The first law of thermodynamics is a conservation law. The second being a dissipation law.
Right now a lot of people think it doesn't matter what you eat, a calorie is a calorie, and if you eat more calories you gain weight, you eat less you lose weight. So the idea is, it is impossible for two different diets, that have the same calories to lead to different results. This is justified by the laws of thermodynamics.
The first law of thermodynamics is an accounting law, that the energy may change, but the total is conserved. It doesn't however tell us if any conversion or phase transitions will occur, and if it does what the distribution of the forms of energy will be.
Meaning if you eat something, the first law of thermodynamics doesn't tell us if it will even convert into energy in your body, if it does, where it will all go or be used for, how much will be stored, and how much of it will come out of you as waste or bi-products. And even how much energy is used like for brain function or cellular maintenance or organ function is still unknown. It's why some people can eat a lot and not gain weight and others do. There is a lot unaccounted for, and the person may not even convert it to energy. And don't say, oh it's genetics or metabolism. That's getting even more scientific without understanding the science. And secondly that's another reason why the laws of thermodynamics do not apply, it doesn't take into account genetics or metabolism.
The second law is about dissipation or efficiency. Meaning there is nothing that is perfectly efficient, there is always seepage. I hit a ball against a wall, it doesn't come back to me at the same speed because it lost some of that energy.
If I ate 1,000 calories, maybe, 500 calories I stored, 200 I used, but 300 is no longer in my body. It still exists and maybe it came out as waste but it got lost somewhere and didn't stay in me, wasn't used, and wasn't stored.
If a calorie is a calorie, how does our body determine how much is used for what? Our body distributes the calories very differently when its a carb calorie, a fat calorie, or a protein calorie.
The laws of thermodynamics is also supposed to only apply to closed systems. We humans are not a machine or a closed system. It also simplifies all the functions the body does and assumes all we do is store or burn calories, and takes everything else that keeps us alive out of the equation. As if we are some kind of beaker that you can poor things into and out of.
Eating a lot of calories will all go just to weight gain. If that's true what about our hormones, food doesn't affect that? Or mood, or brain function, organs, mitochondria, digestive system, immune system, cardiovascular system? Not to mention diseases or illnesses you can cause by eating the wrong calorie types. Ask a person with a peanut allergy if a calorie is a calorie. Why do these same people who think burning calories makes you lose weight also fear if you lift weights you will get bigger? Lifting weights still burns calories right? Actually it can burn more than cardio so how does that work?
Working out all the time only burns off calories? It doesn't affect our hormones, organ function, mitochondria, immune system, our bones, and all the other things that can be affected by everything we do? Also what about illness and injuries and just fatigue that can be caused by over-training? Or cellular damage?
This idea of the laws of thermodynamics in predicting weight was only supposed to serve as a scientific analogy. Not the actual science of weight loss.
To put it simply if counting calories and thinking you can burn it all off, and treating all calories the same worked: