Dear Doctor;I am too thin for my age. Considering that they say junks and fatty foods can lead to weight gain, I have been eating lots of them but I am not getting fat. Is there a peculiar problem with me? Caroline,18.Dear Caroline, Looking thin as a visual impression is different from actually being malnourished or unhealthy. External built of a person is based on many factors like amount and type of food taken, amount and consumption of alcohol and soft drinks, level of physical activity  and most important genetic factors. If you have people in your family towards lean side, well that would be your built also. One would be called unhealthy or weak technically if the BMI (body mass index–calculated as weight in kilo divided by height in square meters) is 18 or less. If you feel energetic enough to carry on your routine work without problem, do not get exhausted readily,  then it is fine.The perception about junk food is not correct. Junk food can lead to obesity as well as asthenia, i.e. both types of malnutrition, as it does not provide any nutrients to the body. What you need is  a carbohydrate and also protein rich food. Carbohydrates provide calories, while proteins help in building muscle mass. Avoid junk food and alcohol. Continue regular exercise and you shall be fine.