I guess (someone correct me if I am wrong), that you can say both "lo zucchero" and "zucchero" (without an article) interchangibly, the same way you would say in English "I eat sugar" or "I eat the sugar", unless you really want to specify that it is THE sugar (and not the salt) that you are eating. in which case it would be "tu mangi lo zucchero" and not "tu mangi zucchero". Correct? That is what I understand.
This is usually true. However, please keep in mind that Italian can vary greatly between regions and especially between North and South, so sometimes you can meet an Italian person who only uses "lo zucchero" and another who only says "zucchero". Strictly grammatically speaking though, you would be correct :)))
Only certain nouns have the quality of not being strict with article use; they are usually those nouns that rarely use a plural form, because the noun is something of an implied plural already. Nouns that would be awkward in English to use the indefinit article, for example, you wouldn't say "I eat a sugar". Sometimes these nouns get "de" or "di", or their respective plural form, in front of them instead.