Descriptions cannot directly lead to prescriptions. Values come from you (the subject).

'Junk food is bad for your health' is a description, but who is it that decides that health is more important than pleasure? The prescription 'Don't eat junk food' is rooted in a value (e.g. that health is more important than pleasure), but you have the freedom to simply reject that value. How much moderation we should have is subjective.