Does a Burnt Piece of Toast Have the Same Number of Calories as a Regular Piece of Toast?

The easy answer is no. If you mean complete pyrolysis of the bread, then there would be less calories because once combustion occurs (even partial) the byproducts are either indigestible or barely so.

If you mean dark toast, the kind you might get at 6 on the toaster, it has the same calories. The Maillard reaction is what drives browning and it is a complex process where proteins denature and bind to other proteins as well as carbohydrates and so forth created an amalgam of mixed molecules. Essentially this is what leads to that caramel/nuttiness you get when things are browned. However, this conformational change and denaturation does not decrease the calories because the overall building blocks are the same and still digestible.

However, if let’s say a byproduct of a Maillard reaction is an indigestible molecule that was previously digestible, you could argue that it is now lower in caloric value because it is no longer bioavailable energy.

Bomb Calorimeter

Side note, a lot of people are talking about measuring calories by using a bomb calorimeter aka burning the item. This is no longer the method used for finding caloric value of food. Instead they find the net average of Atwater bioavailable nutrients and then use standardized values (e.g. 4 Kcal/g for Carbohydrates) to calculate the assumed caloric value. Again, this is obviously dependent on bioavailable sources of energy, not overall stored energy.

Why a Bomb Calorimeter No Longer Works

A perfect example of how a bomb calorimeter is not a feasible option, is Lettuce. Excluding the water (which is 95% of the material) lettuce is primarily fiber. Insoluble fiber in this case or in other words fiber we cannot breakdown (Cellulose). This material has no caloric value to us because it is not bioavailable (aside from small amounts created by gut fermentation thanks to helpful bacteria). So a piece of lettuce has a net caloric value of basically 0 in the Atwater system. In a bomb calorimeter however, it might have a much higher value because inside each of those cellulose walled cells is stored sugars, proteins, and so forth. Additionally, cellulose is essentially Glucose, which means that combustion wise, cellulose and glucose are equivalent in “Calories”.

Again, this is not the case in bioavailability. The only animals that can actually get the full caloric potential from plant material are foregut fermenters and hindgut fermenters, aka Cows and Horses. This is why they need multiple stomachs or a large cecum, in order to host helpful microorganisms to breakdown cellulose. Even Termites are not able to digest cellulose, but usually carry symbiotic organisms that can.