请高手破译这个比较句

gmathacker

In many foods, the natural combination of vitamins with other nutrients makes those vitamins more usable by the body than are vitamins added in vitamin supplements.

这是一道逻辑题的答案(OG11 Riew P533),我想知道的是,如果从改错的角度来说,这句话里,到底谁与谁在比（双方的比较对象各是什么）?如果是双方的主语比，则比较对象好象不对，并且than后面的动词好象也有问题（应用do); 如果是宾语比，为什么后面加are 而不是采用定语从句的格式（即：than vitamins that are added in vitamin supplement)? 

请高手指教，先谢

Icanw

Here is a comparison between those vitamins ( combined with other nutrients) and vitamins (added in vitamin supplements).  And those vitamins is the object of make. 

I think the full comparison is: make those vitamins more usable ... than vitamins...  are (usable by the body)
or you can understand it as those vitamins are more usable than  are vitamins added ...