"The First Law has been hijacked by the nutritionista because it kinda sounds like it should apply to dieting. As a result it is used more today in human nutrition than in any other domain of human endeavour. The First Law is quoted every time you watch or read any information on diet products, exercise or weight loss programs. And thegovernment throws it your face even more regularly than that.
We are told the only way we can lose weight is to either consume less energy (calories) or burn more energy (by exercising). Fat has almost twice the calories per gram as protein or carbohydrate.
Sounds logical, right? And I guess that’s why this simple message has so much traction withadvertisers.
The only problem is that it is utter nonsense.
The First Law certainly says that when we gain weight there will be an accompanying increase in the number of calories consumed (or decrease in the number burned). It is an equation and in equations each side must equal the other, but that does not necessarily mean that increased calorie consumption causes the weight gain.
It is equally logical to say that gaining weight is the driving force in the equation. In other words, weight gain causes us to consume more calories.
It is equally logical to say that gaining weight is the driving force in the equation. In other words, weight gain causes us to consume more calories.
When a child grows they increase their body size and weight. These growth-hormone driven changes cause the child to consume more calories, not the other way around. Not even the most rabid nutritionist would suggest that feeding a 10 year old like an 18 year old would help you get past those awkward teenage years more quickly.
Growth happens because our hormone clock says it should. Calorie consumption just keeps up (so that the energy balance stays, well, … balanced). We are perfectly happy to explain the equation that way when we talk about people who grow vertically. We’re even happy to accept that pregnant women gain weight (other than the baby of course) because hormones tell their bodies to fill the baby-pantry.
Eating is the way we put on weight, it isn’t thereason we put on weight. When a car accelerates, it does so because it has access to more petrol. The petrol doesn’t cause the acceleration, but it does enable it.
But for some reason when Norm grows horizontally, causation is magically reversed. Nutritionists lurch from physics into psychiatry and the cause suddenly becomes greed or sloth (or both).
Just like height gain, weight gain is caused by hormones, but this time it is hormonal dys-function. The fructose half of sugar causes us to become resistant to our main appetite suppression hormones (insulin and leptin). When this happens our appetite is not shut-down when it should be and we just keep eating. And just like a Toyota with a stuck accelerator, our weight accelerates out of control.
Once we understand that weight gain is caused by hormonal dysfunction (a stuck accelerator) many (previously) mysterious things become clear.
A diet that asks you to consume less calories by exercising willpower is doomed to failure. Imagine how successful you’d be if you asked a child to exercise some willpower and stop growing. Fighting hormones with willpower is about as effective as paddling upstream with a barbed-wire paddle. Perhaps this is why the best indicator that you will be heavier in five years is being on a diet now."
TL;dr: The first law of thermodynamics doesn't relate to weight gain because "energy in = energy out" this is a vast misinterpretation.
Source: I'm an engineer is a BSc in engineering and deal with first principles pretty often. I am also 10st before you get on that high horse.