Why do some people struggle with losing or gaining weight? “Hardgainers” claiming that they have a “high metabolism” are generally mistaken when they blame their base metabolic rate (BMR) for the troubles with gaining weight.
Spontaneous physical activity (SPA), however, refers to tiny movements your body makes throughout the day. These movements result in a significant amount of calories burned if you add them all up and the movements are generally made subconsciously and vary from person to person.
In a caloric surplus your SPA will generally increase resulting in more calories burned and counteracting the increase you make in your caloric intake. Conversely, SPA will be reduced in a caloric deficit. The extent to which SPA counteracts changes in your caloric intake varies between individuals.
This means that a hardgainer who adds 500 calories to his/her diet might experience an increase in SPA resulting in 300 extra calories burned per day making it harder for this person to gain weight. The opposite would then be true for someone who easily gains weight. Thus SPA is another factor which proves that adding a certain amount of calories to your diet will not always result in a predictable weight gain.
NOTE : This does NOT mean that “calories don’t work”. All it means is that everyone has DIFFERENT caloric needs to lose weight.
Levine JA. Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science. 1999 Jan 8;283(5399):212-4.