Food, Energy and Entropy.

Successful species

The success of species is related to it's capacity to extract and process information from it's environment in the most beneficial way and  maintaining this capacity through biological adaptation. Humans have adapted better than any other species, being able to modify the environment significantly in our own benefit. A key factor to this success is undoubtedly our highly developed brains which allows us to use language, reasoning, consciousness, etc.

The brain is connected to the external world (environment) through our senses (sight, touch, smell, taste and hearing). By processing signals coming from our senses our brain is capable to develop "consciousness" or "awareness" of what's around us and respond in a presumably coherent manner to these signals. Most of the processes executed by the brain such as recognizing a friend by looking at an image or grabbing the bus bar when it jumps over a big bump might seem trivial to most of us but they wouldn't be possible without a highly developed brain which is capable of processing great amounts of information and transforming it into "knowledge".

Knowledge and information are different things, the first is only reached after the processing of some amount of the latter. After this processing takes place, we became conscious or aware of some rule, fact, pattern, etc., being able to use this knowledge in a predictive way. Thus, for example, when we see grey clouds we know that it will possibly rain because we have experienced many times how grey clouds bring showers. The more information we can retain about something the better (more detailed) our knowledge about it could be. It might also happen that amounts of information reaching our senses never gets transformed in new knowledge due to noise, brain fatigue, distraction, etc. Redundant information never gets converted into knowledge but it might help to retain this knowledge thus avoiding erasure (to forget) from our conscious being.

We all know that food is important to us because we use it as main source of the energy needed to perform our daily physical and intellectual activities. If we are depraved of it for a long enough period time we would weaken and eventually die. Similarly (although not equal) to what happens to a car engine once all available fuel has been used. We see how both living creatures and machines need an external source of energy to function.

Entropy and Negative Entropy

At the absolute zero, the entropy of any substance is zero. This is the state of maximum order. In this state there is only one possible structure so

S = k_Bln1 = 0 

Where  k_B = 1.381 x 10^-23 Joules/Kelvin is the Boltzmann constant.

Therefore the state of the substance is certain. As we raise the temperature there will be more states compatible with the observed macroscopic state (not just one) and therefore for these states of temperature T > 0 K:

S = k_BlnW > 0 (W > 1)

Note that W represents the number of micro states compatible with the observed macroscopic state. Now the uncertainty of the state for the substance has been increased (higher disorder). Therefore, some of the information we had about the substance has been lost (we've gone from one possible state or outcome to several).

An isolated system or a system always evolves in a way that increases its entropy and after more or less time reaches a estate of maximum entropy (greatest disorder or maximum number of possible outcomes). Thus,  in the state of equilibrium there is a maximum number of micro states (outcomes) compatible with the observed macroscopic state. This natural tendency to chaotic states is inherent to all things.

Lets return to the example of of a substance at a temperature T > 0 K. As discussed before, in this state there are more than one micro states compatible with the observed macroscopic state. If we increase the temperature by heating up the substance then the number of micro states will also increase further resulting in an increase of entropy and also there will be a reduction of our knowledge about the state of the substance.  We can see how increasing the entropy of a substance increases its structural disorder.

Lets now consider the case of cooling down a substance by sucking heat out from it. As the temperature approaches zero kelvin, the number of possible micro states (possible outcomes) are reduced and so is the entropy. The process of cooling is equivalent to the absorption of negative entropy, which increases the information associated to the state substance (lower entropy means more certainty about the state of the substance). We can see a strong bond between negative entropy and information.

To quantify this information content we can consider the process of cooling a substance from a state where W micro states are compatible with the actual macro state at temperature T > 0 K to a state of temperature T' < T where W' micro states can be realized, with W' < W. The entropy of these states are:

S = k_BlnW

S' = k_BlnW'

and the information content associated to the process is:

I = S - S' = - ΔS

which is equal to the decrease in entropy of the substance (note that information is always positive or zero). We will see now that feeding upon negative entropy is how living organisms can maintain complex and highly organized states overcoming the natural tendency of things to reach equilibrium.

Feeding upon Negative Entropy

From thermodynamics we know that an isolated system tends to reach a structure of maximum entropy which is the most disordered structure. How is it possible from an statistical point of view the the faculty of a living organism of delaying this 'decay'? The answer is: by eating, drinking, breathing, etc. The technical term is metabolism. Originally the idea of metabolism was an exchange of materials but this is absurd. Any atom is as good as any other of the same kind. What would be gained by exchanging them? In the same manner the energy content of an adult organism is stationary (same as the material content). Since any calorie is as good as any other, a mere exchange of calories cannot be of great help.

Then what is contained in our food that keep us from death?

Every process that take place in nature results in an increase of entropy. Therefore a living organism constantly increases it's entropy and tend to approach states closer to thermal equilibrium (death). Therefore, living organisms can only stay alive by sucking negative entropy from it's environment. It is easy to see that what really fuels an organism is the negative entropy obtained from food, air, sun rays, etc. This negative entropy keep us from dying by driving entropy out of our systems and thus by keeping us in an fairly orderly state.

The key question from a nutritional point of view is how to quantify the negative entropy content available  in  different foods. Presumably highly processed foods that had gone through high heating and some chemical reactions (i.e., irreversible processes) will  increase their entropy contents and therefore decrease the negative entropy content according to the second principle of thermodynamics.