2

votes

Why does water have no calories?

Commented on April 09, 2015
Created October 22, 2012 at 11:41 AM

Explain me the chemistry. Hydrogen and oxygen are full of energy, but water has no calories. Is it only the carbon in food that matters? Why? Science please.

A968087cc1dd66d480749c02e4619ef4

(20436)

on October 23, 2012
at 12:11 PM

Okay calories was a poor choice due to it's specific meaning. Does water not contribute to cellular energy? Water is a necessary input to the krebs cycle, which generates ATP.

A968087cc1dd66d480749c02e4619ef4

(20436)

on October 23, 2012
at 12:10 PM

But isn't water necessary for the krebs cycle? Isn't water providing some of the components of ATP, or cellular energy. Perhaps I shouldn't have said "calorie" in the question, but reather energy (ATP).

3ce6a0d24be025e2f2af534545bdd1d7

(26217)

on October 23, 2012
at 01:36 AM

A calorie is not a unit recognized by the international system of units of measurement. That being said, a kilogram calorie (what we colloquially call a calorie) is roughly the equivalent of 4.2 kilojoules of energy. I took the leap of faith that the OP was specifically interested in the dietary calories (of which water has 0). If this were chemistryHacks, I might have spent more time delving into the details.

510bdda8988ed0d4b0ec0b738b4edb73

(20888)

on October 23, 2012
at 01:06 AM

Too bad upvotes and scientific correctness aren't well correlated here. "Calories" are a unit of energy which water does have. It's that food calories are defined against combustion which defines water (and CO2) at the zero

81181cab058dd652659e4bb2e6f25843

(528)

on October 22, 2012
at 02:33 PM

krebs cycle....

6b365c14c646462210f3ef6b6fecace1

(1784)

on October 22, 2012
at 02:21 PM

I agree, my bad

3ce6a0d24be025e2f2af534545bdd1d7

(26217)

on October 22, 2012
at 02:03 PM

Hydrolysis is not a metabolic pathway. It has nothing to do with whether or not we can convert water to energy.

6b365c14c646462210f3ef6b6fecace1

(1784)

on October 22, 2012
at 01:50 PM

i disagree. We definitely metabolize water in the body. Hydrolysis reactions happen ALL the time in the body.

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5 Answers

7
3ce6a0d24be025e2f2af534545bdd1d7

(26217)

on October 22, 2012
at 12:37 PM

Water contains no calories because the human body cannot metabolize it further to get energy out of it. We have no mechanism of converting water to energy.

This would be like putting a hamburger into a car's gas tank. Why doesn't the car run? The hamburger has all sorts of stuff it in. But the car is not designed to use that type of fuel.

6b365c14c646462210f3ef6b6fecace1

(1784)

on October 22, 2012
at 01:50 PM

i disagree. We definitely metabolize water in the body. Hydrolysis reactions happen ALL the time in the body.

6b365c14c646462210f3ef6b6fecace1

(1784)

on October 22, 2012
at 02:21 PM

I agree, my bad

3ce6a0d24be025e2f2af534545bdd1d7

(26217)

on October 23, 2012
at 01:36 AM

A calorie is not a unit recognized by the international system of units of measurement. That being said, a kilogram calorie (what we colloquially call a calorie) is roughly the equivalent of 4.2 kilojoules of energy. I took the leap of faith that the OP was specifically interested in the dietary calories (of which water has 0). If this were chemistryHacks, I might have spent more time delving into the details.

3ce6a0d24be025e2f2af534545bdd1d7

(26217)

on October 22, 2012
at 02:03 PM

Hydrolysis is not a metabolic pathway. It has nothing to do with whether or not we can convert water to energy.

510bdda8988ed0d4b0ec0b738b4edb73

(20888)

on October 23, 2012
at 01:06 AM

Too bad upvotes and scientific correctness aren't well correlated here. "Calories" are a unit of energy which water does have. It's that food calories are defined against combustion which defines water (and CO2) at the zero

2
7f8bc7ce5c34aae50408d31812c839b0

(2698)

on October 22, 2012
at 02:08 PM

The energy our bodies get from stuff comes from the bonds formed by electrons between atoms in the molecules. Chemistry is all about those bonds, forming them and breaking them. Sometimes breaking a bond liberates net energy, sometimes breaking a bond takes net energy. Since we are not nuclear powered, nearly everything our body does metabolically is related to forming and breaking these bonds. For example converting CarboHydrates (Carbs) to HydroCarbons (Fats) is really just rearranging atomic bonds in molecules.

H2O, water, takes net energy to break the bonds between H and O. So this reaction cannot power the human.

1
510bdda8988ed0d4b0ec0b738b4edb73

(20888)

on October 22, 2012
at 03:16 PM

Here, I answered a similar question a while ago: http://paleohacks.com/questions/41372/calories-combustion-versus-digestion/41533#41533

tl;dr: basically, water does have energy, everything has energy. What we care about is changes in energy relative to a common "zero". When talking about food energy, we're talking about combustion, and water is one by product of combustion, so water defines the "zero" that everything is relative to.

1
Bb3d1772b28c02da2426e40dfcb533f5

(5381)

on October 22, 2012
at 11:57 AM

I am confused by this question. The body breaks down carbohydrates, protiens and fats into energy. These are all complex molecules, not single elements. The body does not power its energy on single elements (not even carbon)

I am not sure exactly the cellular reasoning behind this, but I cant imagine biological cells running on hydrogen combustion, or cold fusion or anything (how would you even get energy from a single element? I think you can get energy from a chemical change/reaction but I dont think you can use that on a single element)

Interesting question though. Maybe someone can illuminate the way glycogen, glucose and fatty acids/ketones are used for energy on the cellular level.

81181cab058dd652659e4bb2e6f25843

(528)

on October 22, 2012
at 02:33 PM

krebs cycle....

A968087cc1dd66d480749c02e4619ef4

(20436)

on October 23, 2012
at 12:10 PM

But isn't water necessary for the krebs cycle? Isn't water providing some of the components of ATP, or cellular energy. Perhaps I shouldn't have said "calorie" in the question, but reather energy (ATP).

0
6b365c14c646462210f3ef6b6fecace1

(1784)

on October 22, 2012
at 01:47 PM

a Calorie is a unit of energy needed raise 1 kilogram of water by one degree C. We quantify the amount of energy in molecules by measuring the amount of heat released using a process called combustion. Combustion involves the exothermic reaction of a molecule with oxygen and in the process, the molecule will end up in a higher oxidation state and molecular oxygen will be reduced. If one considers a water molecule, the hydrogen atom is in its most oxidized state, and oxygen is in its most reduced state. It's essentially inert to combustion, but this DOES NOT mean it's energy-less. It just can't be quantified by combustion.

Long story short, water does not have Calories because it can't undergo combustion. However, it is REQUIRED in many metabolic processes and is therefore just as essential and relevant as the combustible fats, carbs, and proteins we take into our bodies.

One last thing: Relying on Calorie content of food is somewhat foolish because this implies that our bodies actually BURN food in order to access the inherent energy.

Oh, and if you're still awake and interested at this point, this website provides a good basic understanding of Reduction/oxidation-http://www.chemguide.co.uk/inorganic/redox/definitions.html#top).

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