I spend a lot of time around the sports of basketball and rowing and, though dealing with a biased sample, I thought Americans were getting taller. There seem to be more and taller women and men in these programs then in the past.
But Burkhard Bilger reports in this weeks New Yorker that this is not the case across the population as a whole:

By the end of the nineteenth century, however, the country seemed set to regain its eminence. The economy was expanding at a dramatic rate, and public-hygiene campaigns were sweeping the cities clean at last: for the first time in American history, urbanites began to outgrow farmers.
Then something strange happened. While heights in Europe continued to climb, Komlos said, “the U.S. just went flat.” In the First World War, the average American soldier was still two inches taller than the average German. But sometime around 1955 the situation began to reverse. The Germans and other Europeans went on to grow an extra two centimetres a decade, and some Asian populations several times more, yet Americans haven’t grown taller in fifty years.

One possible reason:

In a recent British study, one group of schoolchildren was given hamburgers, French fries, and other familiar lunch foods; the other was fed nineteen-forties-style wartime rations such as boiled cabbage and corned beef. Within eight weeks, the children on the rations were both taller and slimmer than the ones on a regular diet.

This may not be the whole story but it certainly provides food for thought and, I think, individual action.
Via Newmarks Door.