Conventional weight-loss strategies often result in inadequate fat loss, poor sustainability, and unintended lean mass loss. Prolonged water-only fasting followed by dietary change may offer an effective alternative, but its impact on body composition remains underexplored.

This secondary analysis utilized dual-energy X-ray absorptiometry (DXA) to assess changes in fat-free mass (FFM), fat mass (FM), and bone mineral content (BMC) in adults [median (IQR) age: 62 (15) years] with overweight or obesity. Participants underwent a median (IQR) of 14 (6) days of water-only fasting, followed by 6 (3) days of food reintroduction with a whole-plant-food diet and a subsequent maintenance period of 45 (5) days. Study visits occurred at baseline and at the end of each period.

At the end of fasting, FFM accounted for 74% of the total BW lost during fasting. However, at the follow-up visit, the median (IQR) percentage change in total BW, FFM, FM, and visceral adipose tissue (VAT) mass from baseline was -8.2 (5.2), -4.4 (5.6), -14.7 (8.9), and -15.8 (12.8) %, respectively. At follow-up, FM loss accounted for 67% of total BW lost. BMC remained unchanged across all study visits.

This protocol produced sustained FM loss, specifically VAT mass, and FFM recovery, suggesting it may support effective weight management.

This study was a secondary analysis of data collected as part of a clinical trial (NCT04514146) that was approved by the TrueNorth Health Institutional Review Board in Santa Rosa, California (TNHF-2020-2VAT; April 2, 2020) and conducted according to the guidelines of the Declaration of Helsinki. The main study methods were previously published.³¹

Study Details

Participants were recruited from patients at a residential fasting center specializing in inpatient, medically supervised water-only fasting and whole-plant-food refeeding. Informed consent was obtained from all participants before enrollment and throughout the study. Forty participants were enrolled, of which 38 completed the on-site fasting and food reintroduction protocol, and the 29 participants presented here completed the full study protocol.³¹ As previously described, eligibility included males and females aged 40–70 years with a BMI between 25–40 kg/m2 and fasting glucose <7 mmol/L and/or hemoglobin A1c <7% who had independently elected to undergo a medically supervised water-only fast for at least 10 consecutive days followed by the standard food reintroduction period of no less than half the duration of the fast and who were previously approved to do so by a physician. The exclusion criteria were active malignancy, active inflammatory disorders, and stroke or heart attack within the last 90 days.

Upon enrollment, participants attended study visits at baseline (BL), end of fasting (EOF), end of food reintroduction (EOR), and follow-up (FU), at approximately six weeks after the EOR. In addition to anthropometric, serology, clinical, demographic, and dietary adherence data collected as previously described,³¹ DXA scans were performed during each study visit. At the EOR visit, participants were provided basic educational material on maintaining a whole-plant-food diet after returning to their home environment, but no other post-fast dietary support was provided between the EOR and FU visits. Dietary adherence was measured using a brief dietary screener specific to a whole-plant-food diet free of added salt, oil, and sugar.²⁴ At FU, participants reported increased consumption of whole-plant foods and decreased consumption of animal foods, dairy products, and added oil compared to BL.³¹ All study data were collected and managed using REDCap electronic data capture tools hosted at TrueNorth Health Foundation.³²

Prolonged Water-only Fasting and Food Reintroduction Protocol

The prolonged water-only fasting and food reintroduction protocol were detailed in the original publication.³¹ The fasting and food reintroduction protocol was implemented by non-research medical personnel at a residential, fasting facility. Briefly, after being pre-screened before arrival and, if conditionally approved to water-only fast, potential participants were instructed to prepare by eating a diet of fresh fruits and raw or steamed vegetables ad libitum for two days prior to initiating the fast. While fasting, participants were instructed to consume a minimum of 1.2 liters of distilled water per day and limit their physical activity. Participants were monitored twice daily by medical personnel, and serology and urinalyses were measured once weekly and as indicated. The fast was terminated with an established food reintroduction protocol consisting of five phases of gradual food introduction beginning with vegetable broths and/or fruit and vegetable juices and ending with an ad libitum diet of cooked and raw whole-plant-foods free of added salt, oil, and sugar, with one phase for every 7-10 days of fasting lasting a minimum of half of the fasting duration.¹³ Fasting lengths varied (see results below) but were no less than approximately 10 days, followed by a food reintroduction period of approximately five days.

Clinical Measurements

Clinical measurements were obtained at BL, EOF, EOR, and FU by research personnel as previously described.³¹ Briefly, height was measured using a digital wall-mounted stadiometer. BW was measured using a digital body weight scale. BMI was calculated from height and weight.  AC was measured at the narrowest part of the participants’ midsection below the lowest palpable rib cage yet above the top of the hip/pelvic bones using a tension-sensitive, non-elastic tape. Resting SBP and DBP were measured in the morning after blood collection while in the seated position and after resting for five minutes using a digital blood pressure device.

Dual-energy X-ray Absorptiometry (DXA) Measurements

Participants underwent whole-body DXA scans (Horizon Wi [S/N 303354M], Hologic) at BL, EOF, EOR, and FU by certified technicians. Measurements were taken in the morning after drinking one glass of water. The interassay coefficients of variation reported for Horizon Wi were 0.50% and 0.98% for fat-free mass (FFM) and fat mass (FM), respectively, and 7.1%, 7.2%, and 2.83% for visceral adipose tissue (VAT) mass, volume, and area, respectively.^{33 34}

Statistical Analysis

Data analysis was performed using R version 4.3.2.³⁵ Medians, interquartile ranges (IQR), boxplots, histograms, and scatter plots were used to understand the data. The analysis used random intercept linear mixed-effects models with 95% confidence intervals (CI) where clinical parameters were the dependent variables, study visit the main independent variable, participant ID the grouping variable, and age and sex the control variables. The DXA and clinical measurements models were fitted in a Frequentist framework using the ‘lmerTest’ version 3.1.3 R package with confidence intervals obtained from ‘brooms.mixed’ version 0.2.9.4 while the dipstick urinalysis models were fitted in a Bayesian framework using the ‘brms’ 2.20.4 R package with the target average acceptance probability parameter, adapt_delta, set to 0.99 due to warnings about divergent transitions.^36-38 Model diagnostics were assessed as previously described for models fitted with ‘lmerTest’ and ‘brms’.^{20 31} When lmerTest model assumptions did not hold,³⁹ robust score equations⁴⁰ with the Satterthwaite degrees of freedom approximation⁴¹ were employed. Complete case analysis was used to handle missing data. Regression coefficients were considered statistically significant when the 95% CI or 95% credible interval excluded 0.