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Intermittent Fasting – 5 Things You Need to Know

Written by Clay Smith

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Intermittent fasting (IF) helps with weight loss but does not appear better than usual calorie restricted dieting.  Intermittent fasting appears to minimally reduce cholesterol and glucose and it may help slightly with depression.  For patients with non-alcoholic fatty liver disease, it is associated with improved liver function tests. Use caution and consult your doctor if you want to start intermittent fasting and have diabetes.

A Growing Problem

A recent study found that 76% of adults over 25 years of age in the United States were overweight or obese1. Over the years, fad diets have come and gone. One form of dieting with an extensive body of research behind it is intermittent fasting. In addition to weight loss, intermittent fasting is touted to have other health benefits. But what does the evidence say?

The Skinny on Intermittent Fasting

Intermittent fasting is a broad term that encompasses several fasting approaches2.

  • Time Restricted Eating: A common method of fasting is time restricted eating (TRE).  This can be TRE with or without calorie restriction.  An example would be food intake allowed from 8AM to 4PM, with no calorie intake for the next 16 hours.  Some people start with 10 hours of fasting and work up to 16-18 hours over a few weeks.
  • Alternate-Day Fasting: Some choose alternate day fasting (ADF). This could be a full 24 hours of no calorie intake followed by 24 hours of unrestricted calorie intake or modified ADF, with very-low calorie intake days alternating with unrestricted calorie intake.
  • 5:2 – In 5:2, calorie intake is usually limited to 500 calories two days a week and normal food intake the other 5 days.

Other diets that do not involve intermittent fasting include:

  • Continuous energy restriction (CER): This is what most people think of as a diet. It is a 10-20% reduction in total calorie intake, usually with normal timing of meals.
  • There are a host of other diets as well, such as: Mediterranean, vegan, lacto-ovo-vegetarian, pescatarian, low-fat ketogenic/modified Atkins/1:1, ketogenic/very-low-carb/4:1, paleolithic, Dietary Approaches to Stop Hypertension (DASH), and Mediterranean–DASH Intervention for Neurodegenerative Delay (MIND)2.

Fact 1: Intermittent Fasting – The Upside

An exhaustive review article in the New England Journal of Medicine posited several potential benefits of intermittent fasting3. Some of these benefits are less controversial, such as weight loss and reduction in body fat. Other benefits covered in this review article are not as clear, such as an association of reduced risk of heart disease and intermittent fasting, longer life span and potential cognitive benefits with aging. Additional clinical benefits may include lower glucose with intermittent fasting in diabetes patients, improved cholesterol, and possible improvement in patients with some neurodegenerative conditions3. Many of the studies cited in this article are animal-based research, and some of the human studies are preclinical or theoretical in nature. Another New England Journal of Medicine review article calls the evidence for long-term benefit associated with intermittent fasting “scarce and debatable.”2 That said, there are some aspects of intermittent fasting that have been extensively researched in humans, and we will dig into this evidence.

Fact 2: Intermittent Fasting Makes Biological Sense

When in a fasting state, the body relies not on glucose as a fuel but on the metabolism of fats to ketones for energy. There are several reasons why this might have positive health benefits. First, it reduces oxygen free radicals, which can cause cellular injury. It also promotes weight loss, which has a cascade of health benefits. But the switching from a “fed state,” after eating, to a fasting (starvation) state requires a large shift in metabolism.  These metabolic shifts appear to have benefit, as cells release a complex host of chemicals that may make them more resistant to oxidative and metabolic stress and more readily able to repair damage3. In short, there appear to be some solid biological reasons why intermittently fasting might have several benefits for overall health.

Fact 3: Intermittent Fasting – The Downside

When starting out with fasting, it’s important to acknowledge that it can feel awful at first.  Many people have headaches, hunger pangs, reduction in overall quality of life, and irritability (colloquially known as feeling “hangry”).  Most of these adverse effects improve over time, usually after about a month3. Notably, among patients with diabetes, a lowering of glucose levels is very common and may occur as early as 5 days after starting intermittent fasting.  Many times, oral medications need to be adjusted to avoid hypoglycemia, especially in diabetic patients taking sulfonylurea agents (which increase insulin output from the pancreas) or those who are receiving insulin injections.  Work closely with your doctor to plan ahead if you are diabetic and intend to try some form of fasting4. When the adverse effects of intermittent fasting are rigorously studied, a recent meta-analysis of 15 randomized trials found no difference in fatigue, headache, or dropout rate (when compared to a regular calorie-restricted diet). There was a higher rate of dizziness with intermittent fasting, but this did not reach statistical significance5.

Fact 4: Intermittent Fasting Works for Weight Loss

Most people try intermittent fasting to help them lose weight.  An extensive body of research has shown it is effective for this purpose 6–17.  Dr. Mohammed Allaf, a cardiologist in the UK and primary author of the Cochrane Review7, was asked to comment on this post. He graciously replied and noted, “Intermittent fasting is a well-established dietary regime used for thousands of years by many cultures and religions. Studies have shown that it has many benefits including weight loss.” Some of these studies included intermittent fasting alone, and some combined intermittent fasting and exercise. A recent randomized controlled trial did not find a significant difference in the amount of weight loss among patients with obesity who followed a calorie-restricted intermittent fasting regimen (food intake from 8AM to 4PM) versus the control group, which had usual continuous energy restriction (normal meals, ad lib eating times, lower calories). The fasting group had numerically greater weight loss: difference −1.8 kg [about 4 pounds] (95%CI −4.0 to 0.4; P=0.1), but this was not statistically significant6. Although it may not be significantly better than regular low-calorie dieting (continuous energy restriction), several studies have considered various forms of intermittent fasting.  The most reliable meta-analysis of studies7 found that intermittent fasting was associated with significant -2.88kg (-6.3 pound) short-term (<3 months) weight loss compared to no calorie restriction. This difference was not seen with longer term (3-12 month) follow up.  Similar to the single randomized controlled trial we just covered, there was also no difference in intermittent fasting compared to continuous energy restriction (i.e. a usual, low-calorie diet)7.

Fact 5: Intermittent Fasting Has a Smaller Impact on Other Health Outcomes

intermittent fasting has been extensively researched.  Even when the literature search is limited to meta-analyses (which lump multiple studies together) there were still dozens of meta-analyses to comb through over the past ten years.  We are covering the ones we think are the best and most helpful.

Here are some possible additional health outcomes that might be improved with intermittent fasting:

  • Heart Disease and Intermittent Fasting: There is no evidence available on the impact of intermittent fasting and heart risk (such as myocardial infarction, also called heart attack), stroke risk, heart failure, or all-cause mortality 7. We just don’t know. I found this lack of evidence very surprising. Dr. Allaf also commented, “All the studies we looked at had short-term follow up. We are uncertain about the effects of intermittent fasting on clinical events such as mortality, myocardial infarction, and heart failure due to lack of data for these outcomes.”
  • Lowering Cholesterol:  intermittent fasting was not superior to CER but was better than no calorie restrictions at all (ad lib feeding)7,9,10,12,18.
  • Intermittent Fasting and Diabetes: The average decrease in glucose was 0.5 mg/dL (0.03 mmol/L) when comparing intermittent fasting to ad lib feeding, and there was no difference in hemoglobin A1C in intermittent fasting compared to CER7. This also surprised me, as it was a far smaller difference than I expected.
  • Depression: intermittent fasting was associated with improved depression scores but did not show an association with improvement in anxiety or mood19.
  • Fatty-Liver Disease: There was moderate to high-quality evidence favoring intermittent fasting for both weight reduction and improvement in liver function laboratory tests20.
  • Aging and Cancer: There was no reliable evidence to provide guidance on the impact of intermittent fasting on aging, cancer prevention, or cancer treatment. Of note, the Mediterranean diet was associated with reduced risk of cancer21.

Wrap Up

Intermittent fasting has extensive and conclusive evidence that it helps with weight loss. It does not appear to be markedly better than usual calorie restricted dieting. In the end, it comes down to personal preference. Intermittent fasting may have slight advantages over other forms of dieting; it appears to minimally reduce cholesterol and glucose, and it may give a slight lift for patients with depression. There is no data on heart disease and intermittent fasting, and Dr. Allaf added, “Further research is needed to find out longer term benefits of intermittent fasting as well as benefit in certain groups (e.g. patients with diabetes or eating disorders).” For patients with non-alcoholic fatty liver disease, it not only helps with weight loss but appears to improve liver function measures. We would advise caution and close coordination with your doctor if you want to start intermittent fasting and have diabetes.

See our recent JournalFeed Health post: Does Fluoride Impact IQ?

Works Cited

  1. Ng M, Dai X, Cogen RM, et al. National-level and state-level prevalence of overweight and obesity among children, adolescents, and adults in the USA, 1990–2021, and forecasts up to 2050. The Lancet [Internet]. 2024 Nov; Available from: https://linkinghub.elsevier.com/retrieve/pii/S0140673624015484
  2. Yannakoulia M, Scarmeas N. Diets. New England Journal of Medicine. Massachusetts Medical Society; 2024 Jun 13;390(22):2098–2106. PMID: 38865662
  3. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine. Massachusetts Medical Society; 2019 Dec 26;381(26):2541–2551. PMID: 31881139
  4. Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine [Internet]. Massachusetts Medical Society; 2020 Apr 30 [cited 2024 Nov 15];382(18):1771–1774. Available from: https://www.nejm.org/doi/abs/10.1056/NEJMc2001176 PMID: 32348663
  5. Zhong F, Zhu T, Jin X, Chen X, Wu R, Shao L, Wang S. Adverse events profile associated with intermittent fasting in adults with overweight or obesity: a systematic review and meta-analysis of randomized controlled trials. Nutrition Journal. BioMed Central Ltd; 2024. PMID: 38987755
  6. Liu D, Huang Y, Huang C, Yang S, Wei X, Zhang P, Guo D, Lin J, Xu B, Li C, He H, He J, Liu S, Shi L, Xue Y, Zhang H. Calorie Restriction with or without Time-Restricted Eating in Weight Loss. New England Journal of Medicine. Massachusetts Medical Society; 2022 Apr 21;386(16):1495–1504. PMID: 35443107
  7. Allaf M, Elghazaly H, Mohamed OG, Fareen MFK, Zaman S, Salmasi AM, Tsilidis K, Dehghan A. Intermittent fasting for the prevention of cardiovascular disease. Cochrane Database of Systematic Reviews. John Wiley and Sons Ltd; 2021. PMID: 33512717
  8. Silverii GA, Cresci B, Benvenuti F, Santagiuliana F, Rotella F, Mannucci E. Effectiveness of intermittent fasting for weight loss in individuals with obesity: A meta-analysis of randomized controlled trials. Nutrition, Metabolism and Cardiovascular Diseases. Elsevier B.V.; 2023 Aug 1;33(8):1481–1489. PMID: 37248144
  9. Chen YE, Tsai HL, Tu YK, Chen LW. Effects of different types of intermittent fasting on metabolic outcomes: an umbrella review and network meta-analysis. BMC Med [Internet]. 2024 Nov 13;22(1):529. Available from: https://bmcmedicine.biomedcentral.com/articles/10.1186/s12916-024-03716-1
  10. Schwingshackl L, Zähringer J, Nitschke K, Torbahn G, Lohner S, Kühn T, Fontana L, Veronese N, Schmucker C, Meerpohl JJ. Impact of intermittent energy restriction on anthropometric outcomes and intermediate disease markers in patients with overweight and obesity: systematic review and meta-analyses. Critical Reviews in Food Science and Nutrition. Bellwether Publishing, Ltd.; 2021. p. 1293–1304. PMID: 32363896
  11. Cheng X, Sun S, Chen M, Zhou X, Rao M, Guo D, Xie J, Huang Q, Su L. Evaluating the efficacy of intermittent fasting and exercise combinations for weight loss: A network meta‐analysis. Obesity Reviews [Internet]. 2024 Dec 14;25(12). Available from: https://onlinelibrary.wiley.com/doi/10.1111/obr.13834
  12. Dai Z, Wan K, Miyashita M, Ho RS tak, Zheng C, Poon ET chun, Wong SH sang. The Effect of Time-Restricted Eating Combined with Exercise on Body Composition and Metabolic Health: A Systematic Review and Meta-Analysis. Advances in Nutrition. Elsevier B.V.; 2024. PMID: 38897385
  13. Liu L, Chen W, Wu D, Hu F. Metabolic Efficacy of Time-Restricted Eating in Adults: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Journal of Clinical Endocrinology and Metabolism. Endocrine Society; 2022. p. 3428–3441. PMID: 36190980
  14. Elortegui Pascual P, Rolands MR, Eldridge AL, Kassis A, Mainardi F, Kim-Anne L, Karagounis LG, Gut P, Varady KA. A meta-analysis comparing the effectiveness of alternate day fasting, the 5:2 diet, and time-restricted eating for weight loss. Obesity. John Wiley and Sons Inc; 2023 Feb 1;31(S1):9–21. PMID: 36349432
  15. Xie Y, Gu Y, Li Z, He B, Zhang L. Effects of Different Exercises Combined with Different Dietary Interventions on Body Composition: A Systematic Review and Network Meta-Analysis. Nutrients . Multidisciplinary Digital Publishing Institute (MDPI); 2024.
  16. Schroor MM, Joris PJ, Plat J, Mensink RP. Effects of Intermittent Energy Restriction Compared with Those of Continuous Energy Restriction on Body Composition and Cardiometabolic Risk Markers – A Systematic Review and Meta-Analysis of Randomized Controlled Trials in Adults. Advances in Nutrition. Elsevier B.V.; 2024. PMID: 37827491
  17. Enríquez Guerrero A, San Mauro Martín I, Garicano Vilar E, Camina Martín MA. 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. European Journal of Clinical Nutrition. Springer Nature; 2021. p. 1024–1039. PMID: 33293678
  18. Meng H, Zhu L, Kord-Varkaneh H, O Santos H, Tinsley GM, Fu P. Effects of intermittent fasting and energy-restricted diets on lipid profile: A systematic review and meta-analysis. Nutrition. Elsevier Inc.; 2020. PMID: 32428841
  19. Fernández-Rodríguez R, Martínez-Vizcaíno V, Mesas AE, Notario-Pacheco B, Medrano M, Heilbronn LK. Does intermittent fasting impact mental disorders? A systematic review with meta-analysis. Critical Reviews in Food Science and Nutrition. Taylor and Francis Ltd.; 2022. p. 11169–11184. PMID: 35713641
  20. Lange M, Nadkarni D, Martin L, Newberry C, Kumar S, Kushner T. Intermittent fasting improves hepatic end points in nonalcoholic fatty liver disease: A systematic review and meta-analysis. Hepatol Commun. Lippincott Williams and Wilkins; 2023 Aug 1;7(8). PMID: 37534936
  21. Muscogiuri G, Barrea L, Bettini S, El Ghoch M, Katsiki N, Tolvanen L, Verde L, Colao A, Busetto L, Yumuk VD, Hassapidou M. European Association for the Study of Obesity (EASO) Position Statement on Medical Nutrition Therapy for the Management of Individuals with Overweight or Obesity and Cancer. Obes Facts [Internet]. 2024 Oct 30;1–20. Available from: https://karger.com/doi/10.1159/000542155

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