If you are really interested in answering this question, I highly, highly recommend to take the time to read Gary Taubes' book GOOD CALORIES, BAD CALORIES. It's more time-consuming (and more in-depth) than WHY WE GET FAT, but it talks very clearly and scientifically about why calories-in/calories-out is not important when talking about weight loss.
I will try and paraphrase they key points below:
- A common fallacy is that you burn the same number of calories each day. It's pretty easy, I think, to see why this is false. Are more or less active today than you were yesterday? Did you have to move some heavy boxes at work? Did you spend a little more time in the gym? A little more time watching TV? Did you get up from your desk more or less frequently today? Was your body a degree warmer than normal? A degree cooler? Was it working harder to repair cells? Did you excrete more waste? Did you move more? Fidget more? Did you not feel like doing anything? Did you have a lot of energy? All of these have to do with how much energy you are burning in a given day. It does not seem likely that you could accurately estimate how many calories you need in a given day with any kind of accuracy.
- The CW on calories-in/calories-out assumes that the variables in the equation ∆E = Ei-Eo (change in fat = energy in minus energy out) are independent, and that causality runs from the right of the equation to the left (that is to say, that Ei-Eo CONTROLS ∆E). But, in fact, the opposite is true: ∆E CONTROLS how your body handles Ei and Eo. Your body is trying to keep your fat stores at a certain place based on internal regulation controlled by your hormones and then adjusts Ei (by making you hungry) and Eo (by making you energetic or tired) as appropriate. In simple terms, we don't get fat because we eat more; we eat more because we are fat.
- An example: when a 10-year old grows and starts eating a ton, no one thinks "oh, he's growing because he's eating too much." Rather, it's said "He's a growing boy, hormones are making him grow, and that is leading him to eat more." Why would it be different when growing horizontally?
All this to say:
- I don't think you *can* actually estimate your caloric needs with any kind of real accuracy
- Your body will adjust to whatever you take in as cued by your hormones. If lots of insulin is present, your body will cue the storage of fat and impede the release of energy from your existing fat stores. If insulin isn't present, your body will store less fat, and release energy from the fat you already have stored.
- Key point: if you increase your calories, but your body is not getting the signal to store fat, it will find other uses for that energy to keep you in balance: it will give you more energy, raise your temperature, increase cell turn-over and repair, create more excrement, etc.
I really recommend you read the book.
“If you want to build a ship, don't drum up people together to collect wood and don't assign them tasks and work, but rather teach them to long for the endless immensity of the sea” -- Antoine de Saint-Exupery