Are you gaining weight without knowing why? Author of Thin Secrets, Dr Lizzie Kingsley was until she realised that no one is naturally slim, it's all down to making the right lifestyle choices.

Everybody knows a 'naturally' slim person - the sort of person who can get away with eating whatever they please. They never diet; they don't need to because they are slim. We can easily spot them, they're all around us. For example, the girl at work who never refuses the doughnuts at coffee time, yet has a figure to die for. Whereas the rest of the room are exchanging the customary 'ooh I really shouldn't' and 'the diet starts tomorrow' comments, 'Miss Naturally thin' just eats and enjoys. How can she eat like that and get away with it? Is she just lucky? Surely there must be an explanation, but what is it?

Okay, I must admit, I was a skinny teenager. I don't know how many times I heard the phrase 'hollow legs' in my childhood, but once I left home all that stopped. People no longer commented on how slim I was, or asked me how I managed to eat so much without gaining an ounce. A good few years passed before it dawned on me that I had actually put on a fair bit of weight. At the time I didn't really understand how; it just happened slowly and steadily. Obviously, I should have twigged when I had to buy clothes in the next size up, but I always seemed to come up with an alternative explanation for why. The low point came when I realised that I was still gaining weight and I felt like I had no control over it. I couldn't really see what I had been doing to cause it and so I didn't feel like I could ever really lose it.

I tried dieting but never really got very far. I would only manage a week or two at a time and then give up. The joys of losing a few pounds would be quickly overshadowed by piling it straight back on again. Eventually, I got sick of this yo-yo existence so I decided to rebel - to stop dieting. Yet, I still wanted to be slim so I looked for another way of dealing with my weight. I started to try and work out what slim people do differently in order to maintain their enviable figures and along the way I learnt an awful lot about these mysterious creatures.

Nearly everyone I spoke to, both slim and overweight, thought being slim was pretty much down to luck, that some people were just naturally thin. It seemed to be a common way of explaining the unexplainable - how do some people get away with it whilst others aren't so fortunate? However, what I learnt about slim people surprised me.

I started to notice that slim people had certain approaches towards their weight which were different to those people who tended to gain weight. It amazed me that there were actually attitudes which helped to keep people slim, but more importantly, there were other ways of thinking which seemed to keep some people trapped with weight issues. I recognised many of these in myself and realised I had actually been holding myself back from weight loss. Gradually, I forced myself to let go of these and started to adopt some of the attitudes that I could see were successful for thin people.

Over time, I got to grips with this new 'slim' thinking and I began to try and act like a slim person as well. This was when my journey became really interesting. On a day-to-day basis, many slim people do not seem to be very different. This strengthens the idea that they are just naturally slim as it is not immediately obvious what keeps them from gaining weight. However, when I began to really look closely at people's lifestyles, I soon started to spot some small differences. These small, seemingly insignificant things add up over days, weeks, months and even years, and eventually result in quite a substantial weight difference between two people.

At first I wasn't convinced that just making a few small changes to my lifestyle would have a big impact on my weight, but my new slim thinking and my determination not to diet helped me to give it a go. I began to make little changes here and there, starting with tiny things such as not having sugar in my coffee. I built up gradually, really noticing all my old habits; for example, I used to always have cheese on my pasta dishes, now it's a treat every once in a while. It worked - over time I slimmed down, not by dieting, but by adopting some of the little habits that I had learnt from slim people. Eventually, I reached a very healthy weight which I have maintained over the past few years without dieting. I had finally cracked it, I had 'learnt' how to be a slim person.

A little while ago I had a (rather large) dinner with someone who made the comment, 'it's alright for you; you're just naturally slim.' I nearly choked on my chocolate fudge cake. It became clear to me that it was time to start sharing and from that came Thin Secrets - just in case there was anyone else out there who wanted to be slim without dieting.

Dr Lizzie Kingsley's book, published by Bubbly Publishing Limited, is available from www.thinsecrets.com or from www.amazon.co.uk