Is the body’s unmet need for protein causing us to overeat and gain weight? In a paper published on September 4 in the Philosophical Transactions of the Royal Society B, the scientists who first put forth the concept, called the “protein leverage hypothesis,” present more evidence that our built-in protein appetite, unsatisfied by our modern-day diet of highly processed foods, is an important driver of obesity.
Protein is the nutrient that human appetites regulate most strongly, says study coauthor David Raubenheimer, PhD, a professor of nutritional ecology at the University of Sydney in Australia. “We don’t feel full until we’ve eaten our requirement of it, and find it difficult to eat more [protein] than we need,” he says. “That means that if we dilute protein in our diets, we will continue to eat unnecessary calories until we reach our protein target. If it’s fats and carbs diluting protein, then we will eat more of those, leading to overeating energy and potentially obesity.”
The protein leverage hypothesis makes sense intuitively, says Christopher Gardner, PhD, the Rehnborg Farquhar Professor of Medicine at Stanford University in California and a nutrition scientist at the Stanford Prevention Research Center. Dr. Gardner was not involved with the Royal Society study.
“There are a bunch of foods that aren’t great sources of protein: chips, cookies, dessert. If there’s this preset thing in our body that says, ‘Oh no, I haven’t had enough protein yet, I’m going to have to keep eating and eating these foods until I get enough,’ it could lead to consuming way more calories than our body needs. It could contribute to the obesity epidemic,” he says.
Protein Has Become ‘Diluted’ in the Modern Diet
In the new paper, Raubenheimer and coauthor Stephen Simpson, PhD, outline research about protein, how the body uses it and prioritizes it, and how it’s become “diluted” because of the types of foods in the modern diet.
In many ways, this theory aligns with what Gardner has seen happen repeatedly in research studies that attempt to manipulate macronutrients (protein, carbohydrates, and fat).
“In almost every intervention study I see where macronutrients are manipulated, by the time the study gets to 6 to 12 months, the protein intake is very consistently at around 20 percent,” he says. This usually ends up happening even when the intent of the study was to get people to eat a higher or lower percentage of protein, adds Gardner.
This suggests that there is something true about this hypothesis — people tend to choose foods that provide them with that same amount of protein, regardless of how much fat and carbs they are eating, he says.
Can Eating More Protein Help You Manage Your Weight?
In no way, shape, or form does the protein leverage hypothesis mean that the solution to the obesity epidemic is to eat more protein, says Gardner. “That is a different and separate issue. In the United States, most people get more than enough protein. We need to stop obsessing about protein,” he says.
For many years, nutrition science seemed to waffle back and forth — at first encouraging people to eat low fat, then switching the directive to low carb. “The flip side of low fat is that it’s high carb, the flip side of low carb is that it’s high fat, and protein got left out of that debacle. Through all that, some people got the message, ‘Oh, the only thing I can eat is protein. That’s the only thing that they aren’t trashing,’” says Gardner.
Dr. Raubenheimer agrees that most people in the United States and other higher income countries get the recommended amount of protein because “Our biology ensures that’s the case,” he says.
“As our theory states, we don’t feel satisfied until we’ve eaten the right amount of protein, so most people — specifically those who can afford to — eat until they do get the recommended amount. The problem is that protein has been diluted by processed fats and carbs in the food supply, so our bodies cause us to eat more of those to get the recommended intake of protein,” he says.
More Than Half of Our Daily Calories Come From Highly Processed Foods
Foods like cookies, crackers, chips, and frozen pizza typically contain high levels of added sugar, fat, salt, or some combination of all three. These highly processed choices make up a significant percentage of the items in the modern food system. “The average grocery store has 50,000 foods in it,” says Gardner.
Evidence suggests that Americans are consuming more processed foods and less minimally processed ones. An 18-year study published in the American Journal of Clinical Nutrition in October 2021 found that ultra-processed food consumption grew from 53.5 percent of calories in 2001–2002 to 57 percent by the study’s end in 2017–2018.
Theory Could Be Used to Help Tackle the Obesity Crisis
Given that the World Health Organization (WHO) has declared obesity as one of the largest health threats facing humanity, the authors argue that there needs to be a focus on integrative approaches that examine various drivers of obesity — including the protein leverage hypothesis — rather than looking at them as competing explanations.
“This will also help researchers and policymakers understand how to move the field forward and which causes might be most relevant to tackling the rising obesity epidemic,” they wrote.
In the United States, more than 2 in 5 adults have obesity, which is defined as a body mass index of 30 or higher, according to the National Health Institute.
Lack of Fiber and Exercise Are Also Factors in Obesity
According to Raubenheimer, the lack of fiber in our diets is another very important contributor to the obesity epidemic. “Next to protein, fiber is the second most satiating dietary component, and it too is very low in processed food diets,” he says.
There’s no question that eating highly processed, calorically dense, and nutrient deficient foods like fast food and convenience foods has contributed to the obesity crisis, along with the drastic reduction of daily physical activity, says Bonnie T. Jortberg, PhD, RD, an associate professor in the department of family medicine at the University of Colorado School of Medicine in Aurora, who was not involved in this study.
“The latest data from the CDC [Centers for Disease Control and Prevention] shows that only 28 percent of Americans are meeting physical activity recommendations. The obesity epidemic is a result of the confluence of these two issues: Americans consuming too many calories as our food supply has changed so much and it is so easy to eat too much, and Americans getting too little physical activity,” says Dr. Jortberg.
How Much Protein Should You Eat?
In the United States, government health agencies’ daily recommendations for protein intake are:
- Adults: 0.8 grams (g)/kilogram (kg) body weight, or about 50 g/day
- Athletes: 1.0 g/kg body weight or about 73–122 g/day depending upon weight
- Older adults (65 and older): 1–1.2 g/kg
For example, a 40-year-old active woman who is 5 feet, 6 inches and weighs 150 pounds needs an estimated 54 g a day.
People can use the U.S. Department of Agriculture Daily Required Intake (DRI) calculator to determine their protein needs.
From a dietary standpoint, whether protein comes from animals or plants doesn’t matter — it all counts, says Jortberg.
How do you know if you’re not getting enough protein? “The most common symptoms are brittle hair and nails, feeling weak or hungry, and sometimes people get sick more often as protein is needed to boost the immune system,” she says.
Experts Agree: The Healthiest Diet Is Rich in Plant Foods
Although high-protein diets do help people lose weight in the short term, there is now a lot of research suggesting that over the longer-term, high protein diets are associated with accelerated aging and the early onset of diseases like cancers, says Raubenheimer.
“The best diets for healthy body weight, and health more generally, are diets rich in plant foods, like vegetables, grains, fruit, nuts, and beans. The high fiber content satisfies our appetites at lower protein intake, so people on those diets don’t overeat energy, and the lower protein content helps provide long life,” he says.
Blue Zone Populations Have Diets Low in Protein and High in Fiber
It’s no coincidence that the healthiest, longest-lived populations on the planet such as the “blue zone” populations all have diets low in protein, at approximately 10 percent of their calories, but high in plant foods — hence more fiber, says Raubenheimer.
“If you wanted to focus on a deficit, focus on our fiber deficit,” agrees Gardner. Most Americans eat about half of the recommendation for fiber, he adds.
Dietary fiber intake is recommended at 14 g per 1,000 calories of food. For example, at a 2,000 calorie reference level (which is appropriate for some but not all people) the daily dietary fiber intake should be 28 g, according to the USDA.
Gardner advocates eating less highly processed foods like fast foods, chips, and cookies and eating more nutrient-dense foods. “I think what we would end up with is fewer overall carbs, perhaps a modest decrease in saturated fat, and the same amount of protein we get now,” he says.