HR Activity Monitor API recommendations needed for Diabetes Decision Support Tool

I’m developing a decision support tool for Type 1 Diabetes. It facilitates analysis of Continuous Glucose Monitoring data, insulin and carbs consumed, and activity monitoring data such as heart rate and steps, for the purpose of insulin dosing, meal planning, and activity choice. The tool is used in real time throughout the day, and needs data served through an API. I’m looking for recommendations for a Heart rate and Pedometer device that has an excellent API. I need about 5 minute resolution. I value both stability of the company, and size of the user base. Ideas?