The Charles Perkins Centre is a newly established research hub in Sydney that presents regular lunchtime talks in its stunning new premises at University of Sydney.
A recent talk by Professor David Raubenheimer examined the ‘protein leverage hypothesis’, a theory that may hold clues to why we are collectively putting on weight.
Over many years, Raubenheimer and his colleagues have studied the eating habits of monkeys, cats, pigs, insects, fish, birds, mice, mink, and even slime mould, in an effort to find universal laws of nutrition that also apply to humans. What these studies have revealed is that animals exhibit a ‘dominant appetite’ for protein. If they are given food that is low in protein but rich in carbohydrates, they will keep eating the carb-heavy food until it has supplied them with enough protein. This increases their overall energy intake, leading to weight gain.
Does this hold true for humans?
Increasing evidence suggests that the answer is yes. Similarly to animals, humans also appear to have a fixed daily protein target that must be reached for optimum functioning. The amount of protein that we require each day is believed to have evolved over the course of human history and is now programmed into us.
Because of this fixed target, the number of grams of protein that we consume on a daily basis has not changed much in the last 50 years, while in contrast, fat and carbohydrate consumption has been steadily climbing. This means that protein has been largely ignored as a factor in the obesity epidemic.
Raubenheimer stresses that it is important to consider the ratio of protein in our diets, rather than just the average number of grams of protein consumed per day. While the absolute amount of protein we eat hasn’t changed over time, the ratio of protein to carbohydrates/fats in our diets has decreased.
If you eat 100 grams of steak, you will consume about 25 grams of protein. Similarly, 100 grams of lentils contain approximately 25 grams of protein. In contrast, if you eat 100 grams of bread, you will only consume around 12 grams of protein, while 100 grams of potato chips only provide 7 grams of protein.
This means you would have to eat 2-3 times more bread and chips than steak or lentils to reach your daily requirement of protein, because the protein ratio is lower in these foods. Since the protein in bread and chips comes packaged with high levels of carbs and fat, this would increase overall calorie consumption, driving weight gain.
The Food and Agriculture Organisation of the United Nations has been recording the ratio of carbohydrate, fat, and protein in the diets of Americans since 1961. They have found that the percentage of protein in the average diet has dropped by 1.5% (from 14% to 12.5%), and that this has been accompanied by a 14% increase in overall calorie consumption. In other words, it seems that Americans are eating foods that are lower in protein (therefore higher in carbs and fat), resulting in them eating more.
These epidemiological observations are supported by the results of over 30 controlled human studies. For example, Raubenheimer and his team fed a group of 22 lean people 10%, 15%, or 25% protein diets over 4 days (the World Health Organisation recommends that 15% of your daily energy intake should come from protein). The researchers found that the subjects on the 10% protein diet consumed 12% more calories than those on the 15% protein diet. 70% of these extra calories were consumed in the form of snacking, supporting the notion that protein keeps you fuller for longer, thereby reducing the need to graze between meals.
So, why has the protein ratio in Western diets slipped?
One answer is that protein is more expensive. When Raubenheimer and his colleagues examined the relative protein, carbohydrate, and fat contents in over 100 supermarket foods, they found a direct relationship between protein content and price. We are now eating more processed foods, which are high in refined carbohydrates, because they are cheap and readily available.
Another more speculative theory is that fruit, vegetables and grains now contain lower ratios of protein than they once did, as a result of the higher levels of carbon dioxide in the atmosphere. According to Raubenheimer, plants grown in high-CO2 environments convert the extra CO2 into carbohydrates. Because this lowers the protein to carb ratio, he believes we must eat more of these plant-based foods to reach our daily protein requirements. There is evidence to back up his claim: Experiments have shown that elevated levels of carbon dioxide can boost the carbohydrate content in plants by 30-40%.
If this theory is correct, it may help to explain why animals around the world, not just humans, are also getting fatter. This is a phenomenon that has puzzled scientists, since animal weight gain cannot be blamed on sedentary lifestyles, junk food, and advertising. BUT, if the protein to carbohydrate ratio in plants has dropped, animals may have to consume more of these foods to meet their protein needs.
Several other observations appear to fit with the protein leverage hypothesis. For instance, groups that have traditionally lived on high protein diets are now particularly susceptible to weight gain. The stand out example of this is the Inuit, whose diets once contained a whopping 35% protein. Now that they have adopted a Western-style high carb diet, they are one of the most obese populations in the world.
The protein leverage hypothesis may also help to explain why lower income groups have higher rates of obesity. While there are obviously many other factors at play, Raubenheimer notes that lower socioeconomic groups tend to eat foods that are relatively low in protein and high in carbohydrates and fats, since protein is more expensive.
If Raubenheimer and his co-workers’ theories are correct, they support the notion that weight loss is not just about cutting calories – getting the right balance of macronutrients is crucial. According to Dr Alison Gosby, who works with Professor Raubenheimer:
“Counting calories is not enough to manage appetite and body weight. In the western world, where food is abundant, if you reduce your calorie intake but fail to reach your protein target you will find it hard to resist hunger pangs.”
This is a simple piece of advice, but one that may be invaluable for maintaining a healthy body weight.
CPC fosters collaboration between medical scientists and experts from a diverse range of disciplines to better understand obesity, diabetes, and cardiovascular disease, and develop more effective ways of treating these conditions. Learn more about the latest research happening at CPC and around the world at the free lunchtime seminars held every Tuesday from 1-2pm in the CPC auditorium.