Weight loss and successful long-term weight maintenance depend on a number of factors, one of which is controlling the desire to eat. No matter how dedicated you are, if you are always hungry you will eventually succumb to eating more than you should. Increasing satiety by choosing the right foods to eat is essential to achieving successful weight loss and weight maintenance. Dietary proteins, as well as amino acids, play key roles in hunger control.
Protein for Weight Loss
Scientists have known for more than 50 years that when you eat a high-protein meal you end up consuming fewer calories because dietary protein elevates amino acid concentrations in the blood. Since that 1956 study, it has been repeatedly shown that high-protein meals induce satiety.
Several studies conducted on healthy subjects show that a chronic high-protein diet induces continuously higher satiety. Some of these studies carefully controlled energy balance by having the subjects live in a chamber for several days so that their total energy expenditure could be accurately measured. The level of energy expenditure was matched with the same amount of caloric intake. This approach ensured that the only variable was the level of protein intake, so there was no doubt that the greater satiety was due to the higher intake of dietary protein.
How Does a High-Protein Meal Make You Feel Fuller?
There are a few theories as to why a high-protein meal makes you feel fuller. When you eat proteins, your gut releases hormones that could possibly be responsible for satiety. The proteins themselves may also directly contribute to a feeling of fullness. Proteins are made up of amino acids, and when we eat proteins, they cause a spike in the plasma concentrations of their component amino acids—the elevation of plasma amino acid levels can induce satiety.
The satiating effect of increased plasma amino acids was proven in an experiment in which amino acids were infused intravenously. This study demonstrated that intravenous amino acid infusion increased the satiety rating and feeling of fullness, even though nothing was ingested into the gastrointestinal tract. The suppression of appetite was due to elevated plasma amino acid concentrations. When the infusion stopped and plasma amino acid concentrations returned to the basal level the effect on satiety was eliminated.
There are different possibilities to explain the effect of dietary proteins and their component amino acids on satiety. One possibility is that a greater availability of amino acids, particularly the essential amino acids that cannot be produced in the body, stimulates protein synthesis. The stimulation of protein synthesis increases energy expenditure. It has been proposed that the extra use of oxygen associated with stimulated protein synthesis leads to satiety.
The brain is another possible action site of amino acids on satiety. When amino acids were infused into the intestines of cats, either as a mixture or independently, increased signaling from the part of the brain responsible for satiety was recorded. Early amino acid studies that identified the brain receptor that influences satiety are supported by the more recent identification of a chemosensor for dietary essential amino acids in the brains of rats. This area of the brain projects signals to other brain areas that are associated with the control of food intake.
The effect on satiety is dependent on the specific amino acid. In one study tryptophan and lysine increased satiety, arginine decreased satiety, and proline and threonine had no effect. In another study, amino acids were directly injected into the cerebroventricular area of the brain. Leucine, or all three of the branched-chain amino acids (BCAAs), suppressed food intake. The BCAA effect was entirely due to leucine, as the other BCAAs (isoleucine and valine) had no effect.
Not only do certain dietary essential amino acids induce satiety (i.e., tryptophan, leucine, and lysine), but the reverse is also true. A low-protein diet drives increased food intake—there is a brain receptor that identifies a diet that is deficient in essential amino acids! A low-protein diet causes an increase in food intake, and weight is gained until there are adequate essential amino acids. This phenomenon was demonstrated by a study published in Physiology and Behavior. Increased food intake occurs when all essential amino acids are reduced, not just leucine or the BCAAs in the diet.
So, what does it all mean for weight loss?
Satiety is difficult to quantify objectively in humans. A number of factors come into play, and we do not have access to the complete workings of the brain. Consequently, we must rely extensively on data obtained from rats, which differ from humans in obvious ways. That being said, there are important findings obtained from studies on rats that are relevant to human nutrition.
With regard to amino acids, a rat’s brain can sense when essential amino acids are available and modify dietary intake accordingly. Elevated levels of certain essential amino acids (leucine, lysine, and tryptophan) can increase satiety, while a deficiency of essential amino acids increases the amount of food eaten. Following this science, a low-protein diet increases food intake and causes weight gain, and a high-protein diet reduces food intake.
Consuming a balanced mixture of free essential amino acids can have the same effect, with less calories than you’d get from protein food sources. The key to hunger control is a complete essential amino acid supplement with all the essential aminos—supplementing with individual amino acids does not appear to have the same effectiveness.