Objective: To examine the association between protein consumption and subsequent energy intake.
Methods: This study was a 12 day randomized crossover trial of 40 men and 39 women. Subjects visited a university cafeteria for breakfast, lunch, and dinner. The foods offered were similar for each of the three 12 day intervention periods, but varied in protein content (5% of calories, 15% of calories, and 30% of calories). Fat was kept constant at 35% of calories, so the protein was substituted for carbohydrate. Subjects could eat as much at a meal as they wanted.
The researchers measured total daily caloric consumption for each subject when they were eating low, moderate, and high amounts of protein. They also measured subjective ratings of hunger and satiety in each of these three conditions.
Results: The results were striking. When the subjects were eating 5% of calories as protein, they averaged 2,228 calories per day. When they were eating 15% of calories as protein, they averaged 2,298 calories per day. When they were eating 30% of calories as protein, they averaged only 1,722 calories per day! Fluctuations in hunger and desire to eat were attenuated in the high protein group as well.
Discussion: This is a beautifully designed study. I love crossover trials, because you are using the same subjects for each of the conditions being tested, which all but eliminates sampling error. The difference between the moderate and high protein groups was 576 calories per day. This is a dramatic difference that would have a major impact on body weight.
The research is really beginning to mount on how dietary protein promotes satiety and decreases subsequent energy intake. The mechanisms aren’t quite clear, but it may have to do with the high nitrogen content of protein foods. Nitrogen is difficult for our body to process ,so the theory is that when we eat a lot of protein, our body shuts down hunger in order to prevent nitrogen levels from getting too high.
Take Home Message: If you are looking to lose weight and decrease your hunger throughout the day, add a bit more protein to each meal. I have my clients shoot for 20-25% of total calories as protein. I have found this level to be safe and highly effective for weight loss.