Doesn't it become "ARE" instead of "is"?
Many parents and pediatricians do not consider the occasional consumption of junk foods or fast foods to be harmful. It is harmful, however, when these foods are eaten regularly and replace nutritional foods in the child’s diet.

I think it indicates junk foods in the sentence "it is harmful."
Why did the author use "is" verb when junk foods is plural?
Doesn't it become "ARE" instead of "is"?

Please help me!
Thanks!

Question

Doesn't it become "ARE" instead of "is"?
Many parents and pediatricians do not consider the occasional consumption of junk foods or fast foods to be harmful. It is harmful, however, when these foods are eaten regularly and replace nutritional foods in the child’s diet.

I think it indicates junk foods in the sentence "it is harmful."
Why did the author use "is" verb when junk foods is plural? 
Doesn't it become "ARE" instead of "is"?

Please help me!
Thanks!

Answer

Because it is talking about the "consumption" of junk foods. "Consumption" here is singular. I'll give you a few more examples:

1. The father of the two boys is a policeman
2. The group of ducks is at the pond.

Answer

No. It refers to the consumption. Junk foods or fast foods tell us what the consumption refers to. If you were describing or referring to junk foods only you would use 'they are'
Junk foods are not harmful if eaten occasionally. They are harmful if eaten regularly.