Is nutrition an exact science

It depends on where you get your nutrition information from. Only go to reliable sources.

The other thing you should take note of:

When you read about certain studies, take note about who paid for the study as this may show the bias the study has.

If a company produces a certain processed product. They then commission a study to show their food is safe to eat or full of nutrients or whatever. If no other independent study has been done on this, then it would be wise to wait until more studies have been done before trusting the study results.