Most obesity “experts” assume (erroneously) that the big equal sign between the blue and red terms implies a direction of causality.  In other words, they assume that an increase in fat mass (the blue side gets bigger), was CAUSED by the red number being bigger than the green number.

~ Peter Attia from,

Yes, physics always works. Yes, the First Law of Thermodynamics is always true. Yes, “calories in” always equals “calories out”. But that does not explain why we get fat. The equals-sign in the calories-in equals calories-out does not tell you anything about causality.

As with every single thing Peter Attia writes, you should go read this. Twice.