Fine-tuning LLMs in behavioral psychology for scalable health coaching
Personalized, smartphone-based coaching improves physical activity but relies on static, human-crafted messages. We introduce My Heart Counts (MHC)-Coach, a large language model fine-tuned on the Transtheoretical Model of Change. MHC-Coach generates messages tailored to an individual's psychology (their "stage of change"), providing personalized support to foster long-term physical activity behavior change. To evaluate MHC-Coach's efficacy, 632 participants compared human-expert and MHC-Coach interventions encouraging physical activity. Among messages matched to an individual's stage of change, 68.0% (<i>N</i> = 430) preferred MHC-Coach-generated messages (<i>P</i> < 0.001). Blinded behavioral science experts (<i>N</i> = 2) rated MHC-Coach messages higher than human-expert messages for perceived effectiveness (4.4 vs. 2.8) and Transtheoretical Model alignment (4.1 vs. 3.5) on a 5-point Likert scale. This work demonstrates how language models can operationalize behavioral science frameworks for personalized health coaching, showing the potential for promoting long-term physical activity and reducing cardiovascular disease risk at scale.
Authors
- Anders Johnson (ORCID: https://orcid.org/0000-0002-1812-9553)
- Daniel Seung Kim (ORCID: https://orcid.org/0000-0003-2971-1909)
- Abby C. King (ORCID: https://orcid.org/0000-0002-7949-8811)
- Narayan Schütz (ORCID: https://orcid.org/0000-0001-5069-407X)
- Mariana Ramírez‐Posada (ORCID: https://orcid.org/0009-0008-9628-3610)
- Fátima Rodríguez (ORCID: https://orcid.org/0000-0002-5226-0723)
- Alexander Tolas
- Marily Oppezzo (ORCID: https://orcid.org/0000-0001-6668-2508)
- Paul Schmiedmayer (ORCID: https://orcid.org/0000-0002-8607-9148)
- C. Mikael Mattsson (ORCID: https://orcid.org/0000-0002-0642-4838)
- Ritu Doijad
- Euan Angus Ashley (ORCID: https://orcid.org/0000-0001-9418-9577)
- Eleni Linos (ORCID: https://orcid.org/0000-0002-5856-6301)
- Allan Lawrie (ORCID: https://orcid.org/0000-0003-4192-9505)
- Sriya Mantena
Institutions
- University of Washington (US)
- Cardiovascular Institute of the South (US)
- Stanford Health Care (US)
- Stanford Medicine (US)
- Imperial College London (GB)
- Stanford University (US)
Publication Details
- Journal
- npj Cardiovascular Health
- Published
- 2025-09-22
- DOI
- https://doi.org/10.1038/s44325-025-00083-5
- Citations
- 3
- Primary Topic
- Behavioral Health and Interventions
- Type
- article
- Field-Weighted Citation Impact
- 4.63
Funders
- American Diabetes Association
- American Heart Association
- University of Washington
- Imperial College London
- Canada Excellence Research Chairs, Government of Canada
- National Institutes of Health
- School of Medicine, Stanford University
- Stanford Bio-X
- National Heart, Lung, and Blood Institute