Spot on article by Mat Honan, writing for Wired:
We tend to focus on individual metrics in isolation–like heart rate or step counts–because they are easy to measure. It means we sometimes track fitness metrics and mistake it for health simply because it’s something we can measure. Fitness is a component of health, sure, but so is diet. So is your genetic makeup, disease, and your environment. It’s all important, and you need to know how it interacts to get truly meaningful information about your health. But there isn’t a great holistic way to first track all this stuff and then extract meaning from it. Which makes it very tempting to look at how many steps you took yesterday and conclude, “I’m healthy.” Yet change may be coming in the form of a new app called Healthbook.
In my post about health and smartwatches yesterday, I had a bit of fun imagining how health tracking may have practical implementations in the apps we use every day, like Maps. While that may be too futuristic for a first version of Apple's possible wearable device, the underlying concept still applies: health stats alone don't mean much if not given proper context understandable by humans.
Parsing health-related values and summarizing them into something like this could be a good start.