Increased weight loss compared to isocaloric diets (lower insulin and ketogenesis would increase energy expenditure). |
There is no evidence that IF leads to higher energy expenditure: isocaloric studies performed in metabolic chambers show similar weight loss in diets with different proportions of macronutrients (2727. Hall KD, Guo J. Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition. Gastroenterology. 2017;152(7):1718-27.e3.); RCTs show the same weight loss for daily caloric restriction and intermittent fasting, while maintaining similar weekly caloric intake (3333. Trepanowski JF, Kroeger CM, Barnosky A, Klempel MC, Bhutani S, Hoddy KK, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med. 2017;177(7):930-8.,3636. Carter S, Clifton PM, Keogh JB. Effect of Intermittent Compared With Continuous Energy Restricted Diet on Glycemic Control in Patients With Type 2 Diabetes: A Randomized Noninferiority Trial. JAMA Netw Open. 2018;1(3):e180756.,3737. Kroeger CM, Trepanowski JF, Klempel MC, Barnosky A, Bhutani S, Gabel K, et al. Eating behavior traits of successful weight losers during 12 months of alternate-day fasting: An exploratory analysis of a randomized controlled trial. Nutr Health. 2018;24(1):5-10.) and no difference in adaptive thermogenesis (4242. Coutinho SR, Halset EH, Gåsbakk S, Rehfeld JF, Kulseng B, Truby H, et al. Compensatory mechanisms activated with intermittent energy restriction: A randomized control trial. Clin Nutr. 2018;37(3):815-23.). |
Reduced hunger compared to traditional daily caloric restriction. |
Conflicting results in humans (44. Hoddy KK, Marlatt KL, Çetinkaya H, Ravussin E. Intermittent Fasting and Metabolic Health: From Religious Fast to Time-Restricted Feeding. Obesity (Silver Spring). 2020;28 Suppl 1(Suppl 1):S29-S37.). Studies with a proper control group did not show an overall difference in hunger ratings, though individual preferences exist (3030. Welton S, Minty R, O’Driscoll T, Willms H, Poirier D, Madden S, et al. Intermittent fasting and weight loss: Systematic review. Can Fam Physician. 2020;66(2):117-25.,3232. Harvey J, Howell A, Morris J, Harvie M. Intermittent energy restriction for weight loss: Spontaneous reduction of energy intake on unrestricted days. Food Sci Nutr. 2018;6(3):674-80.,3838. Heilbronn LK, Smith SR, Martin CK, Anton SD, Ravussin E. Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. Am J Clin Nutr. 2005;81(1):69-73.). |
Improvement in insulin resistance, independent of weight loss. |
Data derived from animal models (22. Mattson MP, Allison DB, Fontana L, Harvie M, Longo VD, Malaisse WJ, et al. Meal frequency and timing in health and disease. Proc Natl Acad Sci U S A. 2014;111(47):16647-53.,1111. Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46-58.,1616. Anson RM, Guo Z, de Cabo R, Iyun T, Rios M, Hagepanos A, et al. Intermittent fasting dissociates beneficial effects of dietary restriction on glucose metabolism and neuronal resistance to injury from calorie intake. Proc Natl Acad Sci U S A. 2003;100(10):6216-20.). In humans, conflicting results, but many studies without a proper control group. Overall, there is no clear evidence of improvement in IR, independent of weight loss (44. Hoddy KK, Marlatt KL, Çetinkaya H, Ravussin E. Intermittent Fasting and Metabolic Health: From Religious Fast to Time-Restricted Feeding. Obesity (Silver Spring). 2020;28 Suppl 1(Suppl 1):S29-S37.). However, a post hoc analysis suggests benefits in those who had previous insulin resistance (3434. Gabel K, Kroeger CM, Trepanowski JF, Hoddy KK, Cienfuegos S, Kalam F, et al. Differential Effects of Alternate-Day Fasting Versus Daily Calorie Restriction on Insulin Resistance. Obesity (Silver Spring). 2019;27(9):1443-50.). Timing of food intake could also have a role (66. Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metab. 2018;27(6):1212-21.e3.). |
Improvement in glucose control in type 2 diabetes. |
RCTs show similar reductions in weight and HbA1c (3636. Carter S, Clifton PM, Keogh JB. Effect of Intermittent Compared With Continuous Energy Restricted Diet on Glycemic Control in Patients With Type 2 Diabetes: A Randomized Noninferiority Trial. JAMA Netw Open. 2018;1(3):e180756.,6565. Arnason TG, Bowen MW, Mansell KD. Effects of intermittent fasting on health markers in those with type 2 diabetes: A pilot study. World J Diabetes. 2017;8(4):154-64.). However, there is evidence that increased meal frequency is associated with poorer glycemic control (6666. Kahleova H, Belinova L, Malinska H, Oliyarnyk O, Trnovska J, Skop V, et al. Eating two larger meals a day (breakfast and lunch) is more effective than six smaller meals in a reduced-energy regimen for patients with type 2 diabetes: a randomised crossover study [published correction appears in Diabetologia. 2015 Jan;58(1):205]. Diabetologia. 2014;57(8):1552-60.,6767. Jakubowicz D, Landau Z, Tsameret S, Wainstein J, Raz I, Ahren B, et al. Reduction in Glycated Hemoglobin and Daily Insulin Dose Alongside Circadian Clock Upregulation in Patients With Type 2 Diabetes Consuming a Three-Meal Diet: A Randomized Clinical Trial. Diabetes Care. 2019;42(12):2171-80.). |
Anti-aging and anti-inflammatory effects. |
Many animal models demonstrate increased longevity and reduction in chronic diseases with caloric restriction and IF, but data in humans are lacking (11. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. [published correction appears in N Engl J Med. 2020 Jan 16;382(3):298] [published correction appears in N Engl J Med. 2020 Mar 5;382(10):978]. N Engl J Med. 2019;381(26):2541-51.,1111. Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46-58.). Many benefits are alleged to be associated with ketogenesis, but it is unclear whether the level of ketone bodies achieved in popular IF patterns would be enough to elicit the same responses observed in rodents (1111. Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46-58.). Fasting-mimicking diets have been developed to promote these benefits and have shown a reduction in inflammatory and oxidative stress markers, but not clinical outcomes to date (7272. Brandhorst S, Choi IY, Wei M, Cheng CW, Sedrakyan S, Navarrete G, et al. A Periodic Diet that Mimics Fasting Promotes Multi-System Regeneration, Enhanced Cognitive Performance, and Healthspan. Cell Metab. 2015;22(1):86-99.–7474. Brandhorst S, Longo VD. Protein Quantity and Source, Fasting-Mimicking Diets, and Longevity. Adv Nutr. 2019;10(Suppl_4):S340-50.). |
Popular negative claims |
Current status of evidence |
IF would lead to overeating on feast days and could trigger eating disorders. |
IF does not lead to overeating or bingeing on feast days and does not trigger eating disorders (4343. Hoddy KK, Kroeger CM, Trepanowski JF, Barnosky AR, Bhutani S, Varady KA. Safety of alternate day fasting and effect on disordered eating behaviors. Nutr J. 2015;14:44.). Nonetheless, patients with previous eating disorders were excluded from main trials, and IF is not recommended for this population. |
IF could reduce lean mass. |
Conflicting results. Several studies suggest positive effects on lean mass, including one in trained men (4848. Moro T, Tinsley G, Bianco A, Marcolin G, Pacelli QF, Battaglia G, et al. 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. J Transl Med. 2016;14(1):290.). However, a recent randomized trial with TRF observed a higher appendicular lean mass loss after the intervention (4747. Lowe DA, Wu N, Rohdin-Bibby L, Moore AH, Kelly N, Liu YE, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial [published online ahead of print, 2020 Sep 28]. JAMA Intern Med. 2020;180(11):1-9.). More data are needed. |
IF is not sustainable and leads to higher attrition rates. |
Conflicting results, but IF has a slightly higher attrition rate in larger trials (44. Hoddy KK, Marlatt KL, Çetinkaya H, Ravussin E. Intermittent Fasting and Metabolic Health: From Religious Fast to Time-Restricted Feeding. Obesity (Silver Spring). 2020;28 Suppl 1(Suppl 1):S29-S37.,3333. Trepanowski JF, Kroeger CM, Barnosky A, Klempel MC, Bhutani S, Hoddy KK, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med. 2017;177(7):930-8.). One key trial has shown that with MADF, there was a trend of increasing calories on fasting days and decreasing on feasting days, resembling the overall daily calorie restriction pattern (3333. Trepanowski JF, Kroeger CM, Barnosky A, Klempel MC, Bhutani S, Hoddy KK, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med. 2017;177(7):930-8.). |
IF leads to side effects, such as nutrient deficiency and lethargy. |
Trials with different IF patterns show it is generally safe and well-tolerated, with no increase in overall side effects (44. Hoddy KK, Marlatt KL, Çetinkaya H, Ravussin E. Intermittent Fasting and Metabolic Health: From Religious Fast to Time-Restricted Feeding. Obesity (Silver Spring). 2020;28 Suppl 1(Suppl 1):S29-S37.). |