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.
