Discrete units of continuous quantity

Is it proper to speak of units of a continuous quantity as if they are discrete or continuous?

For example, I never know whether I should say that some food has "less calories" than another food (because the calorie is a unit of energy, which is continuous) or that it has "fewer calories" (since the calorie is a unit of measure and spoken of in plural).

Solution 1:

The rule is simple, and you obviously know it: discrete quantities require the use of "fewer" and continuous quantities require the use of "less".

Calorie, as a unit, requires the use of "fewer". Energy, as a continuum, requires the use of "less".

"Fewer calories means less energy."

In the same way, although it's a bit awkward, one should refer to units of volume with "fewer" and volume itself as "less".

"Fewer liters means less volume."