Hola Maggi

The use or omission of the articles is something that creates a bit of frustration to students are there is no clear rule about when exactly to use them and when not. We do have a few lessons on specific scenarios when you really need them but other than that I can only say that when we omit the definite articles (el, la, los, las) it is because we see those nouns in a very generic way, without wanting to be specific. We do use them when we are referring to something a bit more specifically. For example, in this writing exercise we are omitting the article to say: 

She eats fruits and vegetables

We mean fruits and vegetables in general.

When it says:

It improves skin and athletic performance

we use the articles:

Mejora la piel y el rendimiento atlético

Here we see these direct objects as more specific things.

I'm afraid there is nothing written about this where there is a clear answer to this issue.  (I keep looking myself) It is one of the things that are more difficult to master in Spanish. 

Saludos