Learn Russian: Врач рассказал нам о восьмистах калориях в этом десерте. - The doctor told us about the eight hundred calories in this dessert.

When declining hundreds (200–900), both halves of the word change. The preposition о (about) requires the Prepositional case. The first half, восемь, becomes восьми (its Prepositional form). The second half, сот, becomes стах (the Prepositional plural form of сто). Put them together, and you get восьмистах.

Why is калориях in the Prepositional case? Doesn't the number 800 require the Genitive plural?

If the number 800 were the subject or direct object, you would indeed use the Genitive plural (восемьсот калорий). However, when a number is forced into an oblique case (like the Prepositional here, because of the preposition о), the noun stops being in the Genitive and simply matches the case of the number. So, both the number and the noun take the Prepositional plural: о восьмистах калориях.

Why is there an и in the middle of восьмистах instead of a soft sign (ь)?

The base number восемь ends in a soft sign, which means it declines exactly like feminine nouns ending in a soft sign (like тетрадь or ночь). In the Prepositional case, that ending changes to an и. Therefore, the first half of the word becomes восьми-.

Could I use the preposition instead of to mean "about"?