Learn Russian: В этой порции нет трёхсот граммов белка. - This portion does not have three hundred grams of protein.

The word нет (there is no) requires the Genitive case. When declining hundreds like триста, both halves of the word change. The first half acts like the number three (три changes to трёх), and the second half acts like the noun for hundred (ста changes to сот). Together they make трёхсот.

Why is граммов used here, and could I just say грамм?

After a number in the Genitive case (like трёхсот), the noun that follows must be in the Genitive plural, which gives us граммов. For units of measurement like grams or kilograms, Russians often drop the ending in casual speech and just say грамм (even for the Genitive plural), but граммов is the strictly correct written form.

Wait, the dictionary word for 'protein' is белок. Why is it белка here instead of 'белока'?

The word белок contains a 'fleeting vowel.' When you add a case ending, the last о drops out to make pronunciation easier. Since it's in the Genitive case here (because we are specifying an amount of it), белок loses the 'o' and becomes белка.

Why does порции end in -ии? Shouldn't a feminine noun end in -е in the Prepositional case?