Protein is more satiating than carbohydrate or fat.” This statement has passed as fact so often many people don’t question it anymore, even in evidence-based fitness circles. As a data analyst I was hired to go into a new firm, ignore what everyone said, look only at the data and draw my own conclusions. Here I’m going to do just that and investigate if protein is really more satiating than carbs or fats. Let’s look at the facts.
More protein, less hunger?
Many studies find lower appetite with higher protein intakes. Yet outside the lab in free-living prospective studies, the consumption of high protein foods is not consistently associated with fat loss . In fact, in these studies the consumption of many high protein foods is associated with an increased risk of gaining fat and becoming overweight. How can this be if protein reduces our appetite and thereby energy intake compared to carbs and fats?
As it turns out, many studies find no acute effect of protein intake within a meal on satiety.
- Raben et al. (2003) found no difference in hunger suppression or subsequent energy intake after isocaloric meals of either 32% or 12% protein.
- Bligh et al. (2015) found no effect on satiety of adding fish and almonds to a plant-based paleo meal, even though protein content from the meal rose from a paltry 16 grams to 41 grams.
- Giezenaar et al. (2017) found that consuming a whey protein shake before a buffet didn’t reduce unrestricted (‘ad libitum’) energy intake at all.
- Blatt et al. (2011) found that 5 different preload meals ranging from 10% to 30% protein, which were manipulated to look and taste the same, had the same effect on appetite and unrestricted energy intake.
- Wiessing et al. (2015) found that a whey protein shake was no more effective at suppressing energy intake in the next meal than sugar water, regardless of protein content (high vs. low).
- A 2013 meta-analysis confirmed that there is no relation between the protein content and the appetite suppression of meals.
If protein is more satiating than carbs or fats, how come there are so many studies showing the contrary? To answer this, we should look at why high protein intakes may be appetite suppressing in the first place.
How does protein suppress our appetite?
The most likely mechanism of protein’s satiating effects is hormonal. In the gut, amino acids stimulate the release of several hormones that activate satiety centers in the brain, namely glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). There is also mixed evidence for hunger suppression by cholecystokinin (CCK) stimulation and for suppression of ghrelin release, ‘the hunger hormone’. This looks like a simple and convincing theory: more protein -> more appetite suppressing hormones -> less appetite. However, the theory breaks down on all 3 levels in several studies. High protein meals do not always stimulate more appetite-mediating hormone release or suppression than high carb or high fat meals. On the flip side, different meals with the same macros can result in major differences in gut hormone production. Higher protein intakes also do not always result in greater appetite suppressing hormone levels nor lower hunger hormone levels than lower protein meals, even when the high protein meals have a higher total energy content [3]. The response of gut hormone levels to protein also seems to depend on whether someone is lean or overweight.
Moreover, the relation between supposedly appetite suppressing hormones and actual appetite is inconsistent and, as some researchers put it, “too small to use hormone and glucose concentrations as appropriate biomarkers for appetite, at least at the individual level and probably at the group level.” (Lemmens et al. 2011)
Other researchers propose that large neutral amino acids (LNAAs) that can cross the blood-brain barrier directly alter brain activation and neurotransmitter levels with appetite suppressing effects. However, Koren et al. (2007) found that A) the amount of protein in the diet doesn’t affect the level or ratio of LNAAs in the blood and B) unrestricted energy intake changed over time without changes in blood amino acid levels. So it’s unlikely blood amino acid levels directly influence satiety.
In conclusion, the evidence for direct hunger controlling mechanisms of protein is just as inconsistent as the evidence for the actual appetite suppression. One theory offers a plausible explanation for the variability in protein’s effect on our appetite: protein leverage.
Protein leverage theory
Put simply, protein leverage theory states that the body monitors protein consumption to ensure we consume enough of it. Since the body doesn’t have an efficient storage mechanism for amino acids like it does for carbohydrate (glycogen) and fat (adipose tissue), it makes sense from an evolutionary perspective that the body has adapted mechanisms to ensure we consume enough of this vital macronutrient. Specifically, our appetite stays up until protein requirements have been met. We have a ‘Protein-Stat’. In other words, protein leverage theory says protein is more satiating than carbs or fats until we’ve consumed enough protein for our bodily needs.
The exact mechanisms are still being uncovered, but research indicates a link between satiety signaling in the brain and amino acid utilization in our body. Activation of anabolic signaling pathways (mTOR) and suppression of catabolic signaling pathways (AMPK) reduce food intake by acting on the hypothalamus. A key modulator in the brain may be GCN2 (general control nonderepressible 2), which quite directly monitors amino acid balance and thereby basically the protein quality of our diet. We also have receptors in our mouth that detect amino acids. It’s thus plausible the brain can monitor our protein intake, compare it with our requirements and adjust our appetite accordingly.
In rodents and several other animals, including pigs, protein leverage theory has strong supporting evidence. Rats will quite reliably overeat on low-protein diets until they’ve consumed enough protein (= ‘leveraging protein’). After protein deprivation, when given the choice between high and low protein food, they tend to prefer the higher protein food. Rats can even self-select foods with complementary amino acid profiles. This preference for higher protein foods to meet bodily demands occurs independently of energy balance.
Protein seeking correlates surprisingly well with protein needs for muscle growth in animals. Birds will self-select diets to reach a protein intake that’s close to their estimated optimum. They’ll increase this protein intake in periods of growth and when injected with growth hormone. Birds bred to have more muscle also select higher protein diets and male birds eat more protein than female birds.
This protein seeking can be compared to our innate so-called ‘specific appetite’ for sodium and water, which we also cannot effectively store or survive without.
It should be noted the ability of animals to leverage protein is limited and some research finds protein intake has no effect on energy intake at all.
Protein leverage in humans is more difficult to study because it’s unethical to starve people of protein and difficult to restrict their food choices to a narrow selection for a long time. However, the research we have is promising. After low protein diets, people’s preference for higher protein foods increases compared to after high protein diets. We also have a low drive to eat protein sources with an incomplete amino acid profile lacking in essential amino acids, as we cannot meet protein requirements with those foods. These phenomena are impossible to explain with the simple model that protein is inherently more satiating than carbs or fats because it directly stimulates satiety hormones.
Another phenomenon that’s impossible to explain with the traditional ‘protein is the most satiating macro’ theory is habituation. The satiating effect of high protein meals decreases after high protein diets and comes back after low protein intakes. In other words, if you consume a diet higher in protein than you need, protein will lose some of its satiating effect. The body can sense excess protein intake in the form of increased protein oxidation rates. Habituation to protein’s satiation again makes evolutionary sense. If we only have access to low-protein foods, we should keep eating until we’ve consumed enough protein so we can survive. But if we only have access to high protein foods, we should not stop eating before we’ve consumed enough other nutrients. Otherwise high protein environments would cause us to starve ourselves. In this scenario, protein is abundant and high protein foods are just an energy source like carbs or fats. So the body should treat them as such in terms of how much we need of them.
The finding that high protein intakes reduce how well protein controls our hunger also means we should be skeptical of all short-term research. Just because a high protein diet is satiating for a couple of days, doesn’t mean it will have long-term beneficial effects.
With protein leverage in mind, ‘how satiating is protein?’ is the wrong question to ask. We should instead ask: how much protein do we need? I answered that question in my article on the optimal protein intake. The short answer is: most strength trainees don’t need more than 1.6 grams of protein per kilogram of bodyweight per day (0.64 g/lb/d). Protein leverage theory would thus predict that’s the optimal protein intake for satiety as well with no further benefits of going higher in protein. In sedentary individuals, the optimal protein intake for satiety should be lower in accordance with their lower bodily protein requirement. Let’s look at long-term studies that compared diets that were similar in all respects except for their protein intake, with one group consuming at least 1.6 g/kg/d, and see how they match up in terms of satiety.
The effect of higher vs. lower protein diets on our appetite
- Mettler et al. (2010) placed strength trained athletes into two weight loss groups with either just 1 g/kg/d or 2.3 g/kg/d of protein for 2 weeks. In spite of the clearly suboptimal protein intake of the lower protein group, they found no significant differences in appetite ratings between the groups. Fluctuations in hunger and desire to eat were lower in the high protein group though.
- Das et al (2007) studied groups of overweight individuals consuming 2.3 compared to 1.5 g/kg/d of protein for 48 weeks. They found no significant difference in any measure of satiety: daily self-reported hunger, desire to eat or ad libitum energy intake.
- Karl et al. (2018) found no difference in appetite, food preferences or appetite-mediating hormones between 1 g/kg/d and 2 g/kg/d of protein in highly active men in 40% energy deficit at high altitude.
- Gwin et al. (2017) compared 0.6 to 1.6 g/kg/d protein in overweight women. While the higher protein intake self-reported higher satiety, it didn’t actually reduce carbohydrate or fat intake when eating freely, so the high protein group ended up consuming more total calories. This discrepancy between self-reported appetite and actual eating behavior appears in several studies.
- Stubbs et al. (1999) compared a day of eating in 4 groups with different macros with protein intakes ranging from 2.2 to 4 g/kg/d. There were no significant differences in any measure of self-reported satiety or free energy intake over the day.
In contrast to the above studies, when comparing higher and lower protein intakes both below 1.6 g/kg/d, there is reasonably consistent evidence for appetite suppressing effects of higher vs. lower protein diets.
Thus, it appears there’s a ceiling effect after which protein loses its extra satiating effect. This ceiling is evident in studies comparing 3 different protein intakes. For example, Li et al. (2016) performed a long-term cross-over trial of diets with 10%, 20% or 30% protein. The 10% protein group slightly underperformed on satiety, but there were no differences on any appetite measure between the 20% and 30% protein intakes. The researchers overall concluded that a diet’s protein intake has “minimal effects on appetite control”. It appears the brain directs us to consume at least ~15% of energy intake as protein, as hunger increases below this point but satiety does not increase above it [2, 3, 4]. The average optimum protein intake for satiety may be a bit higher for some people though, as several studies find benefits of going higher than 15% in protein for satiety [2, 3, 4]. 15% of energy intake corresponded to only 64-75 g protein per day in these studies and it was often insufficient to optimize body recomposition, so it’s not surprising the average sweet spot for hunger control was higher than that in these studies.
A review of 38 studies concluded protein is more satiating than carbs and fats in the 10-20% of energy intake range but not above that, indicating the average satiety sweet spot is a protein intake of 20% of energy intake, corresponding to about 1.2 g/kg/d for non-strength training individuals. The effect was far stronger for self-reported satiety than actual eating behavior: ad libitum energy intake didn’t reliably decrease even at lower protein intakes. The optimum protein intake for satiety was closely in line with the optimal protein intake for body recomposition and health (1.2 – 1.6 g/kg/d).
The literature is thus in line with protein leverage theory: up to the bodily protein requirement, protein is generally more satiating than carbs or fats per gram, but after protein needs have been met, the superior appetite suppressing effect of protein disappears. It likely also weakens over time when consuming a high protein diet.
This explains why prospective large-scale research finds that “participants consuming an amount of protein above the protein intake recommended by the ADA may experience a higher risk of becoming overweight or obese during adult life… Compared to diets with no more than 14% of energy from protein, diets with more than 22% of energy from protein were associated with a 23–24% higher risk of becoming overweight or obese.”
In conclusion, it’s incorrect to say protein is inherently more satiating than carbs or fats.
Moreover, in practice, it’s silly to even think of ‘protein vs. fats/carbs’.
