A claim you hear on longevity podcasts

Does 12:12 intermittent fasting help you lose weight?

The claim checked: In one doctor's practice, patients who cut daily eating from over 12 hours to 12 lost weight and improved blood sugar Hear it at 1:02:08

What do the studies say?

Leaning for

One small study found that shortening the eating window to 10 to 11 hours cut weight and improved sleep; the other nine found mixed results, many testing 8- to 10-hour windows or other fasting plans rather than 12 hours, and several did no better than the comparison group.

Computed from 10 studies that test this point, each judged by two readers, in full. 3 more studies are waiting for a full read and do not count yet.

What was said on the podcast?

“and I have seen just incredible benefits when patients who were eating in eating Windows more than 12 hours they move down to 12 hours I've seen people lose weight improve their blood sugar control improve immune system function improve their energy their sleep”
Rangan Chatterjee · The Only 5 Food Rules You Need To Know To Heal The Body & Prevent Disease | Dr. Rangan Chatterjee · 1:02:08 on YouTube
Comes up in 2 episodes of the show.
The point checked Leaning for

Moving patients who eat over more than 12 hours (often ~15) to a 12-hour window has, in his practice, led to weight loss, better blood sugar and other benefits

Agree 1

other primary study · 2015 · landmark · cited on tape

“Nevertheless, reducing the temporal eating period in a feasibility study imparted measurable benefits of clinically relevant magnitude in terms of body weight reduction and sleep improvement without increasing the subjective sense of hunger.”

A Smartphone App Reveals Erratic Diurnal Eating Patterns in Humans that Can Be Modulated for Health Benefits. Cell Metab
Who: Healthy overweight adults (BMI >25) with a daily eating duration over 14 h, no shift work
PMID 26411343

Partly agree 9

other primary study · 2020 · landmark

“The observed changes in metabolic measures were not significant when comparing between TRE and non-TRE group; these findings remained not significant after adjusting for pre-intervention differences.”

Time-Restricted Eating Effects on Body Composition and Metabolic Measures in Humans who are Overweight: A Feasibility Study. Obesity (Silver Spring)
Who: 20 overweight/obese adults (17F/3M, mean age 45.5, BMI 34.1) with baseline eating window >=14 h (mean 15.4 h)
PMID 32270927

other primary study · 2019 · landmark

“In conclusion, IF significantly improves glycemic control and insulin resistance with a reduction in BMI, a decrease in leptin level, and an increase in adiponectin concentration in the general population without chronic metabolic disease.”

The Effectiveness of Intermittent Fasting to Reduce Body Mass Index and Glucose Metabolism: A Systematic Review and Meta-Analysis. J Clin Med
Who: 545 adults without diabetes (general population, some obese/prediabetic) in 12 controlled trials of intermittent fasting (ADF, ADMF, TRF, periodic fasting) vs regular diet or continuous calorie restriction
PMID 31601019

controlled trial · 2023

“The current study demonstrated a non-significant benefit of TRE plus BE and TRE alone in improving body weight, blood sugar, fasting insulin, and lipid profiles, when compared to the usual standard of care, albeit subject to reduced statistical power.”

Efficacy of Time-Restricted Eating and Behavioral Economic Intervention in Reducing Fasting Plasma Glucose, HbA1c, and Cardiometabolic Risk Factors in Patients with Impaired Fasting Glucose: A Randomized Controlled Trial. Nutrients
Who: Thai adults aged 18-65 with impaired fasting glucose and BMI >=25 (mean age 54.6, 69% female), Ramathibodi Hospital outpatients and staff
PMID 37836517

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trial or observed group · 2020 · landmark

“With 10 h TRE for 12-weeks, we observed significant reductions in body weight from baseline (−3.30±3.20 kg (−3%), p=0.00028).”

Ten-Hour Time-Restricted Eating Reduces Weight, Blood Pressure, and Atherogenic Lipids in Patients with Metabolic Syndrome. Cell Metab
Who: 19 adults (13 men, 6 women, mean age 59) with metabolic syndrome and baseline eating window >=14 h (mean 15.1 h), most on statins/antihypertensives
PMID 31813824

systematic review · 2021 · landmark

“MADF and the 5:2 diet were the only IF types that were associated with statistically significant weight loss of more than 5% in adults with overweight or obesity.”

Intermittent Fasting and Obesity-Related Health Outcomes: An Umbrella Review of Meta-analyses of Randomized Clinical Trials. JAMA Netw Open
Who: umbrella review of 11 meta-analyses of RCTs in adults with or without medical conditions (mostly overweight/obese)
PMID 34919135

controlled trial · 2023 · landmark

“This randomized clinical trial found that 8-hour TRE without calorie counting was an effective alternative diet strategy for weight loss and lowering of HbA 1c levels compared with daily calorie counting in a sample of adults with T2D and obesity.”

Effect of Time-Restricted Eating on Weight Loss in Adults With Type 2 Diabetes: A Randomized Clinical Trial. JAMA Netw Open
Who: 75 adults aged 18-80 with type 2 diabetes (HbA1c 6.5-11.0%) and obesity (BMI 30-50), mean age 55, 71% women, 53% non-Hispanic Black, 40% Hispanic White
PMID 37889487

other primary study · 2024 · landmark

“In our study, we found no perceptible alteration in fasting glucose levels.”

Intermittent fasting and health outcomes: an umbrella review of systematic reviews and meta-analyses of randomised controlled trials. EClinicalMedicine
Who: Adults (healthy, overweight/obese, prediabetes, type 2 diabetes, metabolic syndrome, NAFLD and others) in RCTs pooled by 23 meta-analyses of intermittent fasting vs CER or non-intervention diets
PMID 38500840

systematic review · 2025 · landmark

“While our analysis did not examine specific eating time windows, these findings were generally consistent with our results for TRE versus ad-libitum diet showing a benefit in fasting blood glucose, insulin resistance (HOMA-IR), and diastolic blood pressure.”

Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials. BMJ
Who: adults, median age 45, mostly overweight/obesity, in RCTs of intermittent fasting, continuous energy restriction or ad-libitum diets
PMID 40533200

controlled trial · 2025 · landmark

“Using an intention‐to‐treat approach with imputed outcomes for missing measures, we found that a 12‐week TRE intervention (8 h, self‐selected eating window) in patients with obesity shortened the eating window and reduced weight relative to baseline but did not improve weight, caloric intake, body composition, resting energy expenditure, metabolic flexibility, or glycemic measures compared with CR or UE.”

Time-restricted eating, caloric reduction, and unrestricted eating effects on weight and metabolism: a randomized trial. Obesity (Silver Spring)
Who: adults aged 18-65 with obesity (BMI 30-55) without diabetes and baseline eating window >= 12 h
PMID 39973006

Disagree 0

None.

What else was said but not tested?

  • Autophagy, a cellular clean-up process, kicks in when you give your body a break from food; eating over 15 hours gives it no chance
  • A 12-hour window is simple and achievable for most people

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