What part of the laws of thermodynamics imply calories in -> calories out? This belies a total ignorance of their actual meaning. The first law of thermodynamics would be satisfied in any of the following scenarios:
1. Your stomach explodes, releasing the caloric content of the food as heat over a brief duration
2. Your digestive tract decides to stop digesting, and passes the food through unmodified
3. Normal digestion
As you can see, conservation of energy is completely meaningless in the context of diet.
The second law of thermodynamics concerns itself with efficiency of processes. To apply it usefully, we'd take into consideration the different metabolic pathways of proteins, carbohydrates, and fats. We know for a fact that the pathway for sugars is thermodynamically more efficient than proteins [1] - the laws of thermodynamics directly contradict "a calorie is a calorie"! Not to mention the fact that consumed protein is metabolized and stored differently than consumed sugars, which are in turn different from fats.
> What part of the laws of thermodynamics imply calories in -> calories out?
The part where you restrict your assessment to more or less normal digestive function? Obviously if your stomach explodes or your intestine just plain stop working, caloric intake/deficit is now one of the least of your worries.
I think you actually hit the nail on the head here though, in using the word "imply". Calories in - Calories out doesn't give you an exact picture, but it comes reasonably close for the purposes of most people.
It's by no means a perfect understanding, but someone who is obese needs simple rules that can successfully get them started more than they need a perfect knowledge of the science involved.
what is that exactly? I have twin girls, one is a full foot taller than the other. One is noticeably pudgier than the other.
They both eat differing amounts, and clearly pretty much the same kinds of foods is used in differing ways by different digestive systems.
what makes you think there is an establushed norm?
"Calories in - Calories out doesn't give you an exact picture, but it comes reasonably close for the purposes of most people."
Im not at all convinced that this is true. It sounds true, and it naturally appeals as a simple truth, but I haven't seen much evidence to support it.
From what Ive seen across a wide range of people, differing bodies process foods in wildly differing ways - some people fart or burp a lot, some people hardly ever, some people prefer eating large amounts once a day, some people prefer eating small amounts regularly. As a teenager I could consume mcdonalds like a horse, now a single burger sits like a lead stone in my stomach for hours.
Are your twin girls adults yet? Nutrition and diet are difficult while people are growing. Your body is actively building itself in a way that never happens again. You need larger quantities, of almost everything, than body mass would suggest. You're right, for children, it isn't simple calorie arithmetic.
That said, how disparate are the "differing amounts" they eat? Are they identical twins or fraternal? Lifestyle differences? There are a lot of factors that could produce the effects you're seeing besides "calorie arithmetic doesn't work".
The essence of telling people "calories in - calories out" is to favor a simple (i.e. "easily applied") rule over an absolutely correct one. While people are growing, the equation changes. Get all of your calories in the form of twinkies and your body doesn't get the nutrients it needs to process food, so the equation changes. Assume "normal" ranges for diet and exercise and it more or less works.
This is a fantastically complicated topic, but it is not immune to human nature. We like simple rules that generally work, even if they're full of loopholes and caveats. For people that are overweight or even obese, calories in - calories out is good enough to get started. There are a lot of factors in managing that equation (satiety, nutrient balance, impulse management) but I think a clear goal is a better start than dumping people in a sea of (correct) information.
I can agree with that opinion, but I also see the value in "appealing" to science.
People will believe all kinds of stupid things if you can imply that science has made truth of them. For the purposes of getting someone started this is a rather innocuous lie.
I agree that there are several factors involved in the process of absorption for "Calories in, calories out" to be taken as a sufficient guideline for losing weight. However, the point I think people try to make when they use it is that the raw amount of calories you consume are an UPPER BOUND for the amount of energy you have available.
I know that the actual values are hard even to estimate, but we only get energy from food: if you need amount X of energy to keep functioning and the raw total of energy contained in the foods you eat is less than X, you simply can't gain weight, "asymptotically" speaking. That's why people invoke thermodynamics here.
1. Your stomach explodes, releasing the caloric content of the food as heat over a brief duration
2. Your digestive tract decides to stop digesting, and passes the food through unmodified
3. Normal digestion
As you can see, conservation of energy is completely meaningless in the context of diet.
The second law of thermodynamics concerns itself with efficiency of processes. To apply it usefully, we'd take into consideration the different metabolic pathways of proteins, carbohydrates, and fats. We know for a fact that the pathway for sugars is thermodynamically more efficient than proteins [1] - the laws of thermodynamics directly contradict "a calorie is a calorie"! Not to mention the fact that consumed protein is metabolized and stored differently than consumed sugars, which are in turn different from fats.
[1] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC506782/#__sec4ti...