Many dieters will tell you: losing weight is easy; it's keeping the weight off that's hard.

But now researchers in Denmark say they've found the perfect diet for keeping it off -- and it doesn't involve counting calories.

The best diet for weight loss maintenance, say researchers at the Faculty of Life Sciences (LIFE) at the University of Copenhagen, is one that is low fat, high in protein and low in food with a high "glycemic index."

The glycemic index is a measure of a food's ability to raise blood glucose levels. Generally, raw or unprocessed foods have a low GI and take longer to digest than refined foods such as white bread and sugary foods.

In fact, researchers say that by avoiding high GI foods and eating more lean meat and beans, dieters can eat as much as they like without gaining weight.

The findings come from a long-term diet study, named Diogene. More than 900 adults from eight European countries took part in the study.

It began by having the overweight volunteers follow a stringent 800-calorie a day diet for eight weeks, so that they lost an average of 11 kilos (24 pounds), or eight per cent of their body weight.

About 770 of the participants were able to lose the weight required. They were then randomly assigned to follow one of five different low-fat diets:

• A low-protein diet (13 per cent of energy consumed) with a high glycemic index (GI)
• A low-protein, low-GI diet
• A high-protein (25 per cent of energy consumed), high-GI diet
• A high-protein, low-GI diet
• A control group followed current dietary recommendations without special instructions regarding glycemic index levels

The volunteers were advised which foods to eat and which to avoid but were not told to restrict their calories.

The researchers then followed the volunteers for six months to see which diet was most effective at preventing weight regain. Throughout the project, the participants received guidance from dieticians and were asked to keep food diaries.

After 26 weeks, the average weight regain among all participants was 0.5 kg (1.1 lbs). But the group in the low-protein-high-glycemic-index group fared the worst. Not only did volunteers in that group see the most significant weight regain (an average of 1.67 kg), they were also most likely to drop out.

The best weight maintenance was in the high protein and low glycemic index diet. Not only did those dieters maintain their target body weight, some went on to lose even more weight.

It should be noted that out of the 773 participants who started the six-month study, only 548 completed it, so overall about a third of the volunteers dropped out. But more people stuck to the high-protein, low-GI diet (37 per cent) compared to the low-protein, high-GI group (about 25 per cent)

The results are published in the New England Journal of Medicine.

Low-GI foods cause blood glucose levels to increase slowly and to lower levels compared to foods with a high glycemic index. Low-GI diets are thought to help with weight control by making people feel fuller for longer and by reducing cravings for sweets. They also help diabetics and pre-diabetics control their blood sugar.

Some examples of foods with a low glycemic index:

• wholegrain breads with many kernels
• wholegrain pasta
• whole oats (not instant oatmeal), muesli 
• brown rice
• most fruits, including: apples, pears, oranges, raspberries and strawberries
• nearly all vegetables, with the exception of corn. Most vegetables should preferably be eaten raw
• boiled new potatoes
• beans and legumes
• yogourt, cheese

Examples of foods with a high glycemic index:

• white bread, buns
• most white rice
• most refined breakfast cereals, crackers
• bananas (especially overripe bananas), grapes, kiwi, pineapple and melon
• mashed potatoes and baked potatoes
• sugary processed foods such as donuts, cookies
• ice cream