If you are looking to lose weight then there are some good reasons why you need both cardio and weights in your program, along with keeping a careful eye on your diet.
Diet alone has the greatest impact on weight loss in the short term. It is attributed with 70% or more of the weight loss effect. In the longer term, however, exercise becomes important for maintaining weight loss. A combination of diet and regular exercise therefore is optimal in both the short and long term.
Exercise during dieting reduces the amount of lean muscle mass lost as part of overall weight loss. In other words exercise helps reduce the breakdown of your lean muscle for fuel while you are restricting your calorie intake. This is beneficial so that once you've lost fat you still have some muscle to show underneath!
Diet alone for weight loss results in about 28% of the weight loss being lean muscle mass - EEK!
Include cardiovascular training and you can reduce this to about 2-3% lean muscle mass lost of total weight loss
Include resistance training and you can reduce this loss of lean muscle mass to 0%, or even result in an increase in muscle mass!
In addition, resistance training boosts your metabolism post-training - indefinitely if training is maintained and much more so than cardio alone. This illustrates that to lose weight but protect your lean muscle mass you must perform some resistance training each week. 2-3 times a week is a good guide.
However, the calorific expenditure of training is much greater for cardio than it is for weight training and since weight loss is about energy out being greater than energy in, this is a key point to consider. Here's an example - a 45 year old man weighing 81 kg who runs for 30 mins at 9.6 km/hr will burn around 406 calories, whereas half an hour of weight training, of which 15 minutes is lifting and 15 minutes is recovery, accounts for a net caloric expenditure of about 122 calories. Now you see why we need cardio?
So in addition to your 2-3 resistance training sessions each week, you should also include cardio-based activities. These are 'non-stop' activities that elevate your heart rate for 30 mins or more. A total of 300 minutes of moderate intensity exercise per week is recommended by the American College of Sports Medince (ACSM) for people wishing to lose weight or maintain weight loss, or 150 minutes of vigorous exercise (or equivalent combination of the two). Activities could include walking, running, rowing, cycling, swimming, X-trainer, or if you prefer weights it could be a 'non-stop-circuit' type format.
Train smart everyone!