his attitude toward the claims made by advocates of health food is neutral

中心结构是his attitude is neutral.（态度是中立的。）

toward the claims是attitude的定语。

而made by advocates of health food是claims的定语。

整句翻译过来应该是他对于提倡健康食品的主张的态度是中立的

主句是
his attitude is neutral
什么态度呢toward the claims 对这些声明的态度
什么声明呢 是由倡议者发出的关于健康食品的声明。