For most people, successful weight loss comes down to two things: eating less and exercising more. But new research shows we may have been underestimating the role protein plays in losing weight and building muscle mass.
What are proteins?
Proteins are composed of amino acids that bind together like a string of pearls in a necklace.
When we digest proteins, our intestine breaks them down into amino acids (the pearls), which are then absorbed from the intestine into the body.
Inside the body, amino acids are used for a variety of purposes, such as providing energy and generating new protein.
Proteins form the tissues in our body – the most prominent being muscle tissue. In fact, muscle is a storage organ for protein.
So if we begin to starve, muscles provide amino acids, which are converted into glucose – the main energy provider in the body.
In a typical Western diet, humans consume about 80 to 100 grams of protein every day. While most food items contain some protein, our major providers are meat, eggs and milk products.
Plant products also contain protein, in particular pulses such as beans, peas and lentils.
The body’s protein absorption
Researchers often manipulate the biology of mice to understand how the human body works – these mice are referred to as “mouse models”.
To better understand how the body absorbs protein, my research team (from the Australian National University and the University of Sydney) created a mouse model, which could no longer absorb amino acids from the intestine. These mice replicate a rare human condition called Hartnup disorder.
We found the Hartnup mice were significantly smaller than normal mice, indicating that body size is dependent on the amount of protein available. If the body can’t absorb protein, it’s likely to be smaller.
Protein for optimal growth
Protein isn’t the only part of our diet that regulates body size. Other factors such as minerals and vitamins also play a role.
Westerners consume a large amount of protein, which provides sufficient amino acids for optimal growth.
But people in under-developed countries and those who prefer vegetarian food may not be getting enough protein for optimal growth.
Even though plants contain some amino acids, the composition isn’t the same as amino acids in animals, which promote growth. This is why certain amino acids are added to the normal diet of cattle to make them grow faster.
Changing protein-intake can cause weight loss
Unexpectedly, the “Hartnup mice” in our study (which couldn’t absorb amino acids or protein) were unable to control their body weight.
When we switched these mice to a diet of lower protein, the mice lost up to a fifth of their weight over a few days. Interestingly, the mice even lost fat when placed on a high protein diet.
This suggests that protein plays an important role in the regulation of body weight, which was previously thought to be mainly controlled by fat and carbohydrate intake.
The phenomenon relates to the concept behind the “Atkins diet”, named after its developer Robert Atkins. The diet isn’t based on reduced caloric intake, but rather, on strict avoidance of carbohydrates (sugars).
Carbohydrates are our major source of energy and are found in high concentration in all staple foods (bread, rice, pasta, potatoes).
Reduced intake of carbohydrates causes a reduced insulin release after a meal. When insulin levels are low, the body starts using its energy reserves, which are mostly fat and muscle.
But since the Atkins diet recommends high protein intake, muscle protein is maintained.
Similarly, the Hartnup mice in our study failed to release insulin after a meal, which is a likely cause of their weight loss.
But while the Hartnup mice share some similar outcomes with those on the Atkins diet, the protein intake differs dramatically: Atkins is based on high protein intake; the Hartnup mice couldn’t absorb protein.
We know that a reduction of protein digestion combined with increased protein intake causes the body to use its fat storage as an alternative energy source, resulting in loss of body weight.
Our next steps are to investigate the mechanism of this weight loss, which could lead to the development of new tools to manage excess weight and obesity.