Weight training, typically, is intense. Regardless of whether you’re lifting 2.5kg or 25kg, if it’s relative to your current ability, it will be intense enough to force your body to adapt.
The same cannot be said for running on a treadmill, or what most people refer to as ‘cardio’. In this sense, ‘cardio’ is actually specific aerobic conditioning, if you run on a treadmill all the time, you’re now just very good at running on a treadmill. You have not adapted to lose and keep-off fat.
Weight training is superior for fat loss for many reasons, most notably, the increase in resting metabolic rate, and the fat burning effect that comes with the recovery process.
The more muscle tissue we have, the more calories we burn when resting. Muscle is a metabolically active tissue, the more we have, the higher our energy requirements and the more calories used to sustain it; rather than being stored as fat. This is also why (ladies take note) that when you embark on a weight training regime, you will not turn into Arnold Schwarzenegger.
Our bodies are very clever, they’ve had to be to ensure we survived our caveman days. Back then, food was scarce. Being able to easily build copious amounts of muscle tissue would not have helped us survive. More muscle, more calories burned, more food needed to make sure we didn’t die. Because of this, the majority of us have a gene known as GDF-8, which basically puts a limit on how much muscle we can develop. Only a small minority of people are born without it.
Also, when recovering from a weight training session (that’s intense enough) our aerobic system is a vital component of this process. In many cases, the aerobic system gets a better ‘workout’ when recovering from an intense weights session than a steady paced treadmill session. As such, weight training is a much more effective way of losing body fat than ‘cardio’.