Originally Posted by sclim
Just my 2c: I'm more on the side of those who say you can't out-run a fork (I love that). If that's true, then it's even more true that you can't out-swim a fork!
For those who advocate swimming as a weigh loss method, citing the caloric cost of swimming, I would advise that while in theory, any small caloric cost will help, to be truly effective, the caloric cost of the activity has to be significant.
One detriment that must be considered is that TI is all about efficiency. If you are an excellent TI student, then your swimming is smooth and waste-free, or, in other words not very conducive to weight loss. The only way for an efficient swimmer who is swimming at a level nowhere near his top speed to burn large amounts of calories is to swim long hours to make up the deficit due to low number of calories per hour of swimming. This would generally only apply to serious or competitive swimmers, although the latter group would be burning more calories per minute because they are training at fast speeds as well as huge distances daily.
Some evidence that this is true is the fact that in my pool (and I'm sure this is true at most other pools) there are significant numbers of individuals with varying levels of excess body fat who, nevertheless, swim quite well, and with respectable durations. This speaks to two facts -- firstly, excess body fat in itself does not impair the ability to swim well or even fast, and secondly, for these people at least, swimming has not been enough to removed the excess fat that we are observing now (although, to be fair, we have no insight as to whether that individual is in the process of successful weight loss or not).
In contrast, in groups of habitual distance runners, the incidence of individuals with residual excess body fat seems to be less, as is the degree of the excess. It is useful to remember that for an average person about 100 calories is burned per mile run (over and above the background metabolic burning of calories).
Working backwards to get calories per minute of running for runners of various speeds, even slow runners burn more calories per minute than most swimmers (I'm talking about non-competitive swimmers here).
sclim, this post is almost a year old, so I doubt you will read my response, but I did want to differ slightly with what you are saying above. The first thing to keep in mind is that most swimmers swim with lousy technique, because developing swimming technique that lets you swim distance at a high exertion level is more difficult than developing running technique that lets you run distance at a high exertion level. I would compare the swimming that most people you see doing laps down at the pool with running in deep snow. That is, it's great exercise, but it's so unpleasant that no one in their right mind does it at the exertion level and for the time that most serious runners put into running. However, a good swimmer who knows how to push himself using his core in a rhythmic fashion can swim 40 continuous minutes or a span of hours at a pace that a lot of runners run for 40 minutes, and that, I think, is a better comparison. Unfortunately, I don't think we have any real data for this, that I know of at least.
The other thing to keep in mind is that there is a selective process that works against fat runners in that they are more likely to develop impact injuries when they put in heavy miles. This fact has nothing to do with how many calories are being burnt.
Finally, I have heard one form of speculation, and I have no idea how true it is, that you need to get your body temperature up to a certain level before you can start to efficiently burn fat, and swimming tends to keep your temperature down, which may hinder that process. No idea if there is any truth to this, it's a folk story, that one might want to consider.
In summary, it's difficult to compare the two activities.