The strange role protein plays in obesity
Tara-Lyn Camilleri-Carter, PhD Scholar, School of Biological Sciences, Monash University
Conservative estimates suggest that over 1 billion of us worldwide are either overweight or obese—this has more than doubled since the 1980s. Obesity is responsible for almost 4 million deaths per year worldwide, and predisposes suffers to a plethora of related diseases such as type 2 diabetes, cardiovascular diseases, metabolic syndrome, chronic inflammatory diseases and certain types of cancer. So why, despite the billions spent, does obesity continue to rise? The answer may be—at least in part—due to our evolved protein appetites. The evidence is now mounting that our bodies regulate the consumption of the macronutrients in specific proportions. Protein intake is regulated more strictly than carbs or fat, and individuals on a protein-restricted diet tend to ingest more food to compensate, putting them at a greater risk of becoming overweight or obese.
The evolution of our diet
Our diet consists of many nutrients. We require these nutrients in various proportions relative to other nutrients. It is generally true that each single nutrient we consume needs to be at a balance, and that over- or underconsumption is harmful to our health. So in order to maximise our evolutionary fitness (generally how well we will survive and reproduce), it is in our best interests to balance ingestion and absorption of nutrients in the right proportions. Precisely which foods we choose to ingest to achieve this balance is determined by how each of these nutrients effect our appetite and satiety. The macronutrients are the three major energy-contributing nutrients in any diet—carbs, fat, and protein.
Over the past approximately 40,000 years, our diet has changed drastically; this change in diet has accelerated even further over the past 50 years. Fossil studies of past humans show us that our ancestors were limited for energy because sources of simple fats, starches, and sugars were much less abundant than they are today. Whereas, complex sources of carbohydrate such as roots or tubers, and sources of protein from fish or lean game meat were the main staples of a hunter-gatherer diet. Hunter-gatherer populations today such as the Hadza, have very low levels of obesity (less than 5%) and rarely experience metabolic or cardiovascular diseases. Importantly, the Hadza people spend a lot longer engaging in physical activity than most of us, accumulating over 120 minutes each day. When many societies transitioned from a hunter-gather to agricultural lifestyle, diets increased in carbohydrate—mainly starchy grains. Although the shift to agriculture occurred at differing times across societies, evidence from multiple regions indicate this rise in starch coincided with a reduction in protein. Our consumption of carbs increased even further during the industrial revolution, although still at this time obesity prevalence was low, and considered a luxury of the wealthy.
In the last 50 years, the macronutrient composition of our diet has changed even more, with most in developed nations having access to an abundance of food. We now have access to an unprecedented amount of simple sugars and fats, and highly processed foods are cheap and plentiful. In tandem with this nutritional shift, we are required to expend much less energy on subsistence activities than did our forebears.
Eating to protein targets
Research from the last decade or so pioneered by Stephen Simpson and David Raubenheimer, shows that organisms have appetites that are specific to each nutrient. These ‘nutrient-specific appetites’ help to maintain homeostasis (a balance within the body) and nutrient consumption. Organisms maintain health by ingesting a range of foods with differing but complementary nutrients. Much data across different species (including non-human primates, flies and mice) indicate that protein is the primary determinant of appetite and satiety. This is probably why we feel fuller when we consume an adequate amount of protein in a meal. Generally, animals ingest food in a manner that ensures their protein consumption is within a strict range of values, but a wider range of carbohydrate and fat can be tolerated. This is because over eating protein puts the body under more stress to excrete it, than does overconsumption of carbs or fat. On the other hand, flies and mice who eat too little protein have fewer offspring, or stop producing offspring altogether. But, if we eat too much fat and carbs, we store that extra energy as body fat. In other primates this can be very useful in the wild when they are not sure when they will get their next meal. Because protein makes us feel satiated, it can shape our feeding behaviour. Which is why the exploitation of high protein and low carb diets work to help lose weight. This does not mean we should employ these methods to lose weight, however, as the over consumption of protein and under consumption of carbs is hazardous to our health, and is linked to a shortened lifespan in some species. By contrast, consuming low protein foods to meet a protein intake target can result in overconsumption of sugar and fat. This has led to the hypothesis that the current obesity epidemic is fuelled—at least in part—by the unprecedented abundance of low protein, energy dense foods that are highly palatable, yet have little satiety value.
Direct evidence that we are innately eating to protein targets comes from a study where lucky participants got put up in a chalet in the Swiss Alps for 6 days. The results showed that participants ate to a protein target at the expense of carbohydrate and fat—those put on a higher protein diet, under-consumed carbohydrate and fat. Whereas the group on the lower protein over-consumed carbohydrate and fat in an attempt to consume more protein. The researchers concluded that when we are forced to make choices about food, protein intake is prioritised over carbohydrate and fat. These results are consistent with many other species studied, including other primates, mice, flies and crickets. This suggests that innately eating to meet protein intake over carbohydrate and fat, is unchanged across species, it is—evolutionarily conserved.
Although there is a lot of conflicting dietary advice out there—especially around protein consumption, knowing that we have evolved to eat to specific nutrient appetites can help us to shape our eating behaviour. These studies indicate the importance of not over or under consuming protein, something that is going to be different for each of us. These studies have laid the foundations for more research into individual protein requirements, and personalised genome-matched diets.
