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Q: How does the swallowing of iced water affect the skin and blood temperature?

Krishna: Swallowing cold water is different from applying cold water press directly to the skin.
 Applying cold water or ice to strategic points on the body where the veins are close to the surface — such as the wrists, neck, chest, and temples — can quickly lower the temperature of the blood running through these veins. This allows the body to feel cooler.
But drinking ice cold water is different. Yes it will affect a human body to some extent immediately. But soon your homeostatic mechanisms will take charge and restore balance. 

 The process of maintaining an optimal body temperature is called thermoregulation, which involves a delicate balance between producing and losing heat.

Humans are warm-blooded or endotherms, which are scientific ways of saying we can control our body temperature independent of the environment. We can do this because our bodies are constantly producing heat as a by-product of internal chemical processes (metabolism).

If we consider  the mass of our body and the mass of the water we drink and the fact that our body isn’t just warm, it actually produces heat. These two things reduce the effectiveness of drinking cold water to lower body temperature.  
The heat our body generates is beneficial when it’s cold, but when outside temperatures rise, we need to avoid overheating. While it may seem logical that introducing something cold, like ice cream or ice water/drinks, into the stomach should help reduce temperature, its initial cooling effect is rapidly replaced by heat generated by digestive processes (7). 

The heat transfer between a cold beverage and the digestive system can directly influence temperature. But, this is only momentary and depends on the quantity and caloric content of the ingested liquid.

Beverages with a high caloric content, such as soft drinks, will have a similar effect as ice cream and kick start our metabolism shortly after ingestion.

A small amount of liquid will lose its cooling effect quite quickly as it gets warmed up by the surrounding organs. And large amounts of cold liquids will cause blood flow to slow, making heat transport less effective.

The cooling effects of cold liquids are more likely explained by their rehydration effects. If heat does build up, the body will attempt to lose excess heat by transporting it away from the vital organs to the skin surface where it is transferred directly to our environment through convection and radiation.

For this to occur, the ambient temperature needs to be lower than our own temperature, or the opposite happens and heat will transfer into our body. Just like the heat radiated from the sun on a hot summer day.

Sweating is the most effective way our bodies lose heat. Sweating occurs when an increase in core body temperature is detected by the brain, which responds by stimulating the sweat glands distributed all over the body to produce sweat.

Sweat on the skin surface evaporates, causing the skin to cool down (also called evaporative cooling). Blood that’s flowing close to the surface of the skin gets cooled in the process and helps reduce core temperature.

On average, an adult can lose up to half or one litre of sweat every day, but in hot environments this can increase to almost a litre and a half an hour. That’s why it’s essential to keep the body hydrated during hot weather.

What happens if you take a cold beer on a hot summer day in an attempt to cool down? Alcohol is a diuretic, which means that it will make your body lose water and so reduce your ability to lose heat through sweating.

Surprisingly, warm beverages might be a good way to keep you cool. Although counter intuitive, drinking a warm beverage causes receptors in your mouth and throat to trigger a sweat response, allowing your body to cool down without having to ingest a large amount of the warm liquid.
Active ingredients in spicy foods have the same effect; they too trigger a sweat response that allows the body to cool down. That’s why these types of foods are popular in warm climates.

The ingestion of cold drinks can reduce body core temperature before exercise but less so during exercise. Temperature of drinks does not seem to have an effect on the rate of gastric emptying and intestinal absorption (4).  

The body has a core temperature of around 98.6°F  the body needs to expend additional energy to restore this temperature after drinking cold water.  
In one study (1) it was found that drinking cold water can significantly mediate and delay the increase in core body temperature during an exercise session in a moderate climate with euhydrated subjects. The ingestion of COLD improved performance for 49% and 51% of the participants in the broad jump and TTE performance tests respectively, but did not reach statistical significance. The researchers found that changing the water temperature affected the sweating response of the participants and how much water they drank. The optimal water temperature in the study was 16°C (60.8°F), which is the temperature of cool tap water because the participants drank more water and sweated less.

The researchers concluded that drinking water at 16°C may be the best temperature for rehydration in dehydrated athletes.

Some research suggests that people with conditions that affect the esophagus, or food pipe, such as achalasia, should avoid drinking cold water. Achalasia is a rare condition that can make swallowing food and drink difficult.

Another study (2) found that drinking cold water worsened symptoms in people with achalasia. However, when participants drank hot water, it helped soothe and relax the food pipe, making food and drink easier to swallow.

One study involving 669 women (3) suggests that drinking cold water may cause headaches in some people.

The researchers reported that 7.6 percent of participants experienced a headache after drinking 150 milliliters of ice-cold water through a straw. They also found that participants with active migraine were twice as likely to get a headache after drinking cold water as those who had never had a migraine.

Research suggests that the temperature of the water that people drink can affect levels of sweating and rehydration. For example, a study (6)found that drinking warm water (40°C) rather than cool water (15°C) may cause people to drink less, which can lead to dehydration.

 There is little scientific evidence to suggest that drinking cold water is bad for people. In fact, drinking colder water may improve exercise performance and be better for rehydration when exercising, especially in hotter environments.

However, drinking cold water may worsen symptoms in people with achalasia, which affects the food pipe. Drinking ice-cold water can also causes headaches in some people, particularly those who live with migraine.

At thermal equilibrium at rest or during exercise, the heat gains and heat losses are balanced by adjustments in skin circulation, metabolic heat production, and sweat secretion (5). 
In conclusion, unless you have certain medical conditions, a healthy body restores balance and drinking water temperature reverberations are negligible. 
Footnotes: 
5. Nielsen BThermoregulation in rest and exerciseActa Physiol Scand 1969323S1– S74.

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