Why is my abdomen much cooler than the rest of my body after a ride?

Question

Background

Not having ridden more than a few miles each day in over a year, I wanted to start exercising again with a short-medium ride (~15km/~9.5 miles). I did so over the course of an hour this morning. The area I ride in is known for its hills (I'm not sure what the grades/elevation changes are, but some of them are very tough.)

I ate a few bites of a Cliff Bar along the way and drank about 12oz of the 24oz of ice water I carried.

Observations

When I returned and began stretching before I showered, I noticed that the skin around my belly/abdomen was much cooler than the rest of my body.

Thoughts

[The following are my guesses at the answer to the question, none of which I'm entirely satisfied with. A good answer will not only address the title question but also address these thoughts.]

At first I thought this was simply the result of the body diverting blood from my stomach to my legs, in order to keep them oxygenated as I climbed.

However, I also know that the body needs to circulate blood between around the stomach to transfer excess heat to the ice water I drink; equivalently, the blood needs to reach the stomach to be cooled off so that, in circulating, it cools the rest of my body. Why then was my stomach not warm?

Then I considered that perhaps the body was diverting some resources to keeping the stomach cool in order to allow it to continue to process and absorb the ice water: intuitively, this makes some sense. Warm water on a ride seldom gets absorbed by the stomach, or so I've been told.

I suppose the coolness could simply be the result of the ice water—but this indicates that either the cold was not being spread to the rest of my body, or that it was maintained as a sort of "ice pack" at my core.

Answer 1

This part of the body isn't doing much, unlike other areas, so isn't warmed by blood flow and muscle activity. It is exposed to the wind, and can get a little sweaty quite easily, even if the ambient temperature isn't all that high, but it's also sheltered from the sun. The climbs probably got you sweating, then the descents would have significant wind chill.

Even if you don't have fat to insulate your digestive system, there's still a layer of muscle between it and the skin, so the skin temperature there isn't the core temperature (if it was, there wouldn't be so much need for internal medical thermometers).

Don't listen to whoever told you warm water doesn't get absorbed. It's hard to prove a negative, but if you've ever ridden all day in the warm with only warm water available and no need to go to the toilet despite drinking loads it will be obvious that it's getting absorbed (even if it does turn straight into sweat).

Answer 2

At first I thought this was simply the result of the body diverting blood from my stomach to my legs, in order to keep them oxygenated as I climbed.

This is where the confusion between stomach and belly comes in: your body does divert blood away from your digestive tract if it's needed more urgently elsewhere, but that wouldn't necessarily affect skin temperature much. Your abdominal muscles will still be doing work closer to the surface, after all, and your liver isn't going to drop below its usual core temperature just because you're twitching your quadriceps.

... transfer excess heat to the ice water I drink...

This will likely be a fairly passive thermal gradient somewhere in the digestive tract. I don't think drinking cold water triggers a histamine-style local inflammatory response in the stomach. Which anyway is not the belly.

Warm water on a ride seldom gets absorbed by the stomach, or so I've been told

Honestly, this sounds like nonsense. It might feel less refreshing, but so long as it's a non-scalding liquid rather than a gas or solid, I doubt anything affects absorption much apart from the usual sugar/salt/caffeine or whatever additives.

There are a finite number of ways heat can enter and leave your abdominal skin:

1. external cooling: the front of your body (chest and belly) bear the brunt of wind chill. It could be your upper body was working harder and generated more heat. Or, it could be the cooling combined with the
2. internal insulation: I know I carry some fat there (OK, let's not talk about how much), and that's a good insulator. Men often (mostly?) have less of this insulation at the front of the chest. Not sure how distribution varies in women apart from the obvious. So, even at the same rate of heat loss, it may be replaced more slowly to the abdominal skin than around the chest. And finally
3. local thermogenesis: here, mostly looking at work by the superficial abdominal muscles. Not sure how much work those are likely to be doing - it'll depend partly on how much force you're having to stabilize, but I don't ever recall finishing a bike ride with my abs tireder than my legs, so let's assume not enough to generate lots of local heating.

Your core temperature is kept pretty constant unless you're very ill or actually dead, so the skin being much cooler in one area can only be because it's externally chilled, better insulated from that core temperature, or less heat is being generated immediately beneath it.

The insulation behaviour of the two sorts of abdominal fat will be slightly different (visceral fat is inside the abs, and subcutaneous fat between them and the skin), but there aren't really any other mechanisms of heat transfer.