Recently, I've been working with the Fitbit API for a personal project and I built a service that refreshes the Access tokens you need to authenticate requests to the API. 

Thanks to Azure Functions and Key Vault, building a solution that does this is pretty straightforward. If you're interested in how I did this, I wrote a blog post outlining the process: https://dev.to/willvelida/building-a-token-refresh-service-for-the-fitbit-api-with-azure-functions-and-c-44mk