A high-protein diet can help to keep you feeling fuller for longer, helping to fight obesity. A new study uncovers the mechanisms involved and offers hope of a safer, more easy alternative.

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Can we replicate a high-protein diet safely?

More than a third of adults in the United States are classed as obese. And with obesity comes an increased risk of a range of potentially life-threatening conditions. Therefore, finding ways to reverse this epidemic is paramount. 

High-protein diets are known to keep you feeling fuller for longer. In some people, this can lead to reduced overall calorie intake and weight loss.

However, diets focused on a heavy protein load can be difficult to maintain and often carry their own health risks. Some of these risks include constipation due to a lack of dietary fiber, increased heart disease risk (with higher red meat consumption), and reduced kidney function for people already at risk of kidney problems.

A protein-free, high-protein diet

Because of the dangers and difficulties of a high-protein diet, researchers are keen to understand how they work with the hope of replicating their effects.

Mariana Norton, one of the researchers from the current study, explains, "Diets high in protein are known to encourage weight loss but adhering to them can be difficult. Identifying the mechanisms that sense the protein may allow us to use drugs or functional foods to hijack appetite regulation and treat obesity."

In effect, the aim is to keep the beneficial effects of protein without the protein.

To this end, Prof. Kevin Murphy and his colleagues — from Imperial College London in the United Kingdom — focused on phenylalanine. They chose this compound because previous studies had shown that it can reduce appetite. It appears to manage this feat by triggering the release of appetite-related hormones in the gut.

During digestion, proteins are broken down into amino acids, and one of these is phenylalanine. It is classed as an essential amino acid because our bodies cannot manufacture it, and therefore need to consume it. 

In the gut, phenylalanine is detected by calcium-sensing receptors. Activation of these receptors stimulates the release of glucagon-like peptide-1 (GLP-1) in the brainstem. GLP-1 helps to improve glucose tolerance, among other tasks.

Although researchers believe that phenylalanine helps to influence appetite through GLP-1, the exact mechanisms have not been uncovered. There seems to be more involved than just one hormone pathway. The current study takes a fresh look.

The study paper is titled "Rectal and oral administration of L-phenylalanine suppresses food intake and modulates neuronal activation in appetite-regulating brain regions in rodents," and the findings will be presented by Norton at the 2017 Society for Endocrinology annual conference, held in Harrogate, U.K.

Phenylalanine and appetite examined

Mice were administered phenylalanine either orally or rectally. The two different routes allowed the team to assess its effects on different parts of the gut. Over the following 24 hours, the mice's food consumption was measured, and the parts of the brain involved in appetite were monitored.

Phenylalanine, given both orally and rectally, reduced the rodents' appetite and increased activity in the parts of the brain known to be involved in regulating appetite. Even when a quantity of phenylalanine 10 times less than the daily levels expected from a high-protein diet were administered rectally, these effects were still measurable.

"Understanding how food is detected in the gut may help to identify ways of treating or preventing obesity. The next step is to establish whether phenylalanine can drive similar appetite-reducing effects in humans."

Mariana Norton

It seems that phenylalanine works to suppress appetite using a number of pathways in the gut. Of course, the study does not prove that phenylalanine has the same effect in humans, so more work will need to be done. However, the findings are intriguing and raise further questions to be answered.

For instance, as previously mentioned, the release of GLP-1 with phenylalanine was expected, but the researchers also measured a reduction in levels of gastric inhibitory peptide, which is a hormone that induces insulin secretion. This was a surprising result and warrants further investigation.