Riddle me this Veeky Forums:

>So how much of that 100 theoretical calories are actually usable by the body as energy?
This varies a lot based on the particular food source and even the individual. There are some attempts at measuring this and calculating averages (with metabolic chambers and such), this being the whole bioavailability thing. In the end we must agree that we burn different foods with different efficiency, and keep in mind that no chemical or physical process of transformation in 100% efficient.
So we must treat the kcals on the food labels as the maximum theoretical energy that can be extracted, not as a precise energy measure.

In a Bomb Calorimeter.

Quick user, delet before you look stupid.

First of all let's clear something up.
Amino acids are not "burnt" by gluconeogenesis, but by the Krebs cycle (+decarboxylation for those that get converted to pyruvate first) and ketogenesis-ketolysis. Gluconeogenesis exists to replenish blood glucose when it's depleted and it only uses amino acids as a skeleton; the energy spent in the process is the energy that is released after glycolysis of the resulting glucose molecule (+some extra energy lost as heat to the anabolic process). Now:
The energy the body needs is derived from oxidation of nutrients, namely carbohydrates, fatty acids and amino acids. The same thing, oxidation, happens when you burn these substances, as in literally set them on fire. There is a difference betweem the two processes, though.
In the first one, some of the energy that is released is "trapped" by a molecule called ATP (kinda simplifying things here) and the rest is lost to heat. The body can't use free energy liberated by oxidation, it can use energy trapped in ATP however.
In the second process, all the energy is released in the form of heat.
What all this means is that the body can't extract and use 100% of the energy that these substances hold. However, all these biochemical processes have a set efficiency of about 42%, somewhat lower for amino acid utilisation (since they need to be converted to other molecules first, a process that requires energy). So, from a dietary standpoint, you don't need to know how many calories your body needs but how many calories you need to eat so that you cover your body needs, since there is a correlation between the two.

Please give me a (You) guys, I spent 30 mins writing this.

So what you're saying is the body doesn't actually need, say, 2000 calories in the form of ATP-generated energy, rather, it needs 2000 calories of ingested energy to create 840 (.42*2000) calories worth of energy that can be utilized in the form of ATP? Or am I completely misinterpreting here?

Appreciated, but at the end of the day calories in calories out is all that matters, just the word means different things in that context

Here's a (you).
And a question: I'm curious to read up on the that 42% figure for efficiency, so where did you find that? I was under the impression that things varied a lot more, so I need some clarification.

Thx, but how do I know how many calories I need to eat to cover my body needs? Isn't it specific from a person to another one?

10%

But it you dehydrate it you lose the calories from water

>The Greeks were right all along

Typical white male. Stealing a Chinese idea and calling it a white idea.

Just track your weight and increase calories if it doesn't go up

>greeks
>white

kek

What about bioavailability? The difference between cooked/raw eggs etc. on how much protein you can get from that, so surely this effects the energy too?

seriously kys

That's right. And since all reactions that use ATP, like protein synthesis and muscle contraction, are also not 100% efficient either, in the end you end up using only a fraction of the energy you ate.

Absolutely true.

They do vary, this number is an estimate that I found here:
chem.libretexts.org/Textbook_Maps/Introductory_Chemistry_Textbook_Maps/Map:_The_Basics_of_GOB_Chemistry_(Ball_et_al.)/20:_Energy_Metabolism/20.5:_Stage_II_of_Carbohydrate_Catabolism
chem.libretexts.org/Textbook_Maps/Organic_Chemistry_Textbook_Maps/Map:_Essential_Organic_Chemistry_(Bruice)/18:_The_Organic_Chemistry_of_Metabolic_Pathways/18.3:_The_Catabolism_of_Fats

Obviously.

The intestines are very good at absorbing nutrients, so it's not worth considering much. Still, this is a limiting factor on how much food energy the body can utilise that I didn't take into account.

But don't hormones like insulin affect the rate of which white fat cells are generated greatly, thus creating a big opportunity for the actual amount of food being processed into heat, fat or atp-stored energy to change vastly?

not the user you quoted, but you're kinda right
and it goes even further

I'm German so please don't mind me using the words I learned in nutrition class
as amino acids are broken into three groups (glycogen/ketogen/glcoketogen) and each have their equivalent ketoacid, they enter at different stages into the cytratcyclus (Krebs cycle?) or are made into ketobodies
so to determine how much energy can be won you'd have to take a look of the percentages of amino acids in the protein and then how much your body needs for building specific proteins like enzymes or muscle itself
then after taking that into account you can start to break down the leftover amino acids which your body could use as fuel
which it probably won't do but put them into your amino acid pool as ketoacids
sadly this whole thing is far more complicated than just protein equals x amount of energy

Here's the you homie.

scientificamerican.com/article/science-reveals-why-calorie-counts-are-all-wrong/

Ignore that the publication is called Scientific American, and seemingly defends fat people. Its low hanging fruit, you are better than this.

Sort of. Insulin is the body's way to keep glucose levels in the blood stable. When you eat, you have too much glucose in your blood that you don't need, so the body produces insulin and forces the liver, the brain, the muscles, fat tissue etc to absorb it. These organs, in turn, either use glucose immediately or store it in the form of glycogen or fat. When you are not eating, the body uses the energy it stored before gthanks to the insulin to keep going. All these conversions require energy, further decreasing the percentage of useful energy you take from food, but it's not wasted, if that's what you're asking.

That's why they weigh it before and after dehydration and then multiply the water weight by water's caloric density.

Yes it's called a calorimeter. But some of the info you see on labels are not measured that way, but are just added together using the calories of the constituents. All in all I would say calories are a big fucking dubious piece of shit.

Calories are a unit of energy. It is the energy needed to raise 1 gram of water by 1 degree Kelvin.

The concept of using calories to balance diets and daily intake may be pseudoscience.

Tell a fat person to eat more veggies to lose weight and even they will disprove this article.

Thanks, I was asking this with low-carb or ketogenic diets in mind.

>calories from water
>calories from water

Thats it, im done. Bye

Calorie counting is literal bullshit.

It implies you shit out some type of calorie free, sugar free carbon brick each day.

dont be a fuckwit.

your body absorbs DIFFERENT amounts from the EXACT SAME food depending on heaps of factors; Sleep, water intake, exercise.

1. Icecream has 500 kcals on its label
2. Eat it.
3. Your body needs to warm it up in order to digest it.
4. Warming it up takes energy.
5. Thus the 500 kcal label is wrong, since its 500-X, where X is the energy required to warm it.

Calorie counters btfo.

>water's caloric density

This is why some people say "don't count calories, just eat a lot", which I disagree with because I don't see a reason to ever not track calories. You could eat a lot of low calorie density stuff. Shit, I could eat 2 kilos of mushrooms and still be at a fucking huge caloric deficit.

You can burn water, you know. It's called peroxide.
2 H2O + O2 = 2 H2O2

>I don't see a reason to ever not track calories
Its a chore, and depending on what you eat, very difficulty.
Not everyone opens 20 packages with labels and shit, and empties their payload into a bowl to mix.
If you go to a restaurant and order a meal, you can't possibly know for sure whats in it.

(You)
Happy now?

Which is why you only go to places which publish their menu and nutrition info. Had no issues maintaining my cut this weekend, and I went to Longhorn, Uno, McDonald's (twice actually). Don't be afraid to ask your waitress to finish out what supplier use for a given ingredient. Generally the chef or manager will be happy to tell you, and how much is used in a given dish. It isn't that hard to track your calories, even when dining out.

It sorta kinda relates, but it's not a perfect metric, there's a pretty decent margin of error.
It's still in use because you'll have so many other errors compounded in when you calculate your caloric in-out, from metabolism to activity burn to portion size and variation within the foodstuffs, that it doesn't make sense to use a more precise measure.

Cp of milk = 3.93 kJ/kg/K
Energetic density of ice cream : 2070kcal/kg
500kcal of icecream is roughly 250g
So to heat up 250 grams of ice cream from 5°C to 37°C you need 3,93*0,25*32 = roughly 32kJ of heat which is equal to barely more than 7,5kcals.

>Barely changes more than 1,5% of the calories you just received

Good thing the body produces heat by doing literally anything.

It also loses heat just by existing in space, unless you live in a furnace.

Found the fatty.
HAES!!

... And it also gets warmer by other objects existing in the same space.

How man objects in your room are higher temperature than yourself?

>In what way does that relate to the bodies use?

Linearly.
If you reduce the calories you eat by 10%, as calculated by calorimetry, the actual energy you receive is reduced by 10%. So the calorimetrically calculated energy is a very accurate proxy for estimating your true energy intake. At least as long as you're just changing the quantities of the foods you eat and not what foods you eat. And even, if you do change what foods you eat, the differences in digestibility are likely to be small as most foods have almost the same digestibility. Protein sources have the biggest variance in digestibility.

...

They don't need to be. All objects with thermal energy radiate a part of it via infrared radiation (you too, obviously). It's the reason why you sweat profusely when the room temperature is 36°C, while you feel way more comfortable when it's 26°C.

The chemistry is fundamentally the same you know.