I suppose you refer to the fact that more and more younger people are overweight and de-conditioned and therefore would not be able to meet the requirements to join the military. This is the only context in which I have heard this mentioned. Since the United States has an all-volunteer army, nobody is forced to serve and the entry requirements are well-known, I am not sure that I would go along with that statement.
If there is ever a crisis to warrant a military draft how many young men will be able to pass the military physical. In law enforcement ( include fire, EMTs, health care, security) how many are medically obesity that might hender them unable to help in a time of crisis. Remember that other medical problems come with obesity, such as diabites, heart, liver, and joint problems just to name a few. National security referres to the military, but in todays world one never knows when or where something is going to happen.