Few have even heard of "poop calories" aka "fecal energy loss" but as a recent research papers says "the fact that calories from food can 'disappear' in feces (and urine) is an understudied component of overall energy balance." That's a quote from a paper called the "Role of Energy Excretion in Human Body Weight Regulation (Lund et al)" from July 2020 Trends in Endocrinology and Metabolism. You can read the whole paper at this link, but here are a couple of key details and lines:
"fecal energy loss could in fact explain a large proportion of the population-wide weight gain variation"
Ranges from 2–10% fecal energy loss reported (~80 kcal/day and ~500 kcal/day).
"Dietary fiber content, food matrices, and mechanical food processing, such as chewing, cooking, and industrial processing, all affect food digestibility" & calorie excretion.
And as we note in "The Science" adding insoluble fiber, especially in an intact "food matrix," to a meal can reduce the calories absorbed.