The effects of poor health in the otherwise-eligible pool recruit pool has been a concern for a few years. As I recall, the health of WWI recruits, especially from the south (mostly due to poor nutrition and parasites), had similar implications for readiness, and this was the spur for a number of public health interventions.

There are other potentially problematic indicators, such as the fact that grip strength in young men has plummeted over the last few decades (avg 18yo male is not as strong as avg 35yo woman was at his age). Grip strength is a good indicator of overall fitness.

https://www.militarytimes.com/news/your-military/2016/09/11/...

https://www.google.de/amp/amp.timeinc.net/time/2938158/youth...