Comment by tsimionescu

At the same weight, fat contains way more calories than sugar, so the difference in difficulty of digestion is irrelevant at this level. It's true that if you were to consume 1000 Cal worth of sugar vs 1000 Cal worth of fat, you'd get slightly less fat from the fat - but this should be seen simply as one of many limitations on the "calories in" measurement. The same kinds of differences likely exist between different sugars, different fats, different proteins - and may well be affected by other aspects of how the food containing these nutrients is consumed; and it almost certainly varies a lot between people or even for the same person based on various factors such as age, activity level, time of day, etc.