Protein, carbohydrate and fat are the three primary macronutrients in the human diet, and one of the long-lasting debates in the nutritional community is whether we should get the majority of energy from carbohydrate or fat. Protein is often forgotten since it constitutes a relatively small percentage of the diet compared to the other two macronutrients, and protein intake has remained near constant within and across populations throughout the development of the obesity epidemic. However, accumulating data show that the amount of protein you eat could determine the total energy consumption, since the appetite for protein is so strong that we keep eating until we get to our target protein intake.
The Protein-Leverage Hypothesis
The Protein-Leverage Hypothesis (PLH) suggests that humans regulate their intake of macronutrients and that protein is prioritized over fat, carbohydrate, and total energy intake. If we eat an unbalanced diet that contains a low percentage of protein, we increase the total energy ingestion in order to get to our target protein intake.
The hypothesis is now getting more and more support in the scientific literature, and few, if any, studies don’t support the role of protein in appetite regulation (1,2,3,4,5,6). In a lot of the studies looking into the protein-leverage hypothesis, participants are assigned to eat as much as they want (ad libitum) from either a high-, moderate- or low- protein diet. The researchers then measure energy intake, body weight changes, and appetite profiles in the different groups.
A recent review collected data from 38 published experimental trials measuring ad libitum intake in subjects confined to menus differing in macronutrient composition, and found that there is strong support for protein leverage in lean, overweight, and obese humans. The 38 trials included in the review encompassed considerable variation in protein, fat, and carbohydrate intake, but showed that percent dietary protein was negatively associated with total energy intake regardless of fat and carbohydrate content of the diet (1).
10% of daily energy from protein is not enough
In 2011, researchers from The University of Sydney published a randomized controlled experimental study in which they tested the protein-leverage hypothesis in lean humans (2). 22 subjects were assigned to consume as much as they wanted from fixed menus containing 25%, 15% or 10% protein. The results show that lowering the percentage of protein from 15% to 10% resulted in higher total energy intake, primarily from snacks eaten between meals. The increase from 15% to 25% protein in the diet did not alter total energy intake, but increased protein consumption was associated with less hunger in between meals.
It’s interesting to see that 10% of daily calories from protein promotes overconsumption, as a protein intake of 10-20% is often recommended in official dietary guidelines. Protein is essential not just for regulating appetite, and therefore the optimal protein intake in terms of health or athletic performance could be higher than 15%.
Unbalanced diets low in protein drive the obesity epidemic
While humans can be lean and healthy on both low-fat and low-carbohydrate diets, a low protein intake (<10% daily calories) is associated with poor tissue repair and weakness. Animal source foods such as meat, seafood, and eggs are especially good sources of protein as they contain all of the essential amino acids the human body can’t produce on it’s own.
Prioritizing protein is especially important for someone who’s trying to lose weight, as low protein intake results in overeating, and the negative energy balance associated with weight loss could lead to lower intakes of protein-rich food.
The western dietary pattern is characterized by high intakes of refined grains, sugar, and vegetable oils, and although processed meats are an important part of the diet for many people, dinner is often the only source of high-quality protein. Protein sources are generally more expensive than carbohydrate and fat, and therefore starchy foods often replace grass-fed meats and seafood for economic reasons. While the avergae consumption of protein today is around 15% in westernized countries, hunter-gatherers consumed high amounts of animal food when possible and got between 19-35% of their energy from protein (7).
Regarding dietary causes of the obesity epidemic, the emphasis has been on carbohydrate and fat consumption, but considering the low protein content in many refined and processed foods, and the effect protein has on food intake, it’s clear that lack of protein in the diet could be an important cause of overeating, weight gain, and obesity.
Increased protein intake could be one of the reasons why low-carb diets are so effective
The majority of studies suggest that low-carbohydrate diets promote greater weight loss compared to low-fat diets. These results seem to apply both in ad libitum- and energy restricted studies (8,9,10,11,12). While it seems that reducing the overall carbohydrate content of the diet is linked to increased weight loss, and that ketogenic diets appear to be especially effective in regards to weight reduction, it’s possible that acelluar and refined carbohydrates are the major offenders that promote an inflammatory microbiota, chronic low-level inflammation, and weight gain. Antinutrients found in cereal grains, legumes, and other foods with a relatively high carbohydrate content could further stall weight loss (13,,14).
In addition to the decreased intake of grains and refined carbohydrates, low carb diets usually have a higher percentage of protein than low-fat diets since they contain more animal source foods. This increased protein intake can further explain why low-carb diets are associated with increased satiety per calorie and lower ad libitum energy intakes than low fat diets.
Even before I heard about the protein-leverage hypothesis I suspected that protein was essential for regulating appetite and food intake. Especially in periods with a lot of physical activity I experience increased appetite and a decline in performance if I eat too little protein. In the future it’s possible that I’ll look into the recommended protein intake for people with different needs (e.g., weight lifters, endurance athletes) and for optimal health…