Background: Ketogenic diets have been highlighted as a way to lose weight while experiencing reduced hunger. The protein-sparing modified fast (PSMF) induces ketosis but may be difficult to maintain.

Objective: To track weight loss for individuals initiating PSMF versus all other diets (e.g., balanced, high protein) for up to 5 years.

Design: Retrospective cohort study PARTICIPANTS: Adults who discussed the PSMF with a clinician between 2007 and 2014 INTERVENTION: Initiating the PSMF diet versus other diets MEASURES: The main outcome was percent weight change up to 5 years. Demographic and health data were collected using electronic health records. We fit regression models including age, sex, race, insurance, new medication prescriptions, and specialist visit to identify the effect of PSMF diet on percent weight change. We grouped patients by percent weight change at each year (≥ 5% loss, 4% loss to 4% gain, ≥ 5% gain) and used Pearson χ² tests to compare proportions.

Results: Of 1,403 eligible patients, 879 (63%) started the PSMF. The PSMF group was slightly younger (52 vs. 54 years, p < 0.01) and had a higher body mass index (41.9 kg/m² vs. 40.4 kg/m², p < 0.001). In the adjusted analysis, the PSMF group averaged 3% more weight loss than the other group over the 5-year follow-up (95% CI - 3.5, - 2.0, p < 0.001). PSMF patients lost more weight initially, but by year 4, there was no difference between diets (1.6% versus 1.3%, PSMF versus other diets, p = 0.12). Patients starting the PSMF were more likely to experience ≥ 5% weight loss at 1 year (55% vs 20%, p < 0.001) and 3 years (33% vs. 23% p < 0.05), but not 5 years (34% vs 29%, p = 0.16, PSMF versus other diets, respectively).

Conclusions: In clinical practice, the PSMF achieves rapid weight loss in the first 6 months, but only a small percentage of patients maintained significant weight loss long term.