Um, the first law of thermodynamics applies to the entire universe. You can't say just because something is not simple the law doesn't apply to it.

Let's go with the statement of the first law:

The increment in the internal energy of a system is equal to the difference between the net increment of heat accumulated by the system and the increment of work done by it.

the "net increment of heat" is the energy in (food) minus the energy out (poop, water in your breath, sweat, etc), and the "work done by it" is you moving around (exercise). All the energy is accounted for there.

Seriously though, physicists came up with this stuff back in the 1800's do you really think something like the human body would be one of those things that 1) doesn't fit a well known law and 2) no one found out about it until now?

I'm gonna let the good Dr. Feinman take this one:

We review here some aspects of thermodynamics that bear on weight loss and the effect of macronutrient composition.

I was about to post this question and found this one on search and noticed that the top answer is incorrect, so this is to bump it.

I gave this question to our engineering coop at work (who is into leangains) and he hacked it in 20 minutes with diagrams. Smart kid.

The first law of thermo has fine print, and people didn't read the whole law!

The first law, conservation of energy, only applies to a closed system. Humans eating, pooping and living are not a closed system. There are more chemical processes going on inside that do things with energy that are not observable from the outside.

Much of nutrition and fitness use the first law as literally true, and they are all mistaken.

It's actually pretty close in the long run, but it's still the wrong argument.

What most people don't take into account is that calories in-calories out is the last step in the causal chain. Trying to make it the first step is why people think it doesn't work.

It's only "sort-of" thermodynamics based on population statistics and empirical testing of activity and food. Look at how glycemic index is derived and you get an idea. A set of 10 student eaters (minimally paid) are fed, then their glycemic response is measured 2 hours later and the results are tabulated. Repeated tests produce variable results, but rice, glucose and wheat always come out high no matter how they're prepared.

I've thought about having my RMR tested to see how closely I conform to the Harris-Benedict population model. A few years ago I was counting calories carefully (both food amd activity) and losing weight, and I back calculated my RMR using the standard 3500 calories per pound over a period of a month. The number was within 5% of the H-B equation prediction. I thought maybe I was a special thermodynamic case, but I'm just run of the mill.

Yes, it applies, because it is a law. But, it isn't really all that applicable to obesity because we are not perfect systems. The CICO model assumes many things. One, that we can extract all calories equally, and extract them with an equal amount of energy input. But, really, does that make sense? We don't have to do all that much to digest sugar, but we have a whole digestive apparatus set up to deal with breaking down proteins, especially. Another big issue is that all the CI do not get translated into usable CO in the same way. Thin people throw off energy in the form of fidgeting, while fat people throw it into their fat cells. No law of thermodynamics is broken here. Thin people get hungry and eat, while fat people get hungry and have to eat even less because they have lowered their metabolism. This makes it look like CICO doesn't work. The real issue is not CICO, but how sometimes our bodies figure out how to balance it all (through appropriate eating signals, throwing off the excess, etc.) and for some of us, we don't balance it well. And since we aren't static, closed systems, the results do not match perfectly to the theory.

Mike's right of course, but I think "thermodynamics," "calorie-in/calorie-out," "if it fits your macros," and "just eat 500 under maintenance" are garbage as far as constructing a sustainable fat loss protocol. The big key is creating a significant amount of net lipolysis while maximizing satiety. Losing fat needn't involve any feelings of deprivation. Eating a trimmed steak and boiled potatoes vs. an isocaloric amount of burgers/fries has a massive difference in satiety.

If you wanted to lose weight, would you rather eat 10,000 healthy calories a day or 1000 of unhealthy calories including chemicals such as endocrine disruptors?

I would go for the former.

The problem with the problem with CICO is oversimplification. Oversimplifying any system can make it appear to produce very counter-intuitive results. Be sure to balance your energy equation with things like:

• Digestion inefficiency, i.e. how many calories are in your poop?
• Thermic effects of food, i.e. How much energy is lost as heat during digestion of various macronutrients?
• Effect of food ingested, i.e. how it affects metabolism via hormonal pathways.

It goes on and on, as you start not ignoring variables, inputs and outputs, you get more and more accurate representation of what is actually going on.

Isn't conservation of mass more important?

You can turn thermodynamics on it's head with thermal loading. It's about the only way there is to increase the 'calories out' portion of the equation. www.hypothermics.com NASA figured it out, you can too!

I'm really sad this thread doesn't have more. I Love this discussion on calories in vs calories out. 1st law applies to everything, but means less in the real world of us humans. There is variance amongst people for calorie expenditure but so there is nutritional lying or underreporting. When coaching people who have trouble losing weight, i tend to always think of underreportinv calories, but these days I only think that after they've improved their food quality.