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Some children have a hard time eating if food upsets their stomach. Sometimes this is due to delayed food allergies. There is an expensive blood test for this, but it is not 100% accurate, but can be a good starting point. Food elimination diets can detect them, but don't use the ones that are too restrictive. Most popular food problems: gluten (wheat), milk, eggs or egg whites, corn, soy, nuts. Lots of books on this.
Celiac disease= no gluten. If your child is really underweight or short for their age, try the celiac disease blood test. The test is not 100% accurate as there are sometimes false negatives if the disease is in the early stage. Other symptoms of celiacs: stomach upsets, IBS, tooth enamel problems, skin problems.... Google for more info and symptoms. Celiac disease destroys the villi in your intestine and food/vitamins are not absorbed. Ergo, if your child is not absorbing their food, they don't grow. Once your child is off gluten, the intestines heal and food is absorbed.... weight is gained.