I used to preach a simplistic equation to my clients with obese pets: if you eat x and burn off y then you gain x-y in weight.
However over the years my experiences of dealing with this issue and working with so many different people and their pets (some in denial, some absolutely transparent weighing and recording everything) that I have come to realise that is really isn't that simple. I suppose there may be more genetic variation between my patients than there is between humans, but they definitely have different metabolic rates and different ways of dealing with whatever nutrition they are fed.
I am a bit of a piglet and always have been. I have always scoffed whole tins of Roses chocolates and things like this over Christmas, and most of the time put on no weight at all until pretty recent times. As a student when I was broke and hungry I used to do the Cadbury's Creme egg bet; get someone to bet me I couldnt eat 10 creme eggs in one go without vomiting and if I did they got to buy the Creme eggs. I could also do 5 king sized Mars bars. I knew I could easily eat them and more
Despite all this gluttony I remained a size 8 no matter what I ate. I am notoriously bad at sitting still though, and have always been told I'm burning it all off with 'nervous energy'.
I really believe that genetics and luck play a
huuuuuuge part in what we can get away with. If it didn't then I would be 15 stone and some of my friends would be very skinny.
Metabolism is an incredibly complex thing to understand. There are literally hundreds of chemicals and hormones that affect the way nutrients are absorbed, broken down, stored and used. I know it isn't an excuse for being obese and failing to lose weight though, whatever hand you are dealt you can still lose weight by reducing what you eat and exercising more. But we are not all alike and it is for some much harder than others.