A small group of people have been thin since birth and have always found it very difficult to gain weight. These individuals are likely born with a condition that is called ‘constitutional thinness’ or just ‘natural thinness’ in plain language (for more, see here).
Their low body weight is a bit of a mystery to scientists like us and cannot readily be explained by their diet and exercise habits.
One can, therefore, speculate whether this type of thinness is caused by a lower calorie absorption within the gut and/or a larger calorie excretion via urine.
One study shows that naturally thin individuals lose the same amount of fat in feces as other healthy humans do.
Another investigation, however, shows that carbohydrates and protein excretion often differ to a greater extent between individuals. Thus, more thorough investigations are needed to test if differences in excretion of carbohydrates and proteins explain the low body weight.
People with a large fat mass are often exposed to stigmatization and discrimination.
Body weight is almost as heritable as height and neither body ‘thickness’ nor ‘thinness’ is about will power.
It is instead about e.g. the biology we inherit from our parents – and calorie excretion might one of the biological factors that explain why we do not all weigh the same.