The popular 'FAT BURNING ZONE' is based on a mistake. The mistake is a misinterpretation of the sports science research, and is based on the fact that the body preferentially uses fat as a fuel source during low to moderate exercise intensities.
While this is true 'metabolically' it does not influence the degree of overall weight or fat loss from exercise.
In terms of weight control, it doesn't matter which fuel source is used as long as calories are expended.
It is essential that clients get the most out of each training session. Interval based training is more effective than low intensity steady state exercise as more total calories are used in the same amount of time. Take a look at the figures.
Exercising at a low Intensity (60% MHR) for 30 minutes will burn 200 kcal, with 50% of calories from fat (100 kcal). If you increase the intensity to 80%MHR then you will burn 500 Kcal with 40% of claories from fat (200 kcals). So for the same workout time you are getting more bang for your buck, and that is before we even start to consider how many calories you continue to burn post workout!
So to summarise:
High intensity interval training will maximise weight loss and fitness results.
High intensity interval training forces the body to become conditioned to more intense exercise.
Increased levels of fitness will make a given level of exercise feel easier and you will be able to exercise at higher intensities which 'burns' more calories.
High intensity interval training will increase Basal Metabolic Rate (BMR), resulting in more calories 'burnt' 24 hours a day.
High intensity interval training strengthens the involved muscles to a greater degree than regular aerobic exercise.
Slow steady state training is not as effective as high intenisty intervals.
If you take part in a sport that requires an element of speed and power then intervals are far more effective, not just for fat loss but for increased sport specific fitness.....unless you want to become slow and steady.