabolic Effects of Intermittent Fasting. Annual Review of Nutrition, 37(1), 371-393. doi:10.1146/annurev-nutr-071816-064634
- Schubel, R., Nattenmuller, J., Sookthai, D., Nonnenmacher, T., Graf, M. E., Riedl, L., . . . Kuhn, T. (2018). Effects of intermittent and continuous calorie restriction on body weight and metabolism over 50 wk: a randomized controlled trial. Am J Clin Nutr, 108(5), 933-945. doi:10.1093/ajcn/nqy196
- Varady, K. A., Bhutani, S., Klempel, M. C., Kroeger, C. M., Trepanowski, J. F., Haus, J. M., . . . Calvo, Y. (2013). Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial. Nutrition Journal, 12(1), 146. doi:10.1186/1475-2891-12-146
- Varady, K. A., Bhutani, S., Klempel, M. C., & Kroeger, C. M. (2011). Comparison of effects of diet versus exercise weight loss regimens on LDL and HDL particle size in obese adults. Lipids in health and disease, 10, 119. doi:10.1186/1476-511X-10-119
- Varady, K. A. (2011). Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss? Obes Rev, 12(7), e593-601. doi:10.1111/j.1467-789X.2011.00873
- Even, P. C., Virtue, S., Morton, N. M., Fromentin, G., & Semple, R. K. (2017). Editorial: Are Rodent Models Fit for Investigation of Human Obesity and Related Diseases?. Frontiers in nutrition, 4, 58. doi:10.3389/fnut.2017.00058
- Baker, D. H. (2008). Animal Models in Nutrition Research. The Journal of Nutrition, 138(2), 391-396. doi:10.1093/jn/138.2.391
- Sutton, E. F., Beyl, R., Early, K. S., Cefalu, W. T., Ravussin, E., & 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(6), 1212-1221.e1213. doi:https://doi.org/10.1016/j.cmet.2018.04.010
- Gabel, K., Hoddy, K. K., Haggerty, N., Song, J., Kroeger, C. M., Trepanowski, J. F., … Varady, K. A. (2018). Effects of 8-hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: A pilot study. Nutrition and healthy aging, 4(4), 345–353. doi:10.3233/NHA-170036
- Moro, T., Tinsley, G., Bianco, A., Marcolin, G., Pacelli, Q. F., Battaglia, G., . . . Paoli, A. (2016). Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. Journal of Translational Medicine, 14(1), 290. doi:10.1186/s12967-016-1044-0
- Ravussin, E. , Beyl, R. A., Poggiogalle, E. , Hsia, D. S. and Peterson, C. M. (2019), Early Time‐Restricted Feeding Reduces Appetite and Increases Fat Oxidation But Does Not Affect Energy Expenditure in Humans. Obesity, 27: 1244-1254. doi:10.1002/oby.22518
- Patterson, R. E., Laughlin, G. A., LaCroix, A. Z., Hartman, S. J., Natarajan, L., Senger, C. M., . . . Gallo, L. C. (2015). Intermittent Fasting and Human Metabolic Health. Journal of the Academy of Nutrition and Dietetics, 115(8), 1203-1212. doi:10.1016/j.jand.2015.02.018
- Barnosky, A. R., Hoddy, K. K., Unterman, T. G., & Varady, K. A. (2014). Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. Transl Res, 164(4), 302-311. doi:10.1016/j.trsl.2014.05.013
- Lv M, Zhu X, Wang H, Wang F, Guan W (2014) Roles of Caloric Restriction, Ketogenic Diet and Intermittent Fasting during Initiation, Progression and Metastasis of Cancer in Animal Models: A Systematic Review and Meta-Analysis. PLOS ONE 9(12): e115147. https://doi.org/10.1371/journal.pone.0115147
- Harvie, M. N., & 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. Adv Nutr, 7(4), 690-705. doi:10.3945/an.115.011767
- Redman, L. M., Heilbronn, L. K., Martin, C. K., de Jonge, L., Williamson, D. A., Delany, J. P., & Ravussin, E. (2009). Metabolic and behavioral compensations in response to caloric restriction: implications for the maintenance of weight loss. PLoS One, 4(2), e4377. doi:10.1371/journal.pone.0004377
- Martin, C. K., Heilbronn, L. K., Jonge, L. , DeLany, J. P., Volaufova, J. , Anton, S. D., Redman, L. M., Smith, S. R. and Ravussin, E. (2007), Effect of Calorie Restriction on Resting Metabolic Rate and Spontaneous Physical Activity. Obesity, 15: 2964-2973. doi:10.1038/oby.2007.354
- Gillette-Guyonnet, S., & Vellas, B. (2008). Caloric restriction and brain function. Current Opinion in Clinical Nutrition & Metabolic Care, 11(6), 686-692. doi:10.1097/MCO.0b013e328313968f
- Shojaie, M., Ghanbari, F., & Shojaie, N. (2017). Intermittent fasting could ameliorate cognitive function against distress by regulation of inflammatory response pathway. Journal of advanced research, 8(6), 697–701. doi:10.1016/j.jare.2017.09.002
- Martin, B., Mattson, M. P., & Maudsley, S. (2006). Caloric restriction and intermittent fasting: two potential diets for successful brain aging. Ageing research reviews, 5(3), 332–353. doi:10.1016/j.arr.2006.04.002
- Witte, A. V., Fobker, M., Gellner, R., Knecht, S., & Flöel, A. (2009). Caloric restriction improves memory in elderly humans. Proceedings of the National Academy of Sciences, 106(4), 1255-1260. doi:10.1073/pnas.0808587106
- Van Cauwenberghe, C., Vandendriessche, C., Libert, C., & Vandenbroucke, R. E. (2016). Caloric restriction: beneficial effects on brain aging and Alzheimer’s disease. Mamm Genome, 27(7-8), 300-319. doi:10.1007/s00335-016-9647-6
- Brandhorst, S., Choi, In Y., Wei, M., Cheng, Chia W., Sedrakyan, S., Navarrete, G., . . . Longo, Valter D. (2015). A Periodic Diet that Mimics Fasting Promotes Multi-System Regeneration, Enhanced Cognitive Performance, and Healthspan. Cell Metabolism, 22(1), 86-99. doi:https://doi.org/10.1016/j.cmet.2015.05.012
- Cherif, A., Roelands, B., Meeusen, R., & Chamari, K. (2016). Effects of Intermittent Fasting, Caloric Restriction, and Ramadan Intermittent Fasting on Cognitive Performance at Rest and During Exercise in Adults. Sports Medicine, 46(1), 35-47. doi:10.1007/s40279-015-0408-6
- Longo, V. D., & Panda, S. (2016). Fasting, Circadian Rhythms, and Time-Restricted Feeding in Healthy Lifespan. Cell Metab, 23(6), 1048-1059. doi:10.1016/j.cmet.2016.06.001
- Rong, S., Snetselaar, L. G., Xu, G., Sun, Y., Liu, B., Wallace, R. B., & Bao, W. (2019). Association of Skipping Breakfast With Cardiovascular and All-Cause Mortality. Journal of the American College of Cardiology, 73(16), 2025-2032. doi:https://doi.org/10.1016/j.jacc.2019.01.065
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One thought on “To Fast, or Not to Fast?”
Very helpful article! My mom used time restricted fasting and after years of struggling with diets and maintained weight loss she is doing great! I was still skeptical of the program even though she saw results so your research is super helpful!
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