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The term Body heat used by grand parent /parents is the increased BMR as few people have because of which their body is more warm as compared to others.They used to relate most of the things to increased body heat but now medicine is advanced & we know the cause of things is not heat :
is due to spicy food;increased acidity;loss or improper sleep.
-burning of eyes due to overstraining;sunstroke.
due to infection or irritation of the trachea & lungs.
This increased BMR is probably body heat medically.