Indeed, it has been proven that 100 calories coming from sugar, oil or seafood are not treated the same by our bodies. Calories are coming from 3 distinct nutriments.

Carbohydrates

Refined sugars and its pairs, sweets, candies… are almost pure carbohydrates, rich and quickly assimilated by our bodies. Starches, even if they don’t taste as sugary, are extremely rich in carbohydrates. Starches includes breads, crackers, cereals, pasta, potatoes, lentils, beans… Bananas, grapes and cherries are among the fruits with the most carbohydrates. Obviously wine and all alcohols are not your friends there.
Carbohydrates provide 4 calories per grams directly assimilated.

Lipids

Lipids are definitely your enemies. There are 2 main sources of lipids: animal and vegetal. You’ll find them typically in their almost pure stage in delicatessens, pate, sausages and so on, but also in the meat, like lamb, goose and duck. Beef is normally way leaner. Vegetal lipids are mainly found in oils (olive, sunflowers, peanuts etc).
Lipids provide 9 calories per grams that get assimilated slightly slower than carbohydrates.

Proteins

Proteins are mainly coming from animals and most specifically their meat. Horse meat contains the most protein. Beef meat, although slightly fattier, are extremely good sources of proteins. Again, lamb, pork, goose and duck have too much lipids to be considered a good source of protein. All the seafood, fishes, shrimp, crab, scallops are very lean and with a good amount of proteins. Eggs are good although, ideally, you should eat only the white…yuck
Proteins provide 4 calories per grams that get assimilated very slowly (at least 3 hours).

Let’s do some maths

Because of the protein molecules size, our body needs to spend some energy to digest them. Typically, to extract 100 calories out of proteins, our body has to spend 30 calories (vs 12 calories for the lipids and 7 calories for the carbohydrates). Also, take into consideration that only 50% of the proteins are really assimilable, the rest is pure waste. So a 100g beef steak will provide only 200 calories (100g * 4 cal * 50%) that will require 60 calories to get assimilated…hence your net calories intake will be only 140 calories….which is the equivalent of a spoon of olive oil that we’re pouring so easily onto our salads…

Calories intakes (100g)
Calories assimilated (100g)
Calories diet efficiency (100g)

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