Thinness and fitness are two different things. One can be thin and yet fit, whereas on can be fat and unhealthy. What is important is that the weight should not be below the normal percentile for your age, waist circumference and height. If that is the case the probably you do not eat well. If you eat well, you may be having worms in the stool. Hence a stool test is a must.
If you are underweight for your height, then the cause is usually genetic. If your family in general is thin and skinny, there is nothing much you can do about it. Even without genetic predominance, some people are light weight. The other factors are presence of diseases like diabetes, hyperthyroidism, tuberculosis, poor diet, eating disorders, malabsorption syndromes, and cancers. Worms in stool is another cause.
You can consult your doctor regarding this and eliminate these causes of low weight. If all is fine, consult a nutrition expert and build up a good diet program. A physical fitness trainer then can help you gain some muscle mass. However do not go for this unless the medical causes are ruled out because you may need to modify the diet and exercise program according to the problem you have. Take care!
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