Should I always opt for the “low-fat” version

No, not always. If you’re trying to lose some serious weight, then sure, a lower-fat option is an easy way to save a few calories—a tub of, say, reduced-fat peanut butter indeed comes with fewer calories and a fraction less fat than the full-fat variety. But the food manufacturers have likely swapped the healthy fat in peanuts for maltodextrin, a carbohydrate used as a filler in many processed foods, so ultimately you’re trading healthy fat for empty carbs and twice the sugar . . .