Cold ice water in summer

Pour some ice cold water on your palm. Once it touches your palm, you will experience that it is cold to touch. A few seconds later it no longer feels the same level of cold. What exactly happened here? 

Heat exchange. 

The cold water after coming in contact with the hot body became warmer. The body lost some heat and the cold water gained some heat.

The same process takes place when one drinks ice cold water during summer. Heat exchange takes place in the throat, esophagus and stomach. The body loses heat in this process. This heat was generated through digestion. It is regulated by the Hypothalamus. An optimal temperature (37 degree Celsius) is essential for digestive enzymes to function optimally. The metabolic rate also depends on the body's thermal environment.

Lower heat in digestive organs this impacts the ability to digest food. From an ayurvedic perspective, vayu increases (sheeta guna increases), pitta decreases (ushna guna reduces), and kapha (sheeta guna increases).

Overall metabolism is impacted and the body has to use its own energy for coming back to balance. If it is unable to come back to its balance on its own then a natural process is triggered to bring back the balance. This natural process is temporary illness that basically resets the body.