New research bolsters the argument that the efficacy of low-carb diets could be due to their high protein content, reports Jess Halliday. So could raising America's protein intake help the nation beat the obesity epidemic?
In the new study, published in the American Journal of Clinical Nutrition, David Weigle from the University of Washington School of Medicine and colleagues set out the test the hypothesis that increasing protein while maintaining carbohydrate intake decreases the appetite, leads to the consumption of fewer calories and results in weight loss.
In an accompanying editorial, Dr Arne Astrup of the Royal Veterinary and Agricultural University in Copenhagen said that preventing weight gain is a more complex matter than simply telling people to eat less and exercise more.
A key take-home message of the Dietary Guidelines for Americans concerns calorie control and exercise, with consumers encouraged to choose foods in order to get the most nutrition out of calories consumed.
But Astrup argues that this is a simplistic strategy which assumes people have conscious control over their appetite and body weight regulation.
"I have never met an obese person who has worked hard to become obese and to maintain an excessive body size," he wrote.
For Weigle's study, 19 subjects followed three different diets, one after the other. For the first two weeks, they followed a weight-maintaining diet where protein accounted for 15 percent of calories, fat 35 percent and carbohydrate 50 percent.
For the second two weeks, they followed an isocaloric diet that was 30 percent protein, 20 percent fat and 50 percent carbohydrate.
Finally, for 12 weeks they followed a diet where there was no restriction on calories but the proportions, again, were 30 percent protein, 20 percent fat and 50 percent carbohydrate.
Their appetite, caloric intake, body weight, and fat mass were measured throughout, and at the end of each phase blood samples were taken to measure insulin, leptin (the hormone responsible for satiety) and ghrelin (the hunger hormone).
Weigle's team found that satiety was 'markedly increased' with the isocaloric diet but leptin was unchanged.
With the ad libitum high protein diet, the participants' average spontaneous calorie intake decreased by between 376 and 504 per day, their body weight decreased by between 4.4 and 5.4 kg and their fat mass decreased by 3.3 to 4.1 kg. Leptin levels 'significantly decreased' during this phase and ghrelin increased.
As carbohydrate remained at 50 percent during all three phases, the effects of the ad libitum diet would appear to be due to the high protein intake.
Astrup cited other research indicating that the high protein content of weight loss approaches like the Atkin's Diet and The South Beach Diet may actually be due to the satiating effects of their high protein content (30 to 40 percent of calories consumed) rather than the low-carbohydrate design.
As to the reason that protein is more satiating than fat, the mechanism has not yet been discovered. Weigle's results did not conclusively show that either ghrelin or leptin have a role to play.
Despite not knowing what triggers the increased satiety response, should we recommend that obese and overweigh people increase their protein intake from 10 to 20 percent of calories to 20 to 30 percent?
The answer, according to Astrup, depends on "the potential adverse effects of a high protein diet". But the Institute of Medicine has found no clear evidence that high protein intake increases the risk of renal stones, osteoporosis, cancer or cardiovascular disease, and sets the acceptable range of protein intake as between 10 and 35 percent of calories.
Conversely, obesity increases the risk of developing cardiovascular disease, type-2 diabetes, arthritis, asthma and back problems.
The trouble is, many of the sources of protein in the American diet - such as red meat, cheese and whole milk - are also high in saturated fats. And saturated fats are known to raise LDL 'bad' cholesterol levels.
"It is preferable to replace sugars from soft drinks with protein from low fat milk, high-fat milk and dairy products with the lean versions, and possibly white bread and pasta with lean meat, without reducing the intakes of fruit, vegetables and whole-grain products," said Astrup.
At present, 64 percent of American adults are overweight or obese, and 16 percent of children are obese.
According to a recent study in Health Affairs private spending on obesity-related health care increased tenfold between 1987 and 2002, from $3.6 to $36.5 billion.
"Perhaps now is the time to consider the economic and environmental consequences of increasing the population's intake of protein," concluded Astrup.
Certainly, the problem needs to be address. But whether protein could be the key to bringing the obesity epidemic to its knees, opinion is divided.
It seems unlikely that policy-makers will jump into bed with the likes of Atkins or South Beach unless protein's satiety mechanism is first unlocked, and the tactic is proven to be safe - irrefutably so.