Imagine if you've a goal to increase height of people from 165 to 190cm with minimum number of generations. How will you go about it.
Easiest and fastest way is to have two tall parents breed. You'll see it happen within 2 generation.
Now, if you improve nutrition of people who are 165cm today. How many generations of better nutrition does it require to make them reach 190cm? Certainly more than 2 generations.
Point is that, if you think "sexual selection" is not fast enough to take people from 165 to 190 within few generations (as is the case with Dutch) then better nutrition certainly is not either.
Cattle/pet breeder do it all time.