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exrunner
August 6th, 2003, 04:24 PM
I've read that regular aerobic exercise actually helps keep hunger in check. I've noticed this myself -- for several hours after a (modest) workout, I am satisfied by small, light meals. Anybody know the physiological basis for this effect? Something to do with neurotransmitters?

mattson
August 6th, 2003, 05:00 PM
There are three reasons I can think of:
1) Your inner organs tend to smoosh around when exercising. It takes a while before your stomach settles down. (I'm thinking specifically of running. How hungry are people after riding a roller coaster?)
2) If you dehydrate, your body suppresses hunger until water needs are met.
3) In high school health class (see, you should have stayed awake!), the three major "blood users" in the body are the brain, motor muscles, and digestive tract. You only have enough blood for 2 of the 3. Assuming that the brain wins out :) , that means exercising and digesting fight it out for the remaining resources.

JOHN PAUL
August 7th, 2003, 08:59 AM
Very interesting

laineybug
August 7th, 2003, 10:24 AM
okay, here's a twist...

exercise like jogging, running, floor/step aerobics do decrease or supress my appetite, BUT swimming makes me voracious / ravenous! Anyone else experience this?

aquageek
August 7th, 2003, 11:31 AM
I might temporarily have suppressed apetite right after a swim, but I am voracious about 30 minutes later. It's very similar to the hunger I get after skiing.

Usually after about an hour of swimming, all I can think about is a huge cup of coffee and fried eggs and bacon. Swim to eat, eat to swim.

exrunner
August 7th, 2003, 03:13 PM
I think I'm getting closer to the answer. After a number of internet searches, I stumbled into the following article:
http://psychology.about.com/library/weekly/aa081000a.htm
It identifies serotonin, a neurotransmitter, as one mechanism by which exercise may moderate appetite levels. Another article I read mentions endorphins (which are also neurotransmitters, and which are released during prolonged exercise).

Now, I'm going to have a sandwich.

mattson
August 7th, 2003, 04:22 PM
Two reasons why you might be hungrier after swimming (vs running):
1) Swimming doesn't jostle your inner organs as much as running. So your stomach will be ready sooner. :p
2) Swimming lowers your skin temperature more (once you stop exercising). I've read (sorry don't remember where) that being cold causes the body to want to build up fat reserves.

All the more reason to jump in a hot tub after practice. :D

sparx35
September 22nd, 2003, 05:12 PM
when at rest ,the body in association with the mind decides that it's a good idea to eat.
this is a survival tactic that the body/mind automatically goes into when given time to rest.
when swimming/exercising the body/mind is adrenalising,using reserves from the blood/fat etc this now being mode 2 stops the body from returning to want to stockup..for a while....
now where did i put that pizza...:D

dpflyer
September 22nd, 2003, 07:09 PM
So that explains why we go out for pizza and beer every Thursday night after swimming!

Shaky
September 23rd, 2003, 05:30 AM
There's some information on this question on the "weight loss" thread a little further down:

http://forums.usms.org/showthread.php?s=&threadid=1673&pagenumber=1

I'm no expert, but I have researched these questions a bit. Below is part of what I posted on the weight loss thread in response to a question about why it's easier to lose weight by running that by swimming. I thought it might be of interest here.

* * *

The muscles in the lower body, mostly "dark meat," have evolved in such a way that the body powers them by burning fat. Body fat provides an almost limitless source of energy, allowing the muscles to be used for long periods of time, while walking long distances. Lower body exercise like walking, running or biking taps into these muscles' ability to burn fat.

The muscles in the upper body are mostly "white meat" and have evolved to be powered by burning glycogen stored in the muscles themselves. These muscles evolved for quick bursts of activity, such as throwing, grasping, tearing or lifting, instead of sustained activity.

Since it takes more time to get energy out of stored fat, the glycogen serves as a quick start system. The white muscles will burn fat, but the body resists letting them do it until their glycogen reserves are depleted. Likewise, the dark muscles will burn glycogen for quick bursts of speed, but are optimized to burn fat. The two types of muscle correspond roughly to aerobic and anaerobic exercise, swimming having a large anaerobic component provided by the resistance of the water.

Since more muscle mass can store more glycogen, working muscles anaerobically will cause the body to react by building muscle. Exercise that is more aerobic will build less muscle, since the the muscle mass isn't necessary to store glycogen to power the muscles that way.

The hunger that comes from swimming is a direct result of glycogen depletion. When you burn the glycogen out of your muscles, your body will want to replenish it as quickly as possible in order to be ready for the next burst of activity. When you burn fat through lower body exercise, the body tends to keep running on the fat reserves for a while once the process has gone into high gear, and it thinks that it doesn't need any additional fuel. Thus hunger is suppressed.

For many swimmers trying to lose weight, the loss of glycogen causes a craving for carbohydrates. Carbs are the quickest way to replenish the glycogen in the muscles. The problem is that eating carbs without a balancing portion of protein causes them to hit the blood system as sugar more quickly than the muscles can absorb them, and the blood sugar level spikes. To counter that spike, the body releases insulin. Insulin triggers the formation of fat; it causes the sugar to be taken out of the bloodstream and stored for future use in the fat cells.

Tell me if this sounds familiar: You get out of the pool starving. You eat something starchy to satisfy your hunger. But half an hour later you desperately want a nap, and if you don't get it your brain goes into a fog.

That's caused by an insulin spike. Your body wants glycogen, so you give it carbs. Insulin is released to get rid of the excess. It works too well, so your blood sugar goes a little low. When your blood sugar goes low, your body responds by trying to shut down for a while to recover.

You can avoid that by eating some protein with your carbs. The protein is more difficult to digest, and when it is mixed with the carbs in the digestive system it prevents them from being absorbed as rapidly. Thus, the body can keep up with the sugar entering the system without releasing an insulin spike, thus avoiding issuing a command to the body to store fat.

Finally, that business about running keeping you from being hungry because it jostles your internal organs is a wives' tale. You can pound on someone's abdomen with a billy club, but if his muscles are depleted of glycogen he'll still be hungry.

exrunner
September 23rd, 2003, 02:13 PM
I did some additional web-surfing on this subject, and it does appear to be the neurotransmitter serotonin that is involved here. Serotonin is secreted at a higher level during extended periods of moderate to heavy exercise. Along with its likely effects on mood (e.g., runner's high), it tends to suppress one's appetite for heavy, fatty foods, and incline one's tastes toward carbs. (This is consistent with Shaky's dissertation above.)

Personally, I find that I eat less overall on the days when I swim, even during the dangerous late evening hours spent in front of the tube. I think the above mechanism might explain why I would prefer lighter meals after workouts. In addition, in my case I think there is an additional psychological component -- I just don't want to mess up the progress I've made (in weight control) by pigging out after a good hard workout.

Conniekat8
September 23rd, 2003, 05:18 PM
What Shaky says is so true!!!

As for swimming making people hungry, there are two factors.

First is, that swimming generally burns more calories then running.

Second is that it does lower the body temperature slightly, and the body does respond by wanting to store some fat.

You can offset some of that by swimming in warmer pools if you can, sit in a jacuzzi for a while after a swim, or taking a long hot shower. (long enough for your body to warm up.)

Backman
September 23rd, 2003, 06:06 PM
Precisely why people who live in cold weather climates tend to crave more fats and carbs when winter kicks into full swing.

Always a good idea to keep hydrated with lots of water. It will keep the full feeling there in the stomach, and the call for food is postponed.

If swimmers burns more calories than runners, what would one have to say about the fact that most runners are built like stringbeans?

mattson
September 24th, 2003, 04:19 PM
Originally posted by Backman
If swimmers burns more calories than runners, what would one have to say about the fact that most runners are built like stringbeans?

At any speed, runner's legs have to support body weight, while swimmers do not. I would guess that non-stringbeans will find more reasons (such as sore knees, tired muscles) to stop running. Swimmers, of any size, can always be peer-pressured into swimming more. ;)