If the description contains '... lean only' then the nutritional values for the food are calculated with the separable fat considered as refuse. This is important for example, because depending on how the meat is cooked, some of the fat will render and be absorbed into the meat.

For more information see here; page 8 in particular.

This is confusing -- after an hour of research, I think I finally have the answer to the mystery.

'Trimmed to 1/8" fat' means trimmed to 1/8" external fat -- this is something that is done early on in the butchering to prepare the meat.

Here is an explanation: "Fat thickness (rib fat or back fat) is a measure of external fat thickness on a carcass. External fat is the most important determinant of retail yield. Fat thickness is measured at a point ¾ of the length of the longissimus dorsi muscle from the split chine bone." See: http://www.thebeefsite.com/articles/1961/beef-grades-and-carcass-information

And in the USDA database, they consider meats that are with different fat thicknesses to be of different types of meat (similar to a different cut).

Then there is the other completely different variable, which is what they are measuring -- "separable lean and fat" (both lean + fat) vs. "separable lean only" (just the lean part). And it's true that when they are looking at the lean only part, the thickness of the external fat should not really matter, but hey, the cut of the meat also doesn't matter so much -- it's just a different cut.

Here is the data on for example a Selec Top Sirloin Steak, Broiled: - separable lean and fat, trimmed to 0" fat - Refuse 4%, 206 cals/100g - separable lean and fat, trimmed to 1/8" fat - Refuse 5%, 230 cals/100g - separable lean only, trimmed to 0" fat - Refuse 10%, 177 cals/100g - separable lean only, trimmed to 1/8" fat - Refuse 16%, 170 cals/100g

As you can see, by far the main variable is whether or not ALL fat is separated.

To my knowledge,

"separable lean and fat" --> you ate it all

"seaprable lean only" --> you ate the lean portion, considering the fat as refuse, as stated above.

The "trim" description is just that, describing how much fat was left on by the butcher (or robot, more likely).

Excellent question! I too have wondered about this apparent contradiction.

I don't know the answer for sure but perhaps it has something to do with the difference between visible fat that lies on the outside of the meat and can be easily separated (i.e. trimmed), and the fat that is infiltrated within the muscle fibres. The latter depends on the cut of meat and the age, feeding, breed, etc. of the animal and can't really be removed in any way. Though that still doesn't explain why they refer to "separable" lean & fat and then follow it up with "trimmed to x"... Hmmm...

Hopefully someone else can provide a more useful answer!