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

metabolism 是由前缀 meta-（表示"变化、转变"）和希腊词根 bole（表示"投掷、抛出"）加上名词后缀 -ism 组成，字面意思是"物质在体内的转化过程"，指生物体内维持生命所需的化学反应总和，即"新陈代谢"。