Critical Reasoning (CR) | Leptin, a protein occurring naturally in the blood, appears to regulat

Ans: A {IMO}
As per the stem Leptin can not be used as a dietary supp to control fat. means so we need a reason for the same to complete the passage that why Laptin can not be used as Dietary Supp.

State A clearly says that because digestive system breaks down the proteins before entering the blood stream and from stem we know Leptin occurs in blood, so if Leptin is not reaching the blood stream feeding it won't make any difference and it won't serve its purpose.

A. the digestive system breaks
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