For years, the most popular diet slogan was cut the fat. Then, it was cut the carbohydrates. Now, the latest mantra is: Beef up the lean protein.

A growing body of evidence shows that eating enough fish, chicken, lean meat, low-fat dairy, beans, nuts or other protein-rich foods helps people feel full longer. Scientists are searching for the biological reasons for the increased satiety.

The latest findings might help explain why people report feeling ravenous after eating only a lettuce salad for lunch. It also could offer insight into why some dieters lose weight without getting as hungry on programs that allow ample protein, such as the Zone Diet and the South Beach Diet, and Weight Watchers, in which the typical member selects 20% to 22% of calories from protein. The Atkins diet is often high in both protein and fat.

Researchers discussed protein's role in controlling hunger at the annual meeting of North American Association for the Study of Obesity (NAASO), an organization of 1,900 health professionals involved in obesity research, treatment and prevention.

'NOT THE END-ALL, BE-ALL' Scientists from Adelaide, Australia, reported on a study in which they followed 57 obese people for 16 weeks. Half were on a diet with 34% of calories from protein, 29% from fat and 37% from carbohydrates. The other group followed a high-fat diet -- with 18% of calories from protein, 45% from fat and 37% from carbohydrates. Both groups ate about 1,400 calories a day.

Overall, more people who ate the high-protein diet reported feeling more satisfied and less hungry for three hours after their meals than did dieters who ate high-fat meals. After two months, both groups averaged a 20-pound loss because they consumed the same number of calories.

"A high-protein meal will sustain people far longer than a high-fat meal or a high-carbohydrate meal," says Manny Noakes, senior dietitian with CSIRO Human Nutrition, a government research organization in Australia.

"Protein is not the end-all, be-all. It is a subtle effect, but it is an effect that can help if you are actively dieting," she says. "Protein can provide a certain edge that will allow you to remain on a calorie-restricted diet for longer."

Other studies support the protein-fullness connection. Researchers at the University of Washington in Seattle had 19 people follow an eating program that provided 30% of calories from protein and 20% from fat with no calorie restrictions. The participants ate an average of 441 calories a day less on the 30%-protein diet than they did on the 15%-protein diet they were on before the program began. They dropped 11 pounds in 12 weeks.

"Protein definitely reduces appetite both in the short term and the long term," says lead researcher Scott Weigle of the University of Washington School of Medicine.

However, Weigle adds, "we need to have good clinical trials that look at possible side effects of high-protein diets, especially on the kidneys and on calcium."

Walter Willett, chairman of the nutrition department at the Harvard School of Public Health, agrees. "It is a promising direction, but before we go whole hog, even a lean whole hog, we should be careful, because we have been down diet roads before that we did not want to go down.

"It is going to be really important to study the higher protein intake in longer-term studies lasting at least a year or two."

In fact, a government-financed study has begun that will follow dieters for two years on different eating regimens that vary the daily percentages of proteins, fats and carbohydrates.

For now, the Institute of Medicine has concluded that there is no clear evidence that high-protein intake increases the risk of kidney stones, osteoporosis, cancer, coronary artery disease and obesity.

SEARCHING FOR THE MECHANISM Researchers are trying to understand how protein works to increase the feeling of fullness. Noakes and colleagues tested levels of the hunger hormone ghrelin but did not find significant differences between the levels in participants on the high-protein diet or the high-fat diet. So, they are examining other appetite-regulating hormones.

"The reason it is important to know the mechanism is because we might be able to develop new protein foods or supplements or identify natural protein foods that have higher levels of satiating effects," Noakes says.

Other scientists are trying to unscramble this same mystery. Protein probably works on several mechanisms in the body, says Harvey Anderson, a professor of nutritional sciences at the University of Toronto.

There also might be a practical reason for protein's satiety value, says Barbara Rolls, a Penn State nutrition researcher. People tend to think meals are more substantial if they contain a meat or other protein, she says.

So how much is enough? The most recent recommendations from the Institute of Medicine offer a range of 10% to 35% of calories from protein. "That is not a license to chow down on fatty meats," Rolls says. "The Food Pyramid suggests more low-fat dairy and fish, and beans to increase protein."

Source: USA Today