Both (a slight built and a heavy built) are two ends of the spectrum of growth and development. Both ends are normal as long as they remain within the spectrum i.e. the weight is in a range, which is normal for that age. For example a child of 1 year is normal if he weighs 8-12 Kgs. Let us consider adults. Some adults may weigh 50 kilos; others may weigh 70 kilos. Some may be of slender constitution; others may have a heavy constitution. Some put on fat easily and so have a tendency towards obesity.
These people struggle to keep their weight in check. There are others who just cannot put on weight despite all out efforts to increase it. Yet both types of adults are normal, as long as their weight remains within the normal, defined range. A similar analogy is true for children also.
Why are some children thin while others are chubby? There are many reasons; some of the important ones are as below:
i. Genetics: Hereditary plays a major role. Children born to parents of slender constitution are likely to be slender and vice-versa. What about those born to parents, where one is thin and the other is hefty? The child will take after one of them and not average both!
ii. Physical activity: Some children are not energetic and spend most of their time sitting, watching TV., reading etc, i.e. like couch potatoes. On the other hand, some children involve in a lot of physical activities. Obviously the 2nd group of children will be thinner.
iii. Mental temperament: Some children are emotionally quiet. They appear relaxed with a smile on their faces and don’t get tensed up easily. On the other hand, some children get tensed up and anxious over small matters. Therefore a lot of energy (nervous) is expended by thinking about something or the other and by being under chronic stress. Such children are apt to be on the thinner side. (It holds true for adults also).
iv. Appetite: Some children are born with a big appetite. They eat a lot and there is little in life that affects this. On the other hand, some children have a low appetite, which is also easily affected by their spirits (you can again compare with adults; some adults eat a lot, others eat less). But, as a golden rule of the thumb, every baby is born with enough appetite to keep him healthy and let him grow at a normal pace. In children, a major factor affecting the appetite is the “urging” or the “pressure” exerted by the parents on the child to eat more, which has a negative influence on the appetite of the child.
v. Stress: Stress takes appetite away. If a child is anxious or overburdened with work, he will eat less as happens when the child goes to school. Apart from the heavy burden of studies, the child is under a lot of mental stress. This leads to a low appetite. No wonder one finds that the child, who used to be of good weight during pre-school days, has now become thin. Parents should be aware of this.