You can have a hereditary disposition to being thin. An insufficient diet, both regarding the amount and the content of it can be contributory and hard competitive sports like triathlon suppress the production of sex hormones and thereby making it impossible to gain weight.
Stress and emotional problems might reduce the appetite and cause weight loss, but this is most often seen in young girls in the form of anorexia. These girls often have a mixed-up self-image and think that they are overweight.
Smokers have a larger fat metabolism than non-smokers often leading to an increase in weight in the time after having stopped smoking; so quitting the cigarettes will be the first step for smokers who would like to gain weight.
Children and underweight
No child, regardless of age, needs large amounts of sugar, but small children need a fat – rich diet. Growing children need more food per kilo bodyweight than adults. Fruit, vegetables, and fiber are filling and children who eat only these foods risk being full before achieving their caloric requirement.