From Dragonchilde: "When you don't strength train, up to 30% of the weight loss you experience can come from lean muscle... which reduces your metabolism and raises your body fat percentage."
Nitpick -- this last part is not true. Take for instance a person who is 200 pounds and 40% body fat to begin with. That would be 80 pounds from fat. Now assume that person loses 50 pounds, 30% (15 pounds) of which comes from lean mass. That still means that 70% (35 pounds) comes from fat. The person now has 45 pounds of fat, which at 150 pounds total is only 30% body fat, a huge difference.
Here's where using your muscles while losing weight still does matter: if that same hypothetical person were able to keep it to just 10% lean mass lost, then 45 of those 50 pounds would come from fat, and at 150 pounds the person would be at only 35/150 = 23% body fat. Yet another huge difference, without having actually lost a single pound more. But it's still not accurate to say that loss of weight without strength training could ever make your body fat percentage go up. The majority of your loss is always going to come from your fat stores, assuming you have them, which most of us do.
As far as reduction in metabolism, I don't think it's fair or useful to say that lack of strength training on its own will necessarily cause any problems. I know that I personally did very little to zero strength training until quite recently, and most of my weight was lost while I was doing purely cardio. My metabolism is still quite ridiculously high to the point that at maintenance and exercising, I can barely keep up with it while eating only healthy foods, and not lose any more.
It would be more accurate, I think, to say that lack of strength training, particularly when coupled with over-cutting of calories relative to exercise intensity, is going to increase the risk that you run into metabolic problems or your body backlashing on you in other ways. (Again personally, I always kept my calories relatively high.) I think that treating it as a given just puts additional barriers in the way of people who are are trying to get healthy but have mental, physical, or other bars in the way of getting the recommended amount of muscle-building exercise. It can be done even without it. You don't always need to be perfect.
Height 5'8 1/2"
GW: Originally 150. Maintaining at about 146 since June 2013.
5K 4/21/11: 31:55
|46 Maintenance Weeks