Scientists at Aberdeen’s Rowett Research Institute have shown that a high protein, low carbohydrate diet is most effective at reducing hunger and promoting weight loss, at least in the short term.

Healthy, obese men were given two different diets during their stay in the Rowett’s specialised Human Nutrition Unit. Both diets had a high protein content (30% of total energy value of the diet) but they differed in the amount of carbohydrate: One diet was low in carbohydrate (4%) and the other contained a moderate amount of carbohydrate (35% total energy value).

“Our volunteers found both diets to be equally palatable, but they felt less hungry on the high-protein low-carbohydrate diet compared with the diet which contained high-protein but moderate amounts of carbohydrate,” said Dr Alex Johnstone, the Rowett’s weight-loss expert who led the study.

“Weight loss during the two four week study periods was greater on the high-protein low-carbohydrate diet, averaging 6.3 kg per person, compared with 4.3 kg on the moderate carbohydrate diet,” said Dr Johnstone.

An important part of this study was to unravel the physiological mechanisms behind this type of diet. It is known that when people eat low carbohydrate diets, within a relatively short time their body has to switch from using glucose as a fuel to using something different called ketone bodies. Ketone bodies are appetite-suppressing and they may have an effect on the appetite centres in the brain. It’s also well known that protein itself is very good at making people feel full-up.

“In this study, we showed that on the high-protein low-carbohydrate diet the volunteers became ketogenic within 1-2 days of starting this diet and so it may be that high-protein, low-carbohydrate diets are particularly effective because of the combined effect of the protein and the ketone bodies,” said Dr Johnstone.

“We showed that the volunteers on the ketogenic diet reduced their energy intake without increasing their hunger and this was a very important factor in their ability to stick to the diet.”

Dr Johnstone sounds a note of caution about her findings: “A paper published last year from the same study showed that low carbohydrate diets may have consequences for the health of the gut by dramatically reducing the numbers of particular types of bacteria. So we will be looking in more detail at the complex way in which we respond to changes in our diet before we can say whether low-carbohydrate ketogenic diets are a suitable tool for everyone who wants to lose weight.”

When the phase of the study which involved the volunteers finished in October 2004, it attracted considerable interest because of its celebrity volunteer, Cameron Stout, who lost just over 12 kg during his nine week stay at the Rowett Institute. At the time, Cameron mentioned his surprise at not feeling hungry during the study and said he had adjusted to eating smaller portions.

The weight-loss study is published in The American Journal of Clinical Nutrition, volume 87(1), pages 44-55.

The study which investigated the impact of the low carbohydrate, high-protein diet on the bacteria in the gut was published in Applied and Environmental Microbiology, 73:1073-8.