he wanted to know whether they could be an equally nutritiou

he wanted to know 【whether they could be an equally nutritious but more affordable alternative】.
【】中句子是个宾语从句,作know的宾语.
but用于连接两个表示转折关系的形容词 nutritious （有营养的）,more affordable （更便宜的）.
whether用于引导宾语从句,表示是否.
此句翻译：他想知道：它们是否能成为一种同样营养却便宜很多的选择.
不懂追问~