Rumour has it that Apple's suppliers are developing a non-invasive blood glucose sensor in time for smartwatches released next year. The sensor relies on infrared light and a photodiode to read blood glucose levels, along with your pulse and blood oxygen statistics.
https://www.notebookcheck.net/Apple-Watch-Series-8-Suppliers-develop-an-infrared-blood-glucose-sensor-for-next-generation-smartwatches.575305.0.html