The growing body of evidence suggesting highly processed and refined foods are the main cause of the rising obesity rates in the Western world is supported by a 12-month study of the eating habits of 9,341 Australians.
The Charles Perkins Centre (CPC) at the University of Sydney recently published a new study that supports the "Protein Leverage Hypothesis" and was based on a nationwide nutrition and physical activity survey done by the Australian Bureau of Statistics (ABS).
The Protein Leverage Hypothesis, first proposed in 2005 by professors Raubenheimer and Stephen Simpson, contends that humans overeat fats and carbs due to their bodies' great demand for protein, which it actively prefers above all other nutrients. People are compelled to consume more energy-dense meals until they meet their protein needs because the majority of current diets consist of highly processed and refined foods, which are poor in protein.
Processed foods lack protein and drive craving
"As people consume more junk foods or highly processed and refined foods, they dilute their dietary protein and increase their risk of being overweight and obese, which we know increases the risk of chronic disease," said lead author Dr Amanda Grech, a Postdoctoral Research Fellow at the CPC and the university's School of Life and Environmental Sciences.
"It's increasingly clear that our bodies eat to satisfy a protein target," added Professor David Raubenheimer, the Leonard Ullmann Chair in Nutritional Ecology at the School of Life and Environmental Sciences. "But the problem is that the food in Western diets has increasingly less protein. So, you have to consume more of it to reach your protein target, which effectively elevates your daily energy intake.
"Humans, like many other species, have a stronger appetite for protein than for the main energy-providing nutrients of fats and carbohydrates. That means that if the protein in our diet is diluted with fats and carbohydrates, we will eat more energy to get the protein that our bodies crave."
Protein essential to good health
Every cell in the body contains proteins, which are also utilised to create new cells or repair damaged ones. It is thought that over a million different types of proteins are required for the human body to operate. Meats, milk, seafood, eggs, soy, legumes, beans, and some grains like quinoa and wheat germ are all sources of protein.
Researchers from the University of Sydney examined data from the National Nutrition and Physical Activity Survey, a cross-sectional study of eating habits and physical activity that included 9,341 people with an average age of 46.3 years and was conducted from May 2011 to June 2012. They discovered that the average daily energy consumption for the population was 8,671 kilojoules (kJ), with just 18.4% of that coming from protein, compared to 43.5 per cent from carbohydrates, 30.9 per cent from fat, and only 2.2 per cent from fibre and 4.3 per cent from alcohol.
The pattern they observed matched that predicted by the Protein Leverage Hypothesis after they plotted energy intake versus time of consumption. While individuals who received the appropriate quantity of protein did not do so and, in fact, decreased their food intake throughout the day, those who received the necessary amount of protein did not raise their overall food intake in future meals.
Overeating has been linked to "protein hunger"
By the third meal of the day, they discovered a statistically significant difference between the groups: those who had consumed more protein at breakfast had consumed much less total energy throughout the day. Those who began the day by consuming low-protein foods then increased their intake, suggesting they were trying to make up for it by consuming more total energy. This is true even though both groups' first meal was the smallest, including the least quantity of food and energy, and their last meal was the largest.
Throughout the day, participants consumed less of the recommended five food groups--grains, vegetables/legumes, fruit, dairy, and meats--and more discretionary items--energy-dense meals high in saturated fats, sweets, salt, or alcohol. Their percentage of protein energy decreased even as their intake of discretionary foods increased, resulting in an overall inferior diet at each mealtime. This phenomenon is known as "protein dilution."
Effect seen in other studies
Professor Raubenheimer and colleagues have seen this effect before in other studies for more than a decade, including randomised control trials.
"The problem with randomised controlled trials is that it treats diet as a disease when it's not," said Dr Grech. "Laboratory studies may not be indicative of what people are actually eating and doing at a population level. So this study is important as it builds on work, showing that people do seek out protein. And it confirms that, at a population level, as the proportion of energy from protein increases in the diet, people eat fewer fats and carbohydrates."
While there are many factors that can cause excessive weight gain, such as eating habits, physical activity levels, and sleep schedules, scientists from the University of Sydney contend that the body's strong need for protein and the absence of highly processed and refined foods are major contributors to energy overconsumption and obesity in the West.
Explanation for Obesity
"The results support an integrated ecological and mechanistic explanation for obesity, in which low-protein, highly processed foods lead to higher energy intake in response to a nutrient imbalance driven by a dominant appetite for protein," said Professor Raubenheimer. "It supports a central role for the protein in the obesity epidemic, with significant implications for global health."
Seeking to understand how protein drives human nutrition has also sought taken Professor Raubenheimer to study the diets of people in some of the most remote places, from the Congo to the Himalayas. "The protein mechanism in appetite is a revolutionary insight," he said. "Obesity, diabetes, cardiovascular disease - they're all driven by diet, and we have to use what we're learning to bring them under control."
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