I had to work through it by a process of elimination, like a Sudoku puzzle: the "fat" symbol is definitely evoking "tape measure around the waist" iconography, perhaps implying (falsely) that "fat is the thing that makes you fat"—but more likely just putting people in mind of their weight and then saying "yeah, that stuff, that's what's in this."

Given that, it'd be redundant for the carbs icon to also be about weight. It's a meter of some sort, though. To me, it's either temperature—or pressure in a storage tank. Temperature would somewhat make sense, in the idea that carbs are the simplest thing that the body burns, and eating more carbs literally makes you hotter. But there are more common icons for temperatue (e.g. an old mercury thermometer with the teardrop base), so it's probably not temperature. Pressure it is.

Given that the referent is carbs, what would the storage tank be filled with? Probably a liquid. Liquid sugar? Sugar isn't green. What's a green liquid? Most hydrocarbons. Hydrocarbons are like carbohydrates, and serve many of the same functions for machines that carbs do for humans.

What's a common hydrocarbon pressure gauge? A car's gas gauge.