Energy in, energy out. Your body can't magically "create" fat; it has to come from somewhere.
You need a certain amount to have bodily functions. If you need 2,000 to maintain weight and only eat 1,500, you'll lose a pound a week (these are fake numbers).
I strongly disagree with this, actually, and would love to debate it with you. Calories in/calories out model is way too simplified for practicality. The body's fat storage is mostly controlled by hormones in the same way that growth is.
A kid eats more because he has hormones telling his body to grow, and feels hunger as a response to that growth, and eats in response to the hunger. Similarly, if a person's body is hormonally telling them to store, they will store, and their body will tell them to eat.
The body's metabolism is capable of ramping up to burn excess calories to a limited degree if it is not trying to store, and similarly slowing down to conserve if it is trying to store. If your body is hormonally trying to store food, undereating by a small number of calories will not cause weight loss. The body can lower body heat, encourage you to be sluggish, etc. If this is the case, you can only lose weight by:
(A) Ignoring all body impulses and maintaining a
significant calorie deficit, not eating when you're hungry, etc
or
(B) Finding a way to get your body to stop storing.
The problem with (A) is simple: if your body is trying to conserve, it will go to fat reserves
last. You will sit there starving your muscles before the body goes to the fat reserves. You have to work out just to keep your muscles from degrading, and working out will make you even hungrier, which you have to fight more.
My solution was (B). There's lots of hormones that control fat storage- testosterone/estrogen among them- but insulin is one of the biggest factors. The point of insulin is to make the body store everything it can to get blood sugar down. When blood sugar spikes, insulin puts your body in full on storage mode, and you get hungry again faster because you didn't use most of the calories taken in. A ketogenic diet is designed to eliminate the storage response of insulin as much as possible- and guess what? People on a keto diet find it almost impossible to gain weight.
This guy on the Reddit keto board demonstrated it by eating >3000 calories a day- on purpose, for a week- and lost eight pounds. All meals logged.
This guy on a carnivore board force fed himself ~4000 calories a day (averaging 3800) for one month and did not gain a single pound.
This doesn't violate thermodynamics, but it demonstrates how poor the calories in/out model is. By eating a diet that minimizes hormonal storage responses from the body, the body ramps up its metabolism to burn any excesses and goes directly to fat reserves for any deficit without complaint. The diet is explicitly muscle sparing (it eliminates all storage response) and people tend to gain muscle easily with any workout while on the diet (I've gained significantly, first time with arm definition!).
IMO, this is actually the most feasible way to lose weight. I've done calorie counting, I've watched my roommate lose all his muscle mass along with 80 pounds calorie counting. Keto works better, for health, muscle strength, and lifestyle (I never deal with hunger, I eat whenever hungry because, as my body is not storing anything, my hunger is directly correlated with what my body needs).
As a side note- everyone has different sensitivity to insulin. People who are resistant to insulin have bodies that produce far more insulin to react to the same amount of blood sugars. For a person who has normal insulin sensitivity, calorie counting is pretty easy to do; the body does not go in to storage mode easily. For an overweight person, eating the same amount of sugar/grains produces a much stronger storage reaction.
This is why skinny people always think it's so easy to
just count your calories; they don't get the same hunger cravings from eating the same food, because their bodies are not hormonally trying to store everything, and fat people must just have poor willpower.
Obviously, if you starve yourself, you will lose weight; I'm arguing that if you eat poor food choices, it will:
* Go slower than the calorie math would predict, because the body will try to store and lower metabolism
* Cost you your muscle mass first before going to fat reserves
* Leave you feeling constantly tired and drained and hungry
* If you ever start satisfying your hunger again, you will regain the weight much quicker than you lost it
And I feel that the above simply makes it impractical, though not impossible (like I said; my roommate did it. 80 pounds over 8 months, all muscle mass lost, constantly feeling sluggish and drained).
