Why high-protein diets work

A key hormone in the human gut could be the reason why high-protein diets enhance satiety and promote weight loss, British scientists have reported.

The results have been picked up by many media outlets and reported as potentially good news for so-called low-carbohydrate diets, like the once fashionable Atkins diet.

Low-carb diets have lost popularity among the public with critics saying that the approach puts followers at a higher risk of clogged arteries and heart attack in the long-term.

'Not advocating Atkins'
However, the lead researcher behind the new study, Rachel Batterham from University College London, told that low-carb does not necessarily mean high protein.

“The Atkins diet is primarily a low-carb diet and it doesn't state how much protein should be consumed. As a result of reducing carbs, this tends to be an increased fat and increased protein diet,” she said.

“I am not advocating the Atkins diet,” she said.

What Dr Batterham is advocating is that a hormone in the gut, peptide YY (PYY), increases on consumption of a high-protein diet in normal and obese human subjects and produces a feeling of fullness (satiety).

Research on mice
To test this directly, the researchers generated two types of mice; one was engineered to be incapable of producing PYY (PYY null mice), while the other mice were normal.

After feeding the mice on regular chow, the researchers found that the PYY null mice were 37 percent heavier than their normal counterparts, and this was equivalent to a 237 percent increase in total body fat.

The obese, PYY null mice were then given daily PYY injections and after 15 days it was found that they had lost about 20 percent of their body weight. Within one week of stopping the daily injections (removing the PYY hormone) the mice had regained 60 percent of the lost body weight.

In an additional experiment, the scientists divided the normal PYY mice into four diet groups to eat a high-fat normal protein (HFNP), high-fat high protein (HFHP), low-fat normal protein (LFNP), low-fat high protein (LFHP).

The results, published in the September issue of the journal Cell Metabolism (Vol. 4, pp. 223-233), shows both high-protein diets caused greater PYY release than the normal protein diets.

Over 16 weeks, both high-protein diets reduced weight gain and enhanced PYY synthesis in the mice.

PYY physiologically relevant
“These findings provide compelling evidence that PYY is a physiologically relevant regulator of food intake and body weight,” wrote the researchers.

The reason why protein is the most satiating of the macronutrients could be due to evolution, said the researchers, since the hunter-gatherer diets contained much more protein than most modern meals.

The research is unlikely to lead to supplements containing PYY as a weight-loss product however, said Dr Batterham.

“PYY is a protein, so if it is taken in tablet form it is broken down and doesn't pass into the blood. Thus, to increase PYY it either has to be given by another route (intravenous, subcutaneous, transdermal, nasally) or we can try and increase the body's own circulating levels by modulation diet,” she said.

Long-term approach needed
By showing that increasing PYY levels in PYY null mice led to weight and fat loss, and subsequent gain on removal of PYY indicates that the diet would have to be long-term rather than merely for several weeks or months in order to gain the full benefits of the high-protein diet

More research is needed to confirm and further explore the implications of this preliminary study, and Batterham told that human studies was the next step.

“One of the next steps is to undertake long-term studies in obese patients both with and without type 2 diabetes to try and establish what is the optimum amount of protein that helps with weight loss but which doesn't have any adverse effects,” she said.

Over 300m adults are obese worldwide, according to latest statistics from the WHO and the International Obesity Task Force. - (Decision News Media, September 2006)

Read more:
Atkins diet not always safe
Atkins Diet - what research shows

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