Australasian Science: Australia's authority on science since 1938

Protein Is the Key to Weight Loss

By Stephen Luntz

A new line of evidence has been produced to support the theory that overeating is largely driven by inadequate protein content in the modern diet.

“Humans have a particularly strong appetite for protein, and when the proportion of protein in the diet is low this appetite can drive excess energy intake,” says Dr Alison Gosby of the University of Sydney’s School of Biological Sciences.

Gosby conducted a study where a group of 22 volunteers spent 4 days each on diets where 10%, 15% and 25% of the energy came from protein while fat content was held constant.

Remarkably, the foods were prepared so that each menu was rated as equally appetising, and participants in the study did not know which menu they were on during the week.

No statistically significant difference was found between food consumption on the 15% and 25% diets, but those on the 10% protein diet took in 12% more energy with 70% of this increase due to snacking.

The findings support the protein-leverage theory of co-author Prof Steve Simpson of the University of Sydney. “Our previous work on slime moulds, insects, fish, birds, rodents, mink, cats and monkeys has shown that animals have separate appetites for protein, fat and carbohydrate. Interestingly, if protein in the diet is diluted, even by a small amount by extra fat and carbohydrate, the appetite for protein dominates and they will keep eating in an attempt to attain their target level of protein,” Simpson says.

It is notoriously difficult to control diets for scientific experimentation, and Gosby said they had to keep the participants locked in a house for most of the day. The boredom and lack of exercise may have made the trial less than perfectly realistic, but Gosby says it was the first time such a study has been done where the effects of differing taste and appearance were removed.

Over such a short period differences in weight gain were not detectable, but the implications of higher energy consumption are hard to ignore.

The study used participants with lean body sizes because Gosby says that “most of the people with the time to take part were university students. The protein-leverage theory needs to be tested amongst the overweight as well.”

Gosby expects that people with insulin resistance will have higher protein requirements, and this may address what she calls the “surprising finding” that there was not a further decrease in energy consumption on the 25% diet.

On average the western diet has slightly more than 15% protein, but this has fallen in recent years so that almost half the population are getting less than 15% of their energy from amino acids.

Gosby notes that while the study lumps all forms of protein together, a healthy diet requires a wide mix of protein sources.