I recently wrote an article debunking the myth that protein is always more satiating than carbs or fats. I knew it would be controversial based on my article debunking the myth most people need more than 1.8 g/kg/d of protein for maximum muscle growth. That idea only slowly crumbled over time, even after repeated experiments verifying it, including a study and a meta-analysis I co-authored. The main thing people cling onto now is an incorrect interpretation of the confidence interval, but there are always more straws to grasp.
However, I was pleasantly surprised by how well my article on protein and satiety was received. It quickly reached over 5000 shares on social media and was picked up by many very smart individuals in our field.
While controversial, the research on protein’s satiating effects in my article was not new. It was simply overlooked or ignored, as data that conflicts with established beliefs often is (all humans are prone to this, myself included). My online PT Course students have been familiar with all this research for years.
What is new though is the following study in Nutrients I co-authored: “Satiating Effect of High Protein Diets on Resistance-Trained Subjects in Energy Deficit”
What we did
As the title indicates, we got a group of serious strength trainees of both genders to participate in a 3-part study on the satiating effect of protein. Across 3 mini-experiments, we compared a protein intake of 1.8 g/kg/d vs. 2.9 g/kg/d (0.82 g/lb/d vs. 1.32 g/lb/d) and measured satiety’s 3 most important markers: satiety hormones, subjective satiety and energy intake. We used a randomized cross-over trial, meaning every participant underwent the full study in both conditions with a wash-out period in between in a random order. This type of design greatly increases statistical power and indeed our study had ample statistical power based on our a priori power analysis.
- Phase 1 was a diet in 20% energy deficit with the high or super high protein intake. Given the same energy intake, do higher protein intakes help reduce our hunger?
- Phase 2 measured the acute satiety response to a high protein (0.7 g/kg) test meal after both conditions to see if habitual protein intake modified the response to protein’s satiating effect. Prior research suggests there is habituation: consuming a high protein diet decreases the satiating effect of a given protein intake.
- Phase 3 was an ad libitum diet: with only protein intake fixed at 1.8 g/kg/d in both groups, the participants were free to eat as much as they wanted while we measured their total energy intake and subjective markers. Would the moderate protein intake group overeat more due to being restricted in protein intake during the diet? Or would the high protein intake group suffer when going back to a more moderate protein intake?
I was extremely relieved to find out we achieved very good adherence in the participants, as verified by body composition changes and reported macronutrient intakes, so we could proceed with the data analysis.
What we found
In phase 1 with the calorie-controlled diets, there were no significant differences in reported satiety between the 1.8 and 2.9 g/kg/d protein groups on any of the following measures:
- desire to eat
- training enjoyment
- training motivation
Only 1 measure differed significantly between groups: the 2.9 g/kg/d protein group reported higher diet satisfaction than the 1.8 g/kg/d group. This difference was only significant during the last 3 days of the diet. Considering we had 16 comparisons, this may be a type I error (read: a fluke), but we did control for this in the significance analysis. I speculate the 1.8 g/kg/d group felt restricted more than then 2.9 g/kg/d group and best results will be achieved with setting 1.8 g/kg/d as a minimum protein intake but not capping it, so that people can exceed the minimum when they want. Based on my experience with clients, this flexibility increases diet satisfaction.
In phase 2, the high protein test meal after phase 1, we found evidence of habituation to the high protein intake, but it was inconsistent.
- In response to the test meal, ghrelin, ‘the hunger hormone’, levels decreased more after the 1.8 g/kg/d protein phase. This supports habituation: the higher protein group no longer achieved the same hunger suppression from a given protein intake. However, when normalizing the data for pre-intervention levels, the difference became insignificant.
- Similarly, the levels of PYY, a peptide that suppresses our appetite, increased more after the moderate protein intake, but this was only significant when normalizing for pre-intervention levels and not when relative to baseline levels.
- The test meal suppressed hunger and desire to eat more after the moderate protein diet, while it increased fullness and satisfaction more than after the super high protein diet. However, when normalizing the change scores for pre-intervention values, none of the differences were significant anymore.
Taken together, these results support prior research that consuming a high protein intake in your regular diet can decrease the satiety you get from a high protein meal. This may be relevant for high protein cheat meals. People on very high protein diets may be more likely to overeat in a steakhouse, for example. However, this finding warrants replication and probably won’t affect regular diet adherence.
In phase 3, the ad libitum diet phase, energy intake did not differ between groups. There were also no differences on any measure of the same satiety & satisfaction questionnaire we employed earlier during the diets. Since neither group ended up overeating more or less satisfied with an unrestricted energy intake, this supports the sustainability of the moderate and super high protein intakes was similar. It also supports there is no detrimental effect of going back down from a super high to a moderate protein intake.
Several measures in 3 diet phases supported that a protein intake of 1.8 g/kg/d is just as satiating as a protein intake of 2.9 g/kg/d (0.82 g/lb/d vs. 1.32 g/lb/d). We found some evidence of habituation to protein’s satiating effects after a test meal, but this did not seem to affect satiety or satisfaction during the diet or during the ad libitum diet phase, so it’s probably not very practically relevant except maybe for high protein cheat meals. In conclusion, our results support that 1.8 g/kg/d is a good target protein intake for many people for maximum satiety. There is no reason to restrict yourself to this if you prefer to go higher in protein, so think of it as a minimum intake if you have enough calories left to still get your fat and carbohydrate targets in.
Interested in the finer details? We made our study paper open-access, so everyone can read the full text for free here.
I want to thank my co-authors, in particular Justin Roberts and my research team, for your amazing work!