The beginning of the year often means New Year’s resolutions, many of which relate to weight loss. Among the many options available, your clients may ask you about intermittent fasting. What does most current science say about the impact of intermittent fasting on weight loss?
What are the types of intermittent fasting?
Intermittent fasting (IF) could be defined as limited periods of abstinence from food and drink (1). It can be divided into several subcategories (1-3):
Complete alternate-day fasting: Days of complete fasting and ad libitum dieting, including alternate-day fasting (i.e., fasting one day of two), and “eat-stop-eat” (i.e., fasting for two nonconsecutive 24-hour fasting days a week).
Modified fasting: Allows eating 20%–25% of energy needs on fasting days. This includes the 5:2 method (e.g., consuming one quarter of caloric needs two days a week).
Time-restricted eating: Ad libitum eating within a limited time frame, resulting in an extended fast during the day. Examples include the popular 16/8 method, which includes 8 hours of eating and 16 hours of fasting, and the Warrior Diet (20 hours of fasting and 4 hours of eating).
Religious fasting: Fasts performed for religious or spiritual reasons (e.g., for Ramadan). We will not discuss religious fasts in this article.
What are the proven impacts of fasting on weight loss?
Several groups have studied the impacts of IF on weight loss. A systematic review published in 2019 reported observation of weight loss in all studies, ranging from 0.8%–13% of baseline. However, the short duration of the studies, the variability of the protocols (e.g., the absence of control groups, the presence or absence of ad libitum diet during the food intake period, the composition of the diet), and the use of several types of fasting techniques (alternating between complete fasting, modified, or time-restricted eating) limit interpretation of the results, as stated by the authors (4). Compared to continuous caloric restriction, most studies report that IF does not appear to result in greater weight loss (4). These findings are consistent with those of a previous systematic review published in 2015 (5).
Several meta-analyses have also been conducted. The study by Harris et al., including six randomized controlled or pseudo-randomized controlled studies, reported significant weight loss compared to a control group, but no difference was observed compared to continuous caloric restriction (6). Wang et al. published a meta-analysis in 2021 comparing the impact of IF on weight loss in type II diabetes. Across five studies, the authors observed greater weight loss with IF than with continuous caloric restriction (7). However, they concluded that additional studies with larger sample sizes are needed (7). A meta-analysis of seven studies published last year concluded that weight loss was greater compared to a control group that was not on a low-calorie diet (8). In parallel to these results, another meta-analysis published this year concluded that there was no significant difference between IF and continuous caloric restriction for weight loss, waist circumference, or fat percentage (9).
Some authors have attempted to separate the effects of different types of fasting. In these comparative reviews, the vast majority of studies observe weight loss in participants, usually with no significant difference between those doing continuous caloric restriction vs. fasting. (1,2). Given the limited number of studies and participants and the short duration of the studies, it is impossible to validate the difference in results between the different approaches.
Thus, IF appears to affect weight loss as a continuous energy deficit diet does, but additional studies are needed.
What might be the mechanisms of action involved with fasting that affects weight loss?
There are four predominant theories to explain the impact of FI on weight loss. The first involves alignment of the circadian cycle with food intake. This cycle allows mammals to adapt their rest/activity cycle to varying daylight. The control centre for this cycle is the hypothalamic suprachiasmatic nucleus (SCN), which changes the levels of certain circulating hormones (e.g., melatonin, growth hormone, cortisol, and glucagon), body temperature, and circulating nutrients (2,10). These changes allow peripheral organs to anticipate feeding and fasting periods and adapt their metabolism (anabolic or catabolic) accordingly (2,10). Aligning the period of food intake with the circadian cycle thus allows better regulation of the wakefulness/rest cycle (10). Some findings—e.g., that the timing of food intake (morning vs. evening) impacts weight gain independently of caloric intake (2) and that nighttime food intake is associated with increased risk of obesity and metabolic syndrome (2,10), independently of caloric intake—support this theory.
A second hypothesis involves the metabolic adaptation to fasting, with the decrease in glucose concentration and depletion of glycogen stores leading to increased gluconeogenesis and free fatty acid oxidation, ketone production, and increased lipolysis in adipocytes (11,12). A third theory concerns the change of the intestinal microbiota during fasting (2). This theory arose from observing that time-restricted feeding allows the restoration of cyclic variation in several families of bacteria affecting the absorption of several nutrients and potentially protecting the metabolic impacts of obesity in mice (13). Increased excretion of complex carbohydrate breakdown products that can only be degraded by the gut flora has also been observed (13,14).
The final theory linking intermittent fasting with weight loss involves the decrease in total energy intake. Indeed, an energy deficit of 2%–9% greater than in the continuously restricted control group has been observed in several studies (1,2).
Several studies are still needed to confirm the mechanisms of action linking the impact of IF on weight loss.
What does this all mean?
Several authors conclude that the weight loss achieved with IF is not superior to continuous caloric restriction (1,2,5,6,9). Again, the short duration of the studies, the very limited number of participants, and the different types of IF limit interpretation of the results. Nevertheless, IF may be a feasible option for some individuals for whom this lifestyle is suitable in the long term, as weight loss is similar to that achieved through continuous caloric restriction.
Beyond dietary patterns, adapting the diet to your patients’ reality is fundamental to lasting health changes. A TeamNutrition nutritionist works with your patients to choose the best approach for their lifestyle.
- Patterson RE et Sears DD. Metabolic Effects of Intermittent Fasting. Annu Rev Nutr. 2017;37:371–393.
- Rynders CA, Thomas EA, Zaman A et al. Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss. Nutrients. 2019; 11(10): 2442.
- Gunnars K. How Intermittent Fasting Can Help You Lose Weight. Healthline. [Online]. 2020. [Accessed January 28, 2022]. Available:
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- Seimon RV, Roekenes JA, Zibellini J et al. Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials. Mol Cell Endocrinol. 2015;418 Pt 2:153–172.
- Harris L, Hamilton S, Azevedo LB et al. Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database System Rev Implement Rep.2018 ;16(2):507–547.
- Wang X, Li Q, Jiang H et al. Intermittent fasting versus continuous energy-restricted diet for patients with type 2 diabetes mellitus and metabolic syndrome for glycemic control: A systematic review and meta-analysis of randomized controlled trials. Diabetes Res Clin Pract. 2021;179:109003.
- Borgundvaag E, Mak J et Kramer CK. Metabolic Impact of Intermittent Fasting in Patients With Type 2 Diabetes Mellitus: A Systematic Review and Meta-analysis of Interventional Studies. J Clin Endocrinol Metab. 2021;106(3):902–911.
- Guerrero AE, San Mauro Martin I, Garicano Vilar E et al. 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. Eur J Clin Nutr. 2020;75(7):1024–1039.
- Longo VD et Panda S. Fasting, circadian rhythms, and time restricted feeding in healthy lifespan. Cell Metab. 2016; 23(6): 1048–1059.
- Malinowski B, Zalewska K, Węsierska A et al. Intermittent fasting in cardiovascular disorders—an overview. Nutrients. 2019;11(3):673.
- Hatori M, Vollmers C, Zarrinpar A et al. Time-restricted feeding without reducing caloric intake prevents metabolic diseases in mice fed a high-fat diet. Cell Metab. 2012;15(6): 848–860.
- Chaix, A et Zarrinpar, A. The effects of time-restricted feeding on lipid metabolism and adiposity. Adipocyte. 2015;4(4):319–324.
- Zarrinpar A, Chaix A, Yooseph S et al. Diet and feeding pattern affect the diurnal dynamics of the gut microbiome. Cell Metab. 2014;20(6):1006–1017.