You're asking and I'm answering . . .
I already have a regular, fairly vigorous weekly fitness routine including spin classes, yoga/Pilates and weight training, but I'd like to incorporate some variety. I was thinking about swimming, but I keep hearing that swimming isn't very good as a cardio/weight-loss activity. I am just wondering if you knew whether the "warnings" against swimming as a "bad" form of exercise are valid?
It is a full body exercise, but most of the work taxes the upper body so the lower body isn't challenged much. Because our leg muscles are the largest muscle group in our bodies, swimming doesn't work your total muscle mass. Swimming is also a non-weight bearing exercise, so while it doesn't help maintain bone density and ward off osteoporosis, it saves your feet, ankles and knee joints from wear and tear. These two reasons are why I think swimming is a perfect complement to running, since running works the legs and can over tax the knee joints.
To see the effects of swimming on weight loss
The main reason that swimming has been found to have a negative affect on weight loss is the post workout appetite it can often produce. Swimming in cold water can increase appetite as swimmers fuel the fire of the body temperature with food after stepping out of the pool. In contrast, running and cycling increase body temperature, which can in fact decrease appetite. Often swimmers end up replacing all the calories they just burned.
If swimming is your favorite form of exercise, the chances of you sticking to exercise are greater and that is what is most important. I think swimming would make a great addition to your workout routine, since you are already doing strength training and spin. Just make sure you get enough "impact" exercise, and walking will do this for your bone density.
See you in the pool!