In many nutrition and fitness circles, you may have heard how protein is the most satiating macronutrient and how it can lead to fat loss while staying full. In the late 1980s to 1990s, it was hypothesized that one of the leading causes of obesity was a percentage reduction in protein in people's diets when compared to carbohydrates and fats. In other words, increasing protein in one's diet would lead to reduced energy intake and consequently leanness among the public.
At first, this hypothesis seems to hold ground when using observational and high-level studies but the hypothesis falls apart under scientific scrutiny. Let us look at the evidence.
High-Level Testing in Humans
Though high-level studies(1) suggestive of protein leveraging exist, there are glaring studies that also challenge this notion. In this study(2) of 22 lean individuals studied over 4 days, incomplete protein leveraging was noticed while the increase from 15% to 25% noted no change in energy intake among the individuals. In another study(3), diets using 5%, 15%, and 30% protein from plant and animal sources were used in a 12-day randomized crossover trial with 79 subjects, total energy intake was lower in the 30% protein group but nearly the same in 5% and 15% group. Another 12-day randomized crossover study(4) with 58 subjects using 5%, 15% and 30% protein diets from beef noted, despite achieving nitrogen balance at 5%, in complete protein leveraging was noted with 5% and 15% group showing similar energy intakes.
Covertly Replacing Protein
What would happen if protein content was covertly manipulated without the participants knowing? In an experiment(5), when protein content was covertly changed from 10% to 30% of the energy intake and replaced with starchy ingredients while energy density, fat content, palatability, and appearance were kept the same, that despite the increase in protein intake, energy intake did not change significantly among 10-30% protein conditions. Hunger and fullness remained the same too.
Isolating The Role of Protein
When you try to understand the thermal conductivity of copper before devising a circuit, you isolate it. This is how you find that a certain material or isolate plays a role on its own in the bigger picture. Many over-the-counter protein drinks and even those used in experimentation in scientific literature use protein isolates that may be confounded by thickening agents and other additives that can independently manipulate energy intake(eg. viscosity(6)(7).) However, luckily, some studies do exist that completely isolate protein.
In this first study(8), protein (Clear Protein 8855, a 90% source of whey protein, colorless, further refined, non-cloudy and odorless) is tested compared to water to note any changes in satiety and hunger. Utilizing 1%, 2%, and 4% protein in water reduced energy intake by a very small amount(8%) in the following meal. This was not sufficient to impact food intake 2 hours later. The study was repeated (9) in the future but this time protein(Clear Protein 8855) up to 4% (348 kJ or 83 calories) was compared with up to 10% sucrose water and sweetened water control. Beverages were matched for volume, color, flavor, and sweetness. A challenge meal was provided 2 hours later after these preloads. Neither protein nor sugar-enriched waters had any impact on ad-lib energy or macronutrient intake with no energy compensation for the caloric beverages.
Another question that begged to be asked was if there was a specific peptide that led to a reduction in intake and induced satiety. Luckily, such a study(10) was conducted using a pure metabolized form of protein(CMP or Caseinomacropeptide) up to 2%. CMP was tested against water at various timings and it was found that it had no impact on hunger, satiety, and total energy intake on the current meals and food intake throughout the day.
In a nutshell, there's nothing specific about protein on its own that induces satiety or reduces energy intake. More than likely, protein's capacity to reduce intake relies primarily on how it interacts with other nutrients and non-nutrient components of foods. This becomes more obvious when we compare protein-heavy foods with foods low in protein but perform superior.
Mushrooms Performing Better than Meat Offers Insight
Several potential studies challenge the notion that foods low in protein can offer a superior reduction in energy intake and satiety. For instance, substituting mushrooms for meat yielded superior results when it comes to energy intake(11) and a 'mushroom' diet, as noted in this study(12), that displaced meat also performed superior when it comes to lowering energy intake, body weight, BMI, total body fat, waist circumference, and blood pressure while maintaining 7lbs lower weight than baseline.
Hunger & Cravings Comparison with Ultraprocessed Plant-Based Meat vs. Animal Meat
In this randomized double-blind crossover study(13) with 30 men, ultra-processed plant-based meat performed equally when it came to hunger, cravings, and fullness despite being lower in amino acids derived from protein. As a side note, in another randomized controlled crossover study(14) of 36 participants, plant-based ultra-processed meat showed improvements in cardiometabolic risk factors compared with meat.
Conclusion
Protein is important as it plays a key role as the building block for our body. However, to make better processed and healthy foods of the future, the protein leverage hypothesis or 'protein is the most satiating macronutrient' myth needs to be busted for consumers and intellects alike. We live in the information age and consumers are easily prone to be misled by social media advocates who have formed their identities and diet plans around singular macronutrients, ideologies, and fabricating processed information. We once had The Rice Diet which consisted of using rice and sugar to obtain results and many high protein diets are similar in the sense that they radicalize people enough to get temporary results but completely miss out on mechanisms and properties in foods that work.
The solution to creating better foods of tomorrow is to improve shelf-stable, tasty and convenient foods. This can't be done if we continue to promote poor science to consumers as many such advocates would lead with. In a nutshell, the path to creating better-processed foods would be by working with fats, proteins, carbohydrates, and many other nutritive and non-nutritive components in food while at the same time, we need better educators who are receptive to quality evidence and critical thinking.
Note: This article will be constantly updated as we gain more evidence.
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