Americans still have an obsession with fat in food, but like most obsessions, it obscures the food we need to eat and enjoy. And some of it is downright fatty.

Most of us think there are really only two types of fat: solid and liquid. Or if you like, saturated and unsaturated, with trans thrown in for good measure and bad heart disease.

For the eaters in the group, this means butter and olive oil, or Crisco and walnut oil. And tallow and lard (a trick: lard is more unsaturated than not, and tallow is for candles or the birds, or both).

But fat is more interesting than that. Fat is also short, medium and long. Length here is molecule size, and longer is better.

Short fats have up to six carbon atoms. Medium have up to twelve. Long have up to 24.

Foods with fat are mostly saturated or unsaturated, and short or long. Long fats are more filling than short ones, because they are metabolized more slowly.

Short fats are used for energy easily and metabolized quickly, right from the bloodstream. They don’t have a chance to stay in your gut, and tell your brain that you’re full. Nearly all short fats are saturated.

But some saturated fats are medium (lauric acid in coconut) and long (stearic acid in cocoa butter). And these fats do not raise LDL, and like other medium and long fats, help you feel full faster because they stay in your intestine longer.

Classic long, liquid fats are DHA, ALA, EPA: an alphabet soup of fish, walnut and flax. Olive and nut oils are primarily long too.

So the crucial difference—for feeling full and fully satisfied—may not be saturated or not. But instead short instead of long!