what does able refer to

Question

در عبارت زیر 

Determination of the nutritional efficiency of protein in the diet is in most cases based on estimating the extent to which dietary protein nitrogen is absorbed and retained by the body and is able to balance daily nitrogen losses. Net Protein Utilisation (NPU) is the percentage of ingested nitrogen that is retained in the body, and the Biological Value (BV) gives the percentage of absorbed nitrogen that is retained. 

آیا able به dietary protein nitrogen بر می گردد؟

Answer 1

Determination of the nutritional efficiency of protein.

Answer 2

Yes Sarakam. You're right.