Not all calories are the same — even if the number on the label matches. Your body burns energy just to break down food. This is called the thermic effect of food. Protein uses the most energy to digest: about 20–30% of its calories get used just in the digestion process. Fat uses the least.
Where food comes from also matters. A study found that people who ate whole almonds absorbed fewer calories than those who ate almond butter, even when the portions were the same. The whole nut's cell walls slow down how much fat your body can absorb.
Ultra-processed foods — things like chips, packaged pastries, and fast food — are pre-broken down. Your body barely has to work to digest them, so you absorb almost every calorie. A government study found people eating ultra-processed foods took in 508 extra calories per day without even trying.
Fiber also plays a role. It adds "bulk" to food but your body can't digest it, so those calories don't fully count.
The bottom line: where your calories come from changes how your body uses them. Two foods with the same calorie count can affect your body very differently.