Andy Petro wrote:
How do they arrive at the number of calories in a specific food item.
How is it measured . Can we do it ?

It is done by building a list of the calorific value of the ingredients and
calculating their sum. The calorific values per weight of standard
ingredients is (sugar, oil etc) is known. These were derived from
measurements where the substance was burned under controlled conditions to
measure the heat produced per unit of weight.

Yes you can do it by getting a calory counter book which has tables of basic
substances and of some prepared foods and tediously weighing each component
of your meal and doing the sums.

David