Does Protein Increase Satiety?

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Is protein, in comparison to carbohydrates and fats, truly more filling?

Is protein, in comparison to carbohydrates and fats, truly more filling?

"Protein fills you up better than carbs or fat." Even in evidence-based fitness circles, this statement has become so commonplace that many people no longer question it. As a data analyst, I was hired to go into a new firm, disregard what everyone else said, and look only at the data to draw my own conclusions. I'm going to do just that here, and see if protein is truly more satiating than carbs or fats. Let us examine the facts.

More protein, fewer hunger pangs?

Many studies have found that eating more protein reduces appetite. However, outside of the lab, in free-living prospective studies, high protein diets are not consistently associated with fat loss. In fact, eating a lot of high-protein foods has been linked to an increased risk of gaining weight and becoming overweight in these studies. How is this possible if protein, in comparison to carbs and fats, reduces our appetite and thus our energy intake? Many studies, it turns out, find no acute effect of protein consumption during a meal on satiety.

Raben et al. (2003) discovered no difference in hunger suppression or subsequent energy intake after isocaloric meals containing either 32% or 12% protein.

Bligh et al. (2015) discovered that adding fish and almonds to a plant-based paleo meal had no effect on satiety, despite the meal's protein content increasing from 16 grams to 41 grams.

Giezenaar et al. (2017) discovered that drinking a whey protein shake before a buffet had no effect on unrestricted ('ad libitum') energy intake.

Blatt et al. (2011) discovered that 5 different preload meals ranging from 10% to 30% protein and manipulated to look and taste the same had the same effect on appetite and unrestricted energy intake.

Wiessing et al. (2015) discovered that, regardless of protein content, a whey protein shake was no more effective than sugar water at suppressing energy intake at the next meal (high vs. low).

A meta-analysis published in 2013 confirmed that there is no link between protein content and meal appetite suppression.

Why are there so many studies that show protein is less satiating than carbs or fats? To find out why high protein intakes may be appetite suppressing in the first place, we should look at why high protein intakes may be appetite suppressing in the first place.

What is the mechanism by which protein suppresses our appetite?

The hormonal mechanism is the most likely explanation for protein's satiating effects. Amino acids stimulate the release of several hormones in the gut that activate brain satiety centers, including glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). There is also conflicting evidence for the suppression of hunger by cholecystokinin (CCK) stimulation and the suppression of ghrelin release, dubbed "the hunger hormone." This appears to be a straightforward and convincing theory: more protein = more appetite suppressing hormones = less appetite. Several studies, however, show that the theory fails on all three levels. High protein meals do not always stimulate the release or suppression of appetite-mediating hormones more than high carbohydrate or high fat meals. On the other hand, different meals with the same macronutrients can result in significant differences in gut hormone production. Higher protein intakes do not always result in higher levels of appetite suppressing hormones or lower levels of hunger hormones than lower protein meals, even if the high protein meals have a higher total energy content.  

Furthermore, the relationship between ostensibly appetite suppressing hormones and actual appetite is inconsistant and, in the words of some researchers, "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."

Other researchers believe that large neutral amino acids (LNAAs) that can cross the blood-brain barrier directly affect brain activation and neurotransmitter levels, resulting in appetite suppression. Koren et al. (2007) discovered, however, that A) the amount of protein in the diet has no effect on the level or ratio of LNAAs in the blood, and B) unrestricted energy intake changed over time with no changes in blood amino acid levels. As a result, blood amino acid levels are unlikely to have a direct influence on satiety.

In conclusion, the evidence for direct hunger-controlling protein mechanisms is as contradictory as the evidence for actual appetite suppression. Protein leverage is one theory that provides a plausible explanation for the variability in protein's effect on our appetite.

The theory of protein leverage

Simply put, protein leverage theory proposes that the body monitors protein consumption to ensure that we get enough of it. Because the body does not have an efficient storage mechanism for amino acids like it does for carbohydrate (glycogen) and fat (adipose tissue), it stands to reason that the body has evolved mechanisms to ensure we consume enough of this vital macronutrient. Specifically, our appetite persists until our protein needs are met. We've got a 'Protein-Stat'. In other words, protein, according to protein leverage theory, is more satiating than carbs or fats until we've consumed enough protein for our bodies' needs.

Although the precise mechanisms are still unknown, research suggests a link between satiety signaling in the brain and amino acid utilization in our bodies. By acting on the hypothalamus, activation of anabolic signaling pathways (mTOR) and suppression of catabolic signaling pathways (AMPK) reduce food intake. GCN2 (general control nonderepressible 2), which directly monitors amino acid balance and thus the protein quality of our diet, may be a key modulator in the brain. Amino acid receptors are also found in our mouths. Thus, it is possible that the brain can monitor our protein intake, compare it to our needs, and adjust our appetite accordingly.

Protein leverage theory has strong support in rodents and several other animals, including pigs. Rats will overeat on low-protein diets until they have consumed enough protein (= 'leveraging protein'). When given a choice between high and low protein foods after protein deprivation, they tend to prefer the higher protein food. Rats can even choose foods with complementary amino acid profiles on their own. This preference for higher protein foods to meet bodily needs happens independently of energy balance.

In animals, protein seeking correlates surprisingly well with protein requirements for muscle growth. Birds will self-select diets that are close to their estimated optimum protein intake. They will increase their protein intake during growth periods and when given growth hormone injections. Male birds eat more protein than female birds, and birds bred to have more muscle eat higher protein diets as well.

This protein seeking can be compared to our innate'specific appetite' for sodium and water, both of which we cannot effectively store or survive without. It should be noted that animals' ability to leverage protein is limited, and some research indicates that protein consumption has no effect on energy intake at all.

Protein leverage is more difficult to study in humans because it is unethical to deprive people of protein and difficult to restrict their food choices to a narrow selection for an extended period of time. However, the research we have so far is encouraging. People prefer higher protein foods after low protein diets more than after high protein diets. We also have a low desire to consume protein sources with an incomplete amino acid profile deficient in essential amino acids because we cannot meet our protein needs with those foods.

These phenomena defy the simple model that protein is inherently more satiating than carbs or fats because it directly stimulates satiety hormones.

Another phenomenon that the traditional 'protein is the most satiating macro' theory cannot explain is habituation. After a high protein diet, the satiating effect of high protein meals decreases and returns after a low protein diet. In other words, if you eat more protein than you need, it will lose some of its satiating effect. Excess protein intake is detected by the body through increased protein oxidation rates.

We would starve if we were exposed to high protein environments. Protein is abundant in this scenario, and high protein foods are simply an energy source like carbs or fats. As a result, the body should regard them as such in terms of how much we require of them.

Because high protein intake reduces how well protein controls our hunger, we should be skeptical of any short-term research. Just because a high protein diet is filling for a few days doesn't mean it will be beneficial in the long run.

With protein leverage in mind, the question 'how satiating is protein?' is incorrect. Instead, we should ask, "How much protein do we require?" That question was addressed in my article on the ideal protein intake. The short answer is that most strength athletes require no more than 1.6 grams of protein per kilogram of bodyweight per day (0.64 g/lb/d). According to protein leverage theory, this is also the optimal protein intake for satiety, with no additional benefits from increasing protein intake. Sedentary people's optimal protein intake for satiety should be lower to match their lower bodily protein requirement.

Let's take a look at long-term studies that compared diets that were identical in every way except protein intake, with one group consuming at least 1.6 g/kg/d, and see how they compare in terms of satiety.

The impact of higher protein diets vs. lower protein diets on our appetite

Mettler et al. (2010) divided strength-trained athletes into two weight-loss groups for two weeks, using either 1 g/kg/d or 2.3 g/kg/d of protein. Despite the lower protein group's clearly suboptimal protein intake, they found no significant differences in appetite ratings between the groups. The high protein group, on the other hand, experienced fewer fluctuations in hunger and desire to eat.

Das et al. (2007) studied groups of overweight people who consumed 2.3 g/kg/d of protein versus 1.5 g/kg/d of protein for 48 weeks. They discovered no statistically significant difference in any measure of satiety: daily self-reported hunger, desire to eat, or energy intake ad libitum

In overweight women, Gwin et al. (2017) compared 0.6 to 1.6 g/kg/d protein. While the higher protein group reported greater satiety, it did not reduce carbohydrate or fat intake when eating freely, so the high protein group consumed more total calories. Several studies have found a mismatch between self-reported appetite and actual eating behavior.

Stubbs et al. (1999) compared a day of eating in four groups, each with a different macronutrient profile and protein intake ranging from 2.2 to 4 g/kg/d. Over the day, there were no significant differences in any measure of self-reported satiety or free energy intake.

As a result, it appears that there is a limit after which protein loses its extra satiating effect. This limit has been demonstrated in studies comparing three different protein intakes. Li et al. (2016), for example, conducted a long-term cross-over trial of diets containing 10%, 20%, or 30% protein. The 10% protein group performed slightly worse on satiety, but there were no differences on any appetite measure between the 20% and 30% protein groups. Overall, the researchers concluded that protein intake in a diet has "minimal effects on appetite control.It appears that the brain directs us to consume at least 15% of our energy intake as protein, because hunger increases below this point but satiety does not. However, the average optimum protein intake for satiety may be a bit higher for some people, as several studiesfind benefits of going higher than 15% in protein for satiety. In these studies, 15% of energy intake corresponded to only 64-75 g protein per day, which was frequently insufficient to optimize body composition, so it's not surprising that the average sweet spot for hunger control was higher than that in these studies.

According to a review of 38 studies, protein is more satiating than carbs and fats in the 10-20% of energy intake range but not above that, indicating that the average For non-strength training individuals, the satiety sweet spot is a protein intake of 20% of energy intake, or about 1.2 g/kg/d. The effect was far stronger for self-reported satiety than for actual eating behavior: even at lower protein intakes, ad libitum energy intake did not reliably decrease. The optimal protein intake for satiety (1.2 – 1.6 g/kg/d) was very similar to the optimal protein intake for body composition and health (1.2 – 1.6 g/kg/d).

The literature thus supports protein leverage theory: protein is generally more satiating than carbs or fats per gram up to the bodily protein requirement, but once protein needs are met, the superior appetite suppressing effect of protein disappears. When eating a high protein diet, it most likely weakens over time.

This explains why prospective large-scale research finds that "participants consuming an amount of protein above the ADA's protein intake may experience a higher risk of becoming overweight or obese during adult life... Compared to diets containing no more than 14 percent protein, diets containing more than 22 percent protein were associated with a 23–24 percent increased risk of becoming overweight or obese."

Application in practice

Assume your protein intake is already adequate for body recomposition, but you're experiencing hunger during your cut. Should you up your protein intake even more?

Most likely not, and here's why. Let's start with everyone's favorite high-protein food: chicken breast. Is it difficult to eat 200 g of chicken breast? Unless you have the appetite of an 80-year-old sarcopenic, the answer is: very easy. After cooking, the food volume is negligible. When paired with a tasty sauce, even 500 g of chicken breast becomes nothing more than a snack for most big guys.

(Tip: Coca Cola Light can be used to make a delicious almost zero-calorie sauce.) In general, 200 g of chicken breast contains more than 250 calories. You could eat about 3 pounds of zucchini for those calories, as it only has 17 kcal per 100 grams. Which is more filling and satisfying: the baby's handful of tender chicken or the mountain of fibrous zucchini? The answer should be self-evident.

Vegetables, on average, are far more filling than high-protein foods. A given volume of mushrooms in a lunch meal, for example, is just as satiating as a given volume of meat, despite the fact that the meat contains far more protein and total calories.

Meals based on beans and peas are just as satiating per calorie as meals based on higher protein sources like veal and pork. The combined effects of energy density and fiber on satiety can easily overpower protein's satiating effect.

Thus, talking about 'protein' and 'carbs' makes no sense. This is what I call IIFYM-speak: obsessing over macronutrients to the point of forgetting there's more to food. Focusing solely on macronutrients is fundamentally misguided when it comes to our appetite. Food volume, texture, palatability, fiber content, viscosity, and other factors all influence how we perceive food.

Last word 

Protein, according to conventional wisdom, is more satiating than carbs or fats because it stimulates greater appetite suppressing hormone production in the gut. Higher protein intakes, on the other hand, do not consistently alter gut hormone levels, gut hormone levels are not consistently associated with self-reported satiety or unrestricted energy intake, and, most importantly, higher-protein meals and diets do not always lead to greater satiety than lower-protein meals and diets.

Protein leverage theory explains the disparity in results: the brain has adapted to monitor protein intake and adjusts protein's satiating effect accordingly to ensure we get enough protein. So, once enough protein has been consumed for bodily functions, the extra satiating effect of protein fades, and it wanes with habitual high protein consumption.

So, for satiety, make sure you consume the optimal protein intake for maximum progress, but don't be concerned if you need to consume more. After this point, things like energy density and fiber are far more important than protein intake for satiety. Protein is not inherently more satiating than carbs or fats, so if you don't enjoy high protein foods, you can be just as satisfied with other foods you prefer. Being lean does not necessitate a diet of chicken breast and protein shakes. Potatoes, beans, vegetables, and most fruits are all good options for satiety and your wallet. Experiment with foods other than protein, and you may find that you are not only more satiated, but also more satisfied.

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