Does the first law of thermodynamics apply to nutrition? It certainly does. Can it be applied in many constructive ways? Probably not.
Let's take the example of a car. In this case some interesting inferences can be taken based on thermodynamics alone. If I put in twice as much gasoline how much farther can I go? About twice as far. If I let the car sit around unused, how much gas do I need? None. Or even a little bit more sophisticated: If my car weighs 1000 kg and I put in 1000 kJ worth of gas how far will this take me up a hill? Certainly less than 100 vertical meters. But also in this case thermodynamics has its limits: What happens if I add the same number of calories using a slightly different type of hydrocarbon (Diesel instead of gas)? Better not try that out ...
How about nutrition? Are there any constructive ways to use thermodynamics here: If I eat a second egg for breakfast how much farther can I run? If I sit around doing nothing, how much energy do I need?
It seems more like the interesting questions are way beyond thermodynamics: Which foods provide energy in a way that is readily usable? How do different foods affect satiety? Which foods contain harmful substances and what does their metabolism lead to? How does food intake affect the rate of metabolism?
I am not going to discuss these questions now. All I am saying is: if your nutritional advisor says that all you have to worry about is "calories in - calories out" fire him, just as you would fire your financial advisor if she tells you that all you need for succesful investment is knowing how to add and subtract.
On the other hand of course: Just as your financial advisor should be able to add and subtract, your nutritionist should have basic knowledge of physics. I watched this talk on fructose metabolism by Robert Lustig. He started out asking his audience whether they believed in energy conservation. This made me cringe a little bit. There is certainly no reason to disbelieve in energy conservation. What he actually meant to say is that while it is a correct law it just cannot be applied constructively. Well, I forgive it because the remaining talk was interesting ...
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