Cal in = Cal out is always true. However, the Cal in side has a very important % energy extracted modifier. Simarly, the Cal out side is modified by metabolic rate and exertion modifiers.

Those individuals with high energy extraction and low metabolic rate will find it very difficult to eat as little as required to balance the energy equation without constant hunger and living with excessively small portions. While this is achievable, it is hard and failure is frequent...especially given all the other challenges in life. So naturally, asking whether and how these modifiers can be adjusted is pressing. There is a lot to learn, but thereare known factors that can influence these factors. For example, co tinued cold exposure leads to activation of brown fat that generates heat, which dramatically raises metabolic rate (e.g. Californian's feel cold in 12°C weather, but someone in New England would wear shorts in that weather). Interestingly, cold weather increases gut caloric absorbancy to compensate for these expenditures..(sorry Id provide references, but I dont have it bookmarked on my current device).

see what i'm saying?