One of my brother-in-law's coworkers claims that drinking hot water (straight from the tap) aids in digestion. I've been able to find a debunking of cold water causing cancer, but can't find anything related to drinking hot water (at least nothing backed by scientific evidence, or even the appearance of scientific evidence). So, is there any truth to this claim?
There is some scientific evidence that the temperature of meal/drink has an effect on digestive system activity. See Effect of meal temperature on gastric emptying of liquids in man:
Serial studies were carried out on six healthy volunteers (19-24 years) to investigate the effect of meal temperature [either 4 degrees C (cold), 37 degrees C (control) or 50 degrees C (warm)] on the rate of gastric emptying of a radiolabelled isosmotic drink of orange juice.
Warm and cold drinks appeared to empty from the stomach more slowly than the control drink. The initial rate of gastric emptying of the cold drink was significantly slower than the control drink (p less than 0.05) and the difference in emptying rates between cold and control drinks were significantly correlated with the differences in intragastric temperatures (p less than 0.01).
Anecdotally, in my experience, drinking half a liter of hot water (boiled cold tap water) feels like it has an effect on digestion, but I wouldn't claim that it aids digestion. This is so easy to try out for yourself that I'd suggest doing so.
Drinking hot tap water is not recommended. I guess it might have an effect on digestion too, depending on the plumbing and water quality. I hope this isn't the effect the claimant feels is aiding digestion.
Here's Dr. Steven Novella's take on this issue at the Neurologica blog:
To summarize all of this – the various studies that directly look at this question found variable responses to warm or cold liquid or solid meals, but with a tendency to find delayed gastric emptying in response to food at other than body temperature. This effect, if real, may be a nonspecific vagal response, which can either be due to temperature or any pain or trauma.
What not a single study found, however, was accelerated gastric emptying in response to cold water or food. The actual scientific results, if anything, are the exact opposite of what many of the nutrition sites are claiming.
the scientific data indicates that water, of any tolerable temperature, does not have an adverse effect on digestion. Water, in fact, is critical for digestion. So, drink as much as you want (within reason, of course) during meals.