Fasting is not a new concept. You can find writings from ancient Greeks promoting fasting for healing when diseased.1 Many religions (Christianity, Buddhism, Muslim) promote fasting as a practice of spiritual and physical cleanse.1 You can find intermittent fasting (IF) in popular health literature dating back as early as 1911,2,3 but the science to back those health claims has been slow. Yet the claims keep on coming and sometimes it feels like a game of catch up (wait, does the research actually say that??). So let’s look at some of the most recent literature on IF and see if the claims hold up.
First, it’s important to understand what exactly the research is looking at. In the first installment in this series, we discussed how fasting is a pattern of when to eat and not a specific restriction on what to eat. While this is true, there is naturally going to be energy restriction in fasting. For example, if you are following a 5:2 fasting regimen, then for two days out of the week you are only consuming about 25% of your estimated caloric needs. So, if you are eating 100% of caloric needs for 5 days, and only 25% of caloric needs for 2 days, then you have an average intake of 80% caloric needs each week, which is hypocaloric or calorie restricted.
This is helpful to understand because you’ll find that the majority of current IF research is comparing IF to “continuous calorie restriction” (CCR). CCR means that subjects are simply eating up to 80% of their calorie needs every day without fasting. By comparing IF to CCR, researchers can hopefully identify what benefits come from fasting itself as opposed to the overall calorie reduction.
Common Health Claims for Intermittent Fasting
Improved Heart Health
One of the first “non-weight” claims to be made about IF was the benefit it has on heart health. This came from studies that showed statistically significant decreases in labs like homocysteine, C-reactive protein (CRP), and total cholesterol:HDL cholesterol ratios,4 as well as total cholesterol, triglycerides, and LDL levels.5 However, the impact of IF on these labs has been inconsistent in the literature and no benefits have been identified when a subject is not in an active fasting regimen.4,5,6 In 2017, a randomized controlled trial that included obese adult participants concluded that there was no difference between IF and CCR in blood pressure, heart rate, triglycerides, CRP, or homocysteine.6 In fact, during a 12 month follow up, subjects who followed IF during this study were found to have elevated LDL levels.
As far as heart health goes, there isn’t enough research to conclude that IF has any long-term heart healthy benefits. And as far as we can tell right now, differences between IF and CCR have been inconclusive or inconsistent.
Improved Blood Sugar Metabolism
Once heart health became a hot topic, the claimed benefits of IF on diabetes and blood sugar metabolism were not far behind. One study found statistically significant improvements in fasting insulin and insulin resistance markers for overweight or obese premenopausal women who were following a 5:2 IF regimen.7 Another study looking at men with pre-diabetes concluded that IF alone (without weight loss) had statistically significant improvements in insulin sensitivity.8 However, this research shows some inconsistencies, as other studies and reviews have concluded no significant differences in glucose, insulin sensitivities, or other circulating metabolic biomarkers between IF and CCR.6,9,10 More research is being done to assess IF in the role of different diets and metabolic outcomes. But the most current research is still preliminary mice studies and cannot yet be applied to humans.11
Improved Body Composition
Many people interested in IF are intrigued by the claim that IF decreases body fat and increases lean muscle mass. There are studies and reviews that do conclude body fat reduction when following an IF regimen, but again, these do not compare IF to CCR.1,13 Authors of a review in 2015 agreed that more research needed to be done in determining long-term effects of IF on body composition, as well as IF combined with exercise.13 While research looking at IF in combination with exercise is underway, so far it has only been concluded that IF does not adversely affect lean muscle retention or muscular improvements.14 Other research so far has been in mice and not yet conclusive for humans.15
Studies that looked at subjects during Ramadan did find reductions in IL-6 and CRP,4 suppressed proinflammatory cytokine expression, and decreased circulating levels of leukocytes.16 However, these were observational studies, and researchers could not draw these same conclusions when comparing IF to CCR. A 2018 study looked at effects of IF while controlling for weight loss and concluded that IF reduced oxidative stress among other benefits.8 These conclusions are promising but will require additional research to verify the true efficacy of IF.
Overall, there is not enough research yet to conclude that benefits from IF are not due to the calorie restrictive nature of this eating pattern. While there does appear to be some metabolic benefits to IF in the research, more high-quality studies are required to determine the impact of IF alone while controlling for weight loss.
Note to readers: The information discussed in this blog is not intended to replace medical advice. Please meet with your physician and dietitian before making any changes to your diet.
Juliette Soelberg, MS, RD, LDN works as a clinical dietitian, and her experience ranges from long- and short-term rehab to outpatient counseling to critical care. She believes that evidence-based practice is the most effective way for dietitians to become assets to their organizations and advocates for their patients.
Dietitians On Demand is a nationwide staffing and recruiting company for registered dietitians, specializing in short-term, temporary and permanent-hire positions in acute care, long term care and food service positions. We’re dedicated to dietitians and helping them enhance their practice and excel in the workplace. Check out our job openings, request your coverage, or visit our store today!
Fung J, Fung, Anderson J, et al. Fasting – A History. Intensive Dietary Management (IDM). https://idmprogram.com/fasting-a-history-part-i/. Published July 15, 2019. Accessed September 29, 2019.
The Fasting Cure by Upton Sinclair. Goodreads. https://www.goodreads.com/book/show/6496944-the-fasting-cure. Published July 16, 2008. Accessed September 29, 2019.
Bernarr Macfadden. Wikipedia. https://en.wikipedia.org/wiki/Bernarr_Macfadden. Published August 2, 2019. Accessed September 29, 2019.
Aksungar FB, Topkaya AE, Akyildiz M. Interleukin-6, C-Reactive Protein and Biochemical Parameters during Prolonged Intermittent Fasting. Annals of Nutrition and Metabolism. 2007;51(1):88-95. doi:10.1159/000100954.
Varady KA, Bhutani S, Church EC, Klempel MC. Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults. The American Journal of Clinical Nutrition. 2009;90(5):1138-1143. doi:10.3945/ajcn.2009.28380.
Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults. JAMA Internal Medicine. 2017;177(7):930. doi:10.1001/jamainternmed.2017.0936.
Harvie MN, Pegington M, Mattson MP, et al. 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. 2010;35(5):714-727. doi:10.1038/ijo.2010.171.
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 Metabolism. 2018;27(6). doi:10.1016/j.cmet.2018.04.010.
Barnosky AR, Hoddy KK, Unterman TG, Varady KA. Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. Translational Research. 2014;164(4):302-311. doi:10.1016/j.trsl.2014.05.013.
Schübel R, Nattenmüller J, Sookthai D, et al. Effects of intermittent and continuous calorie restriction on body weight and metabolism over 50 wk: a randomized controlled trial. The American Journal of Clinical Nutrition. 2018;108(5):933-945. doi:10.1093/ajcn/nqy196.
Dedual MA, Wueest S, Borsigova M, Konrad D. Intermittent fasting improves metabolic flexibility in short-term high-fat diet-fed mice. American Journal of Physiology-Endocrinology and Metabolism. October 2019. doi:10.1152/ajpendo.00187.2019.
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. The American Journal of Clinical Nutrition. 2005;81(1):69-73. doi:10.1093/ajcn/81.1.69.
Tinsley GM, Bounty PML. Effects of intermittent fasting on body composition and clinical health markers in humans. Nutrition Reviews. 2015;73(10):661-674. doi:10.1093/nutrit/nuv041.
Tinsley GM, Forsse JS, Butler NK, et al. Time-restricted feeding in young men performing resistance training: A randomized controlled trial. European Journal of Sport Science. 2016;17(2):200-207. doi:10.1080/17461391.2016.1223173.
Moraes RCMD, Portari GV, Ferraz ASM, Silva TEOD, Marocolo M. Effects of intermittent fasting and chronic swimming exercise on body composition and lipid metabolism. Applied Physiology, Nutrition, and Metabolism. 2017;42(12):1341-1346. doi:10.1139/apnm-2017-0435.
Faris “MA-IE, Kacimi S, Al-Kurd RA, et al. Intermittent fasting during Ramadan attenuates proinflammatory cytokines and immune cells in healthy subjects. Nutrition Research. 2012;32(12):947-955. doi:10.1016/j.nutres.2012.06.021.