Fighting the flab inside, as well as out Being overfat is more common than you might think - with research at Imperial College London estimating that up to 40 per cent of the UK population is carrying dangerous levels of internal fat, even in organs such as the liver, heart and pancreas. And, like me, these people don't necessarily look fat or overweight on the outside. Indeed, research by Prof Jimmy Bell, from the Medical Research Council's Clinical Sciences Centre at Imperial College, has found that many people who look healthily slim on account of their diet or genes, still carry unhealthy pads of fat around their internal organs. Such visceral fat is particularly dangerous around the abdomen, where it is now thought to disrupt normal hormone and chemical functioning, increasing the risk of diabetes and cardiovascular disease (abdominal fat may also show up on the outside, resulting in the unhealthy apple shape). "People shouldn't think they are healthy just because they look thin," warns Prof Bell. Exercise rather than dieting is the way to burn off internal fat, he adds. Researchers aren't sure why, but yo-yo dieting can give rise to inflammation of fat cells, with the body laying down fat around vital organs. According to Prof Bell's research, 11 per cent of men and 15 per cent of women have a normal Body Mass Index (BMI) - which measures weight according to height - but are nevertheless overfat. Despite my inner fat, my BMI is 22.8, well within the normal range. As Prof Bell says: "People have become obsessed with dieting, but doing this without exercise simply means they may be laying down fat in the wrong places." While there are no official guidelines on the optimal percentage of fat for the human body (and no fail-safe way of measuring it), anything over 25 per cent is not to be encouraged. Glenys Jones, a nutritionist from the Medical Research Council's Human Nutrition Research unit, says women should aim to keep their body fat between 25 to 28 per cent; men should aim for around 20 to 25 per cent. She calmly informs me that I am what's known as a TOFI: Thin on the Outside, Fat on the Inside (wonderfully, the acronym was inspired by the original Quality Street toffee stick - a thin tube filled with fatty caramel). Women carry a lot more fat than men. They need to for when they become mothers," explains Jones. While women's body fat percentage is at its highest during pregnancy, Jones says many women put on a lot more weight than they need to at this point. "If they can, women should breastfeed after childbirth as it's a great way to lose calories and, crucially, fat," she says. But too many women think it should all happen instantly. If it took nine months to put the weight and additional fat on, it should take at least six months to come off. Fun times ahead then, if I make it through this fat-burning regime and the wedding. According to Prof Steve O'Rahilly, director of the Medical Research Council's Centre for Obesity and Related Metabolic Diseases, the hormonal shifts that take place during pregnancy can also interfere with fat levels, making it harder for some women to return to a pre-pregnancy condition. Doctors are still unsure why this takes place. Prof O'Rahilly also warns that the issue of too much internal fat is common among the over-sixties, as lean muscle tends to reduce at this age: "People in their sixties need to keep active to keep their muscle mass up." Back to the gym, where Mark explains his "six-week body plan"" to burn off my inner fat (and hopefully any external fat, too). "Your 30 per cent fat is about average for women in the UK, but I want to get it down to around 25 per cent,"" he tells me. Whether I lose weight or not is irrelevant - in fact, more muscle may mean my weight could increase. So for the following six weeks, I exercise with a trainer for an hour, four times a week. This sort of treatment costs £1,800 in all, and I soon see why. My 24 sessions include intense strength training, abdominal crunching and high-paced 20-minute walking sessions at a painful gradient, which Mark swears will burn off my fat reserves. In my final session, feeling agile for the first time since leaving school, my body fat measurement is taken. Fat levels have dropped by 2 per cent - not bad, though I haven't reached the target 25 per cent fat. As a welcome by-product, subcutaneous fat has also disappeared and the circumferences of my thighs, stomach and hips have all dropped by a couple of inches. There are still seven months until the big day. I am determined to remain slim - both inside and out.