I've seen the connection made before between Si and health, but then, as has previously been noted, there are Si dominants who seem to neglect their health.
The connection arises in the first place perhaps because it feels good to be healthy. But then there are some things which feel good right now but in the long term may result in poor health.
Now maybe someone has come up with this before, but maybe the health-focus is more likely to be evident in intuitive types (rather than Si types) who have a concern for Si (either Si-valuing or perhaps even Si-role), because they might be more likely or more able to think about how things will play out in the future (Ni?). Then they might show more concern for their health in the present, because they know if they don't do things to keep healthy, then in the future they will feel physically bad. Whereas an Si type might be less likely to think about the future, and so they do things which feel good right now but might make them feel bad in the future.
please rip this theory apart at your will