The figure is a schematic diagram of the metabolism of three nutrients in human body, and the following questions are answered according to the figure: (1) The sources of amino acids in human body can include ① and ④ in the figure. (1) The process ⑤ in the figure is the process in which protein in food is hydrolyzed into amino acids. This process needs to generate peptides under the action of protease and then amino acids under the action of peptidase.

(2) The process ① in the figure is the process in which sugars generate nonessential amino acids through amino conversion, which occurs in the liver.

(3) Among the 20 common amino acids in protein, except for 8 essential amino acids, the rest 12 are non-essential amino acids. From the diagram analysis, it can be seen that the amino acids that must be obtained through the process ⑤ are essential amino acids, and there should be 8 kinds.

(4) Lipid neutral hormones can maintain the secondary sexual characteristics of human body; Glycogen in sugar can store energy, so the three nutrients in human body, sugar, lipid and protein, are not only the main nutrients in the body, but also the important substances in the body structure and metabolism regulation.

(5) According to the stem information, the function of Clenbuterol is to promote the synthesis of protein and accelerate the transformation and decomposition of fat, so it can be inferred that after pigs eat Clenbuterol, Clenbuterol promotes the transformation of fat in pigs into protein, accelerates oxidative decomposition, releases energy and produces CO2 and H2O；; Fat is composed of three elements, C, H and O, and contains no nitrogen, so it will not produce urea after oxidative decomposition, so it is selected as D. 。

So the answer is:

(1) protease and peptidase

(2) Non-essential amino acids? Amino conversion? liver

（3）8

(4) Sex hormones? glycogen

（5）D