Ever since I became a Personal Trainer I've had people telling me about how they've lost weight in the past but then put it back on.
The reason for this is that the person has usually gone on an extreme calorie restricted diet and is constantly burning a lot more than what they are putting into their body.
This translates into weight loss and that is why they get the results they are aiming for.
There is a big BUT and this comes after you've reached your goal and got to the weight you're happy with!
Towards the end of your diet you start to feel tired and irritable, you feel stressed and hungry all the time.
When you finish your diet and start eating normally again, this is when you start to realise that you are gaining weight.
You may be eating healthy foods but because you're consuming more calories then you are burning your body is storing the excess calories in case it ever needs it again
e.g. When you are on an extreme calorie restricted diet.
With my clients I don't like to change their diet in an extreme way, by changing their diet straight away and then pushing them further in the gym will cause them to tire easier, not recover as well as they usually would, and then are more likely to GIVE UP.
The best way to change a persons diet is to think of the long term.
Sure, it won't have the quick results that extreme calorie restricted diets achieve, BUT by thinking of the long term and changing small things first will help change the persons habits and this is important in maintaining the weight they are happy with.
By slowly changing the habits of the person they aren't going to be getting the cravings that others will that are on a calorie restricted diet.