To investigate, researchers from the University of Copenhagen enrolled over 700 men and women and their families in eight different western European countries in the study.
Families were randomly assigned to one of five different weight-maintenance diets for 26 weeks. None restricted calories, but four of the diets did dictate the proportion of proteins, fats and refined carbs that should make up daily food intake.
One group with no food restrictions served as a control, the rest were assigned to eat either a low-protein, low-GI diet; a low-protein, high-GI diet; high-protein and low-GI; or high-protein and high-GI.
People in all of the groups could eat as much as they liked.
Among the 548 people who completed the study, only those who ate a low-protein, high-GI diet gained a significant amount of weight (about 4 pounds).
When the researchers looked separately at people in the high-protein diet groups, they found these individuals gained about a kilogram less than those in the low-protein groups; the same was true for the low-GI versus high-GI groups.
People in the high-protein, low-GI diet were the most successful at keeping the weight off.
Researchers say adding a serving of nuts and beans to the diet every day and cutting back on refined grains will produce at least as much dietary change as they obtained in the study.
The study was published in The New England Journal of Medicine.
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