The saying goes: 3,500cal = 1lbs of fat.
But does the body only burn the fat first?
Surly the body is breaking down more than just fat. So how many calories do you really have to go negative to burn a lb of fat?
Is there any general measurement?
Your body primarily uses carbohydrates as fuel and some fat. These ratios change based on the intensity of the work you are doing (sitting at a desk vs doing a HIIT workout for instance) and the available carbohydrates (stored as glycogen) in your body.
When your body is depleted of glycogen it begins burning more fat and if you’ve ever heard the expression “hitting the wall”, this is the feeling you get when you are running primarily on fat, the fat is converted into a couple different fuel types, one of them actually being glucose (for the brain). It is common in endurance athletes when the competitions run longer than their body can maintain a glycogen store. This is when gels and other sugars from things like gatorade become useful. The simple carbohydrates from those sources are easily digestible by you and can get to your muscles faster for more energy.
Regardless, your body is always burning fat in some quantity for pretty much everything you do, even at rest.
When you eat an excess of calories (when your body is full up on glycogen and other stores), this is when your body is most likely to store those excess calories as fat.
Your body has 2 energy stores, carbohydrates (stored as glycogen in the muscle and liver) and fat stored as adipose tissue under your skin and around your organs.
There are a lot of caveats and subtleties and slight gotchas depending on how good your body processes food and the quality and bioavailability of the food you are eating but general speaking if you eat an excess of 3500 calories you will store that as fat in your adipose tissue along with some water and other molecules, weighing roughly 1 lb. Similarly if you run a calories deficit of 3500 calories, you will generally speaking permanently (until you overeat) lose 1 lb of weight, comprised mostly of fats, some carbs, water and other molecules.
Hope that helps.
Edit: And to be more even more clear. Your body is super complex and it is constantly burning and saving fats and carbohydrates. As you tip the scales one way or the other is when you start to see reductions/additions in fat/carb stores.
Double Edit: Another useful thing, that huge chunk of weight you lose at the beginning of a diet - this is actually all of the weight stored as water (along with carbohydrates) as glycogen in your muscles. That big chunk of weight you lose right at the beginning of a restrictive calories diet is you moderately depleting your glycogen stores. It comes back as soon as you eat carbohydrates though as your body REALLY wants to use carbs as energy.
As others have mentioned, when you lose weight, you lose more than just fat. If you do all the right things, it would be primarily fat, but there would still be some reduction in lean tissue. If you’re just on a sustained caloric deficit, you will absolutely lose lean tissue, including a non-trivial amount of muscle tissue.
1 lbs = 453g, 1g fat = 9 kcal, thus: 1 lbs fat = 4077 kcal
My guess: That 4077 discrepancy probably accounts for the lean tissues. Or put a different way, from the approximately 1 lbs you lose from a 3500 kcal deficit, some percentage will be lean tissues.
Anecdotally, in my weight-loss journey, a sustained 7700 kcal deficit (what 3500 per lbs equates to in kg) tracked pretty accurately. I was lifting 6 days a week, and 3-monthly DXA scans showed a small but consistent increase in muscle mass and bone density. So yeah, the mix will vary based on your activity.
1lb or fat does have about 3500 calories stored in it yes.
1g of fat = 9 calories of stored energy. We always think about this as a value in food, but fat in the body in the same.
So in theory a deficit of 3500 calories per week should lead to around 1lb per week fat loss. It’s a kind of rule of thumb that works pretty well on the whole.
But the body is a bit more complicated than that and you’re right you could burn from other sources E.g. Muscle if needed which can happen particularly when in a defecit but it’s not as quick as people often think (depending on activity and nutrition).
Your body won’t use fat first as a fuel source as it will primarily use carbohydrates (glucose), but the way to get your head around it is if it doesn’t have what it needs from other sources it must then take from reserves which will generally be your fat stores. So if you’ve eaten say 1500 calories of whatever but you need 2000 to run all your processes and activity that day, regardless of what the body uses first it physically needs to find that 500 calories else or you wouldn’t survive and run. So where does it go to grab it? Stored energy aka fat!
Ideally if you want as much as possible taken from fat you need to be resistance training as well, because although muscle is healthy, it’s not essential for survival so if you are in a defecit, it is possible if not ebevitable without resistance training to lose a little in the process and ideally keeping protein adequate (I say adequate because it really depends on what someone is trying to get out of it goal wise). I don’t mean you have to be a meat head and actively trying to gain loads of muscle, but resistance training a few times a week helps preserve it and ensure more energy used is from fat.
It’s worth noting that at the beginning of a diet the scale will usually drop quicker in the beginning but some of this is attributed to water loss. The reason for this is that glycogen stores become a little depleted and 3g of water is stored with every 1g of carbohydrate (glycogen) so with the glycogen goes some water weight. It make sit look like the equation is way out but it’s due to this. Weight fluctuations will happen through out a period in a defecit.
Academics won’t be the reason you are successful at weight loss. Grit, determination, focus and dedication are the attributes you’ll need to sustain a desirable weight. Sure you’ll need a basic nutrition comprehension but people who are well versed in nutrition are out of their ideal weight bracket. Spend less time on the numbers and more time figuring out how you can make fewer dietary mistakes throughout the week.