# Biochemistry answers for common weight loss questions: What are Calories?

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Posts: 7,122 Member
edited November 2017
So what is a calorie exactly?

Calories are units of measure for energy, just like a kilometer is a unit of measure for distance and a kilogram is a unit of measure for mass. Also just like there are multiple types of units for distance (feet, inches, meters, kilometers) and for mass (pounds, kilograms, stones) there are different types of units for energy as well (calories, joules, foot-pounds, horsepower). Certain units tend to be associated with certain uses. For example in weights your weight might be measured in kilograms while the weight of a diamond might be measured in carats and the weight of a salt packet might be measured in grams. All of these units are interchangeable and you could technically express your weight in carats if you wanted to but since that is an atypical use people wouldn’t have a framework for which to judge how “heavy” that was.

So what is the common use for calories with regards to energy then? Calories tend to be the unit of measure for energy chosen for heat production. One calorie is defined as the amount of energy necessary to heat one gram of water (or one mL of water) by one degree celsius. When things burn they are essentially undergoing a chemical reaction whereby molecules within them are being rapidly oxidized releasing heat and light. You can measure the amount of energy in an oxidizable molecule by burning it in a controlled system and measuring the total heat output. Such a controlled system is known as a calorimeter. If you take 1 gram of glucose, light it on fire and trap the energy within a calorimeter to heat a known amount of water it will release enough energy to heat 400 mL of water by 10 degrees Celsius. That is the same as 4000mL by 1 degree Celsius. That would be 4000 calories of energy. But wait, we at MFP know our macros here…1 gram of glucose is 1 gram of carbohydrate and there are 4 calories in 1 gram of carbohydrates not 4000, so what is going on here? Well, as it turns out, the energy production of foods are quite large when measured in calories and are in the range of thousands of calories. As such food calories are actually kilocalories (1 food calorie is 1000 calories). So 4000 calories is 4 kilocalories which is 4 food calories which is the same amount of energy necessary to raise the temperature of 1 liter of water by one degree Celsius (1 liter is 1000 mL). Needlessly complicated I know so from here on out when I say “calorie” I mean a food calorie which is in terms of energy a kilocalorie.

So if you literally burn a gram of glucose it releases 4 calories of heat energy. So what happens to it in your body? Does your body light it on fire? No, but your body does carry out the same chemistry that fire does…just in a different way. When you light a molecule of glucose on fire this is the reaction that occurs. 1 glucose molecule (C6H12O6) interacts with 6 oxygen molecules (6 O2) to form 6 water molecules (6 H20), 6 carbon dioxide molecules (6 CO2) and energy. The energy in terms of fire is released as heat and light at the rate of 4 calories per gram. When your body metabolically digests 1 glucose molecule it does the same reaction. 1 glucose molecule is combined with 6 oxygen molecules to form 6 water molecules and 6 carbon dioxide molecules releasing again 4 calories per gram. This, by the way, is why you breath in oxygen and breath out carbon dioxide…the oxygen is needed for oxidation and the carbon dioxide is the waste product you exhaust. The difference between a fire and your body is that in your body this process is carried out by enzymes (proteins that catalyze chemical reactions) in a controlled step by step process that combines the release of energy with other chemical reactions or electron transport to lock that energy into chemical bonds in a common “currency” molecule of ATP. Those ATP molecules can then be coupled with other reactions in the body to utilize that energy to perform work. In addition to this energy captured in ATP some of it is also lost as heat. So while fire converts glucose to heat and light, your body converts it to ATP for storage plus some waste heat.

The example I gave here is a glucose molecule but all food is comprised of oxidizable chemicals and although the metabolic pathways vary and are complex they all end up doing essentially the same thing. Molecules comprised of carbon, hydrogen and oxygen (proteins, fats, carbohydrates) get combined with oxygen to produce water and carbon dioxide and energy. Energy is contained in ATP molecule and some is lost as heat. The amount of calories in a given amount of food is a known quantity from calorimeter measurements and/or the gram quantities of digestible/metabolizable components in the form of macros (proteins, fats, carbohydrates). So when someone says a calorie is a calorie they are correct in the sense that a calorie is a unit of measurement that refers to a specific amount of energy and we can calculate the amount of energy present in any given food item in terms of calories. Calories listed for food items have real world meaning in that if you took all the energy in that foot and converted it to heat it would heat a specific amount of water by a certain temperature. It is a “gold standard” and calories on food items are quite accurate. A calorie always refers to that amount of energy and therefore a calorie is always a calorie.

So is a calorie a calorie in terms of everyone’s bodies? Well, yes and no. No machine is 100% efficient and your body is no different. The 100 calories you eat will not be converted into exactly 100 calories of usable energy. It is technically possible that for different people the amount of calories they get from a 100 calorie meal of carbohydrates might vary slightly depending on how efficiently those people digest the carbohydrates and how much of it ends up being metabolically processed versus just going out as waste. In theory differences in an individual’s microbiome or digestive processes or overall health or medical conditions could factor into how efficiently one can process different types of foods and as such one person might get more from carbohydrates than another. Now this is about how efficient ones digestion is, after digestion and absorption at the point molecules enter metabolic pathways to be catabolized into energy everyone’s has the same metabolic pathways that derive the same amount of energy from the same molecules (ignoring alternate metabolic pathway states such as ketosis).

With that said there is no person on earth that can eat 100 calories of carbohydrates and derive 110 calories of energy. 100 calories is the most anyone can get because that is the complete energy content of the food itself. You can (and are) less than 100% efficient in processing, but you cannot be 110% efficient. So what does that mean? It means that although there might be small variations in the population with regards to how many calories one can derive from a meal, there is no person whose metabolism or digestion is “different” in such a way that they can derive more energy from food than is available within the food and therefore put on fat because they get more calories from meals than there are calories to get. You can’t subside on 500 calories because you have some super-efficient metabolism.

So how do calories relate to weight gain or loss? Ultimately your body requires a certain amount of energy to perform the work it needs to perform and to maintain its temperature at 37 degrees C. That amount of energy can be expressed in calories (although you could express it in horsepower if you wanted to it doesn’t matter they are just units). If you eat an amount of calories that exactly balances the amount your body needs then your weight will not change. If you eat an amount of calories that exceeds the amount of energy your body needs then some of the food molecules you ingest will not be metabolized and will instead be converted into energy storage molecules such as fatty acids or glycogen for long term energy storage. Those molecules have weight and therefore you will gain weight. If you eat an amount of calories that is less than the amount of energy your body needs then your body will mobilize some of those stored energy molecules and metabolize them into energy releasing water and carbon dioxide which will exit your body. As a result you will lose molecules and you will therefore lose weight. What this ultimately means is that your weight is directly tied to the amount of calories you intake versus the amount you expend, commonly known on these forums as Calories in Calories out or CICO.

But what about the possibility of variations in efficiencies in digestion producing more or less energy from the same amount of calories? Doesn’t that mean CICO is wrong? No, it doesn’t mean that. If Person A requires 1500 calories of energy and they eat 2000 calories but derive 1500 calories due to being only 75% efficient they will not gain or lose weight. What Person A then can deduce is if they eat 2000 calories when performing that amount of work they will be at maintenance. They will see they ate 2000 calories and that they didn’t gain weight so they will assume their body required that 2000 calories. It doesn’t actually matter that they really only needed 1500 but were 75% efficient, regardless of whether or not their body actually utilized that amount of energy the end result is the same. Person A will continually be able to predict their weight loss or weight gain on the basis of their caloric intake and caloric expenditure. If Person B requires 1500 calories of energy and they eat 2000 calories but derive 1800 from it due to being 90% efficient then Person B would gain weight over time. If they were tracking their calories and weight, they would realize that eating 2000 calories is above their maintenance level and could adjust accordingly. They could then maintain or control their weight as they saw fit through tracking their calories same as Person B. So CICO still applies even if there are significant differences in how efficient different peoples’ digestion is. Also understand most people will have the same efficiencies or at least be very very close to one another. I am just acknowledging at least the possibility that through digestion inefficiencies people might not derive exactly the same amount of energy from the same number of calories of food. But ultimately I don’t think that actually matters. Either way people can still accurately predict their weight loss or gain from tracking caloric intake and expenditure observing their bodies over time and learning what effect that number of calories has on them.
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## Replies

• Posts: 906 Member
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Loverly as usual =D
• Posts: 7,337 Member
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This is a great explanation.

I try to remember to use the customary convention to capitalize the "c" in food Calories, to distinguish it from the small "c" in the standard calorie.

1000 calories = 1 kilocalorie = 1 Calorie
• Posts: 17,564 Member
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Bookmarking this for the next time our CALORIES ARE FAKE NEWS buddy shows up
• Posts: 7,122 Member
edited November 2017
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CyberTone wrote: »
This is a great explanation.

I try to remember to use the customary convention to capitalize the "c" in food Calories, to distinguish it from the small "c" in the standard calorie.

1000 calories = 1 kilocalorie = 1 Calorie

Yup that is the convention (you'll notice at least in the title I capitalized it), but I figured if I tried to employ that convention it would confuse a lot of people as I bopped back and forth between talking about calories and Calories. As such I just stated I was going to go with what people conventionally use which is calories lower case meaning Calories (ie food calorie, kilocalorie). Almost no one knows about/cares about the physical unit of heat measurement after all, lets be real.
• Posts: 7,722 Member
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Aaron, thank you for taking the time to make these informative, enlightening and refreshing posts. They should be required reading for everyone before posting in the future.
• Posts: 7,122 Member
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malibu927 wrote: »
Bookmarking this for the next time our CALORIES ARE FAKE NEWS buddy shows up

I'm kind of curious to see if they show up here or not and if so what exactly they will contradict about my post.
• Posts: 7,122 Member
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You have a spelling error in there somewhere.

• Posts: 17,564 Member
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Aaron_K123 wrote: »
malibu927 wrote: »
Bookmarking this for the next time our CALORIES ARE FAKE NEWS buddy shows up

I'm kind of curious to see if they show up here or not and if so what exactly they will contradict about my post.

Doubtful. They usually make a few posts yelling at us before they get banned.
• Posts: 7,122 Member
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You have a spelling error in there somewhere.

If I didn't I'd be impressed. I don't exactly copy-edit these to death, I'm not publishing them.
• Posts: 7,122 Member
edited November 2017
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Noel_57 wrote: »
malibu927 wrote: »
Bookmarking this for the next time our CALORIES ARE FAKE NEWS buddy shows up

I miss the physics dude.

The scream-in-all-caps guy who claimed calories are BS because physics somehow? The guy who would create a new profile with a name like RichardFeynmansBalls to rant for a bit before getting banned again and again? He stop making accounts or something?
• Posts: 7,122 Member
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Aaron_K123 wrote: »
Noel_57 wrote: »
malibu927 wrote: »
Bookmarking this for the next time our CALORIES ARE FAKE NEWS buddy shows up

I miss the physics dude.

The scream-in-all-caps guy who claimed calories are BS because physics somehow? The guy who would create a new profile with a name like RichardFeynmansBalls to rant for a bit before getting banned again and again? He stop making accounts or something?

He got his prescription for another week. Give him time for the meds to wear off and he will be back.
• Posts: 7,122 Member
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I'm going to have to Google how to start reporting my weight in carats. I'd kind of like to think of myself as a precious gem when winter sets in and my spirits are low.

I'm ~ 494400 carats.
• Posts: 7,122 Member
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I'm going to have to Google how to start reporting my weight in carats. I'd kind of like to think of myself as a precious gem when winter sets in and my spirits are low.

Go for it