[ctrl_freak]

To add on to this, my understanding is that technically "calories in, calories out" is correct in the sense that weight gain/loss depends on the net caloric intake for your body. But your base metabolic rate, and as a result your daily energy expenditure, is affected by many different things, including the composition of the calories and the timing of the consumption. Insulin plays a major role in weight gain/loss, and simple carbohydrates have a tendency to spike it.

[wastedhours]

This is the hard thing about the "calories in/calories out" statement, it's accurate for a specific thing: the calories of energy in a burnable form available to your body/the direct burned calories as a combination of RMR and exertion.

The issue of course is that we use it in the context of different kinds of food and a simplistic assumption about how effective we are at converting a calorie in each into a burnable form.

2000 calories of energy from custard will be processed, stored and used differently than 2000 calories of energy from chicken.

[betageek]

Exactly, no breakdown of thermodynamics needed, any calories not efficiently processed into usable energy will be emitted as waste.

[gonzo]

Or ... Stored. Which is the problem.

If we really did eliminate the excess, nobody would be overweight.

[amelius]

No thermodynamics needed to disprove it, but we probably need something more complicated than thermodynamics to get a good picture of the real balance.

[UweSchmidt]

Nothing more complicated than a scale in your bathroom is needed. Check daily and see if you lose weight with whatever combination of food/activity/metabolism you have going on. If you somehow gain or lose weight, eat accordingly.

[garrettdc]

Using the scale is great if you understand proper nutrient timing and partitioning. What if you don't know the basics of nutrition though? Where do I cut from? What can I add in that will assist with satiety but still reduce overall energy absorbed? Where do I go to understand that these are questions that need to be asked in the first place?

[r1chard5mith]

I agree. I think the problem stems from how we determine the caloric content[0] of a food stuff. We add the known caloric content of its ingredients, which we determine by burning them. This is probably just not a good model for how the body uses food.

[0] https://www.scientificamerican.com/article/how-do-food-manuf...