Hi there
Its complex. Same with exercise.
So while 3500 cal will create 1lb fat, 3500 calolies of food can come in differing constitutions. The basis on which SlimmingWorld works is a good example. Slimming world is about satiety - feeling full - so if you are feeling full you wont be so tempted and as a result will eat fewer calories. So to do this they use, inter alia, Energy Density (ED) which is the energy per unit weight of a food and is usually expressed as kcal per gram. So a highly satiating, low energy density foods is what they refer to as ‘Free Foods’.
ED foods range from 0 to 9 kcal/g. water is at one end on and 100% fat is at the other. Fruit and vegetables are low ED, while some snacks like crisps are around 5kcal/g. Ive got a top 10 good and bad ED somwhere from the science team at Slimming World, I know mushy peas do well on ED ratings.
ED is heavily influenced by H2O so that drier foods, e.g. biscuits are more energy dense than wetter foods such as fruit and vegetables and lean meat and fish. Furthermore ED low density foods tend to have quality macronutrients. It also helps that low ED foods like protein tend to need more chewing which triggers satiety in the appestat in your brain - I think there was some stuff on this on the BBC last week.
Slimming world take low satiety, high fat, sugary, high ED foods and give them 'syn' values. I think they accept that they are tempting but also that they are not beneficial in terms of weight control.
Its still more complex with insulin effect, and other factors such as oxidation of fatty acids which means fat produces 25% more energy than carobhydrates, tehn theres carbohydrates Vs cellulose/hemicelluose carbohydrates (think fibre - they are carbohydrates that we dont have the enzymes to break down). There's more but I thought Id frame my reply around Slimming World.
Hope this explains a little bit of it.