随着年龄增长，人体的新陈代谢会逐渐减慢。

As people age, their ___metabolism naturally slows down.

他的新陈代谢比一般人快，所以很难增重。

His ___metabolism is faster than average, making it hard for him to gain weight.

这种药物在体内的代谢速度因人而异。

The ___metabolism of this drug varies from person to person.