Cardio is not inherently bad. The problem is that people use it incorrectly.
Over a long period of time cardio will break down the body. It just happens from consistent battering against hard surfaces in that way. Shin splints, tendinitis, muscle wear, etc.
Yes, cardio can and does help people lose weight, but people aren’t using it to their advantage. Most will just aimlessly go and run for a given period of time thinking that’s what they should do to lose weight. Unfortunately, the majority of the time this “weight” is fat and muscle, not just fat.
Weightlifting is less time consuming, increases muscle mass which allows you to burn fat more rapidly and increases metabolism, and is much safer. Yes, safer. People try to use cardio to make up for shit diets and they waste all their time on the treadmill.
In the hierarchy of effectiveness it goes: Diet > Weightlifting > Cardio
I’m so confident in an approach of diet and heavy lifting that I can guarantee you that if you give up cardio, focus on your diet first and get on an effective weight training program you will lose more fat than you would of with cardio, look better and feel better. Guaranteed.
Here are some quotes from Martin Berkhan, one of the most brilliant minds in fitness and nutrition today, that help to illustrate my views:
"Some people rely heavily on cardio in order to maintain their leanness. This allows them to be somewhat more lenient with their diet. I am however no fan of cardio and don’t use it to stay lean. In my view cardio as a strategy to maintain a low body fat percentage is not only time-consuming, but also a sure-fire way to hamper muscle and strength gains. If nor time or muscle gain is a concern then by all means continue your cardio regimen. But considering my priorities and those of my clients, I focus on the macrocomposition of the diet to maintain the lean state."
"Never attempt to train yourself into a caloric deficit. Don’t spend hours on the treadmill. Diet comes first, cardio second. The dumbest fat loss strategy ever devised is used by people that wake up early in the morning before going to work to do cardio and follow that up with a “recovery shake.” Congratulations, you just wasted two hours of your life. Cardio is good for cardiovascular health, but most people use cardio as a fat loss tool - and force themselves through regimens that aren’t very conducive to their daily routine (or mental sanity). Next time, skip the shake and the cardio. Sleep two hours longer, but skip breakfast and fast until lunch time. This way you can create the same caloric deficit with the added bonus of feeling more rested and having saved more time. You’ll be much better off."
"Your diet is where you fix things first and foremost. Adding more cardio when your diet is suboptimal is an inefficient and time-wasting strategy that will result in an increased risk of burnout and overtraining."