DESPITE what many nutritionists have preached for years, low-calorie diets can be healthy if you do them right, and can work wonders on pounds and inches in just a few weeks. Clinical experience shows that somebody with a serious commitment to weight loss can lose up to 20 pounds - and two to three dress sizes - in two months. Thats a lot of weight, and an enormous change in appearance for most of us. Best of all, if a crash diet is done right (and you make permanent changes to how you eat) it can yield results that will stand the test of time just as well as those slow and careful, long-term diets that emphasise depressingly incremental drops in weight. Conventional wisdom says that rapid weight loss leads to rapid weight regain, but a new generation of science is showing that slow isnt necessarily better. In fact, fast weight loss - if achieved with a healthy, calorie-cutting food-based diet - can bring long-term success equivalent to the more gradual weight-loss programmes, which is reason for procrastinators everywhere to rejoice. In fact, for some people, healthy crash dieting may work even better than a diet that lasts all year. A recent study from my laboratory at Tufts University in Boston, Massachusetts, found that the slow and careful approach seems to be sustainable only by those dieters who are not sidetracked by rich food, party snacks and other common food challenges in daily life. Exercise doesnt necessarily make you thin National surveys show that people who do manual jobs - construction, farming and domestic work - are often heavier than people who sit in front of a computer screen all day. Indeed, physically strenuous jobs carry a 30 per cent increased risk of obesity in the US when compared with office jobs. Of course, comparisons like this dont factor in social class, or whether you eat chocolate or take a run after work, but thats the whole point - compared with factors like what we snack on, hard manual labour just doesnt make as much of a difference. Even if your day is spent shovelling gravel, youre still going to develop a potbelly if you lunch on pizza and fizzy drinks every day. This is not to say exercise is bad - exercise is, of course, important for maintaining health, strength and vitality. But when it comes to weight loss, it doesnt seem to be the panacea that it is often made out to be. And the evidence isnt just anecdotal. My laboratory summarised 36 years of published studies on exercise and weight, conducted between 1969 and 2005, and found that adding even an hour of exercise per day results in an average fat loss of just six pounds over the course of several months - hardly the benefit one would expect from all that work. Perhaps more importantly, most of the studies managed to get people to exercise only 30 minutes a day, at which point the average weight loss goes down to three pounds. One research study in The Netherlands also highlighted the problem that simply starting and sticking with a serious programme of exercise is easier after youve lost some weight. Eat the right food at the right time The first principle of successful dieting is to get calories low enough to cause ongoing, serious fat loss. In practice, this means getting daily calorie intake down to 1,200 calories a day if your starting weight is 8.5 to 11 stone, or to 1,800 a day for those weighing 14 to 17 stone. Several studies have shown that at this level of intake, calorie requirements dont decrease anywhere near enough to make your weight plateau, meaning fat continues to be pulled from fat cells and real weight continues to slide off. The second principle involves eating the right foods at the right time; if you dont, counting calories wont cut it because you will be too hungry or unsatisfied to see it through. Based on research studies, its clear that liquid calorie diets - from meal replacement drinks like Slim-Fast - do work, but they are so desperately boring that few people can stick to them. To enjoy yourself more, go the real-food route and maximise benefits by ensuring every meal and snack you eat combines at least two of the properties that numerous research studies have shown cut hunger and increase the feeling of being full: high fibre, high protein, high volume and low glycaemic index carbs. Mail And keep those good foods coming. My experience of helping people lose weight showed that eating three meals and two to three snacks every day and spreading calories evenly from morning to night is about as important as choosing the right foods when it comes to suppressing hunger. What about even faster weight loss? You know those diets: the ones that promise to dissolve 21/2 stone from your belly in one month, or a stone in four days. Unfortunately for anyone longing for results by next weekend, such diets are pure snake oil. Crash dieting can work, but there is a threshold. Its a physiological fact that the human body is capable of losing only a maximum of about three pounds of actual fat per week, even if you eat nothing at all. Greater weight losses than this might occur for a week or two if you put yourself through the wringer of fruit juice fasts, purges, or harsh detox programmes, but you wont lose any more fat - just water, intestinal contents and sometimes muscle. Thats the kind of weight that will bounce right back after a good party or two. The bottom line is that regular workouts are great for health and strength, but if you want to lose weight, the really important thing is what you eat. Stock up on the good foods that help with weight control, and then keep those calorie counts down to make weight loss a reality this time. Start now, and theres still time to put your new body on a collision course with summer. Mail