By Eleanor Bradford
BBC Scotland health correspondent
Some people struggle with the battle of the bulge
Why is it that some of us struggle to maintain our weight and others seem to stay naturally slim?
It's a question no-one has managed to answer fully, but science has given us a fascinating insight into the role of what is often described as our "sweet tooth".
Dieter and bride-to-be Lisa Jane Laurie has been finding her sweet tooth hard to resist.
Over the past three weeks she's lost 4lbs, but put back on 1lb 7oz (0.7kg).
She's on a Weightwatchers diet. "I've been eating chocolate," she admits.
"Instead of using my points to have a bit of chocolate, I've been using all my points up, and then eating chocolate."
At Loanhead Equestrian Centre in Aberdeen, manager Shona Still has noticed preferences for certain foods in the horses.
Her own horse, Arnie, loves turnips and will choose them over sugar lumps, but he's unusual.
Most of his stablemates love sugar lumps and some take a lot of coaxing to take a bite out of a turnip.
Lisa Jane, the bride-to-be, and Arnie the horse may share the same in-built sweet tooth.
"Some people will find sugar more rewarding," says Julian Mercer, head of obesity research at the Rowett Institute in Aberdeen.
Some horses need a lot of coaxing to eat healthy foods
"For example, if you had to give up either chocolate or cheese, some people will choose chocolate and some will choose cheese."
Studies on rats have suggested that when they're offered a sugar solution for the first time, some will consume lots and others just a little, and this will continue for the rest of the study.
It's likely humans have the same biological preferences.
"On top of that," says Julian, "there's our habitual diet, with a huge choice of foods on offer, and through that we're probably reinforcing our liking for sugar."
Surveys have shown that some people consume 10-times as much sugar as others, but what we don't understand is why some people seem to have such a sweet tooth.
Julian has just received a grant to carry out more work on this.
"We need to understand what drives people's preferences if we're going to help them take up a diet that's better for them in the long term," he added.