It’s certainly possible that swimming may make you hold onto fat. Although as someone mentioned we’re not seals or dolphins, our fat deposition pattern and type is much more similar to aquatic mammals than it is to say, a dog, a monkey, or an ape.
Google aquatic ape hypothesis for more on that.
Swimming is a fantastic form of cardio and IMO fat loss or not depends on the type of swimmer you are. I was a breaststroker/50m freestyle sprinter. The breaststroke is a VERY fast twitch dependent stroke. And most breaststroke specialists look the part.
With that ‘sprinter’ look. The 50m sprint takes an olympian around 20 seconds. A good interval for anaerobic dominance. So even though the movement of the freestyle stroke is not as explosive, its still going to work in that context.
If you watched the summer olympics you may have noticed the same thing, the guys who weren’t sprinters, and who weren’t breaststrokers ‘looked’ a lot like endurance athletes of any type. Long slow movements, exercise times of well over a minute, makes sense.
Lap swimming is like any form of steady-state cardio. Not great for fat loss. For all of the same reasons. It’s worse in some respects for the lifting population.
I would also be concerned about lap swimming in the weight lifting population for a different reason. Fiber type conversion.
Cressey has talked about that at length in ‘cardio conundrum’ or was it ‘cardio confusion’?
Swimming uses more muscles at a higher intesnity than just about any other form of steady-state cardio. Which means, while its great for the heart, you are MORE THAN LIKELY going to have a higher degree of Type II–>Type I fiber conversion. Which is, umm, not good if you’re a weight trainer.
if you want to use swimming for fat loss, I would suggest 25 yards on, 50 yards off intervals. No offense, but unless you know how to swim, have great technique, and have times consistent with those of a FAST high school or college swimmer, 50 yards will take too long for a proper interval.
The breaststroke is probably the best stroke since its the most explosive. Butterfly is probably next, followed by freestyle.