However, no previous studies have looked into the effects of specific proportions of protein, carbs and fat in a systematic, dose-response manner.
For this reason, scientists examined the effects of five drinks containing different proportions of these nutrients.
Protein is generally considered to be the most filling macronutrient, whereas fat is the least filling. However, not all studies support this.
This may be because protein and carbs suppress the hunger hormone, ghrelin, more than fat.
Another hormone that may be involved is the satiety hormone, glucagon-like peptide-1 (GLP-1). However, it’s currently unclear which macronutrient has the greatest effect on GLP-1.
Scientists from Lund University, in Sweden, examined how different macronutrient proportions affected calorie intake, appetite hormones and appetite.
This was a randomized, crossover trial examining the effects of liquid meals — varying in protein, fat and carbs — on appetite, appetite hormones and calorie intake.
A total of 36 healthy men and women participated in the study.
They were assigned to five different liquid meals, which they received on separate days in a random order:
- Low-protein, low-carb, high-fat drink (LP/LC:HF): 8.9% of calories from protein, 28% from carbs and 63.1% from fat.
- High-protein, low-carb, medium-fat drink (HP/LC:MF): 40% of calories from protein, 18.5% from carbs and 41.5% from fat.
- Low-protein, high-carb, low-fat drink (LP/HC:LF): 8.9% of calories from protein, 71.1% from carbs and 20% from fat.
- High-protein, medium-carb, low-fat drink (HP/MC:LF): 40% of calories from protein, 46.8% from carbs and 13.2% from fat.
- Medium-protein, medium-carb, medium-fat drink (MP/MC:MF): 24.4% of calories from protein, 50.4% from carbs and 25.2% from fat. This drink was replicated three times.
These liquid meals were based on milk protein isolate, rapeseed oil and a mixture of maltodextrin and table sugar. All of them contained 502 calories (2,100 kJ), and had the same volume of 670 mL.
After finishing the drink on each of the seven test days, the participants rated their appetite every 30 minutes until they had a lunch test meal.
During the same period, the researchers collected blood samples every hour. The blood samples were analyzed for the appetite hormones, ghrelin and GLP-1.
The purpose of the lunch test meal was to measure calorie intake. It was served 3.5 hours after the breakfast, and was based on pasta. The participants were encouraged to eat as much as they wanted.
Bottom Line: This randomized, crossover study examined the effects of liquid meals, varying in protein, carbs and fat, on appetite hormones, self-rated appetite and calorie intake.
Finding 1: Protein Tended to Lower Appetite
Every half an hour after finishing the test drink, the participants were asked to rate their feelings of appetite on a visual analog scale (VAS).
The VAS included the following feelings:
- Desire to eat.
- Prospective food consumption.
The researchers found that subjective ratings for fullness were significantly greater after HP/MC:LF than after LP/LC:HF.
Similarly, prospective food consumption ratings were 12% lower after the intake of HP/MC:LF, and 11% lower after the intake of HP/LC:MF, compared to LP/LC:HF.
The findings are presented in the chart below:
However, there were no significant differences in any of the ratings between HP/MC:LF and LP/HC:LF, indicating that carbs may have some filling effects as well.
Overall, protein appeared to be the most filling macronutrient, while fat was the least filling. No significant differences in hunger or desire to eat were seen.
Bottom Line: Protein reduced self-rated appetite more than carbs and fat, and appears to be the most filling of the three macronutrients.
Finding 2: Calorie Intake Was Unaffected
There were no significant differences in how the meals affected calorie intake at lunch, 3.5 hours after they were consumed.
Some evidence indicates that liquid calories have weaker effects on appetite and calorie intake, compared to solid calories.
Also, the effects of these drinks on appetite may have faded 3.5 hours afterwards.
Bottom Line: The various liquid meals had no significantly different effects on calorie intake at a lunch eaten 3.5 hours afterwards.
Finding 3: Protein Increased Levels of GLP-1
Circulating levels of the appetite-suppressing hormone, GLP-1, were higher after the HP/LC:MF drink, compared to the other liquid meals.
This can be seen in the chart below:
Specifically, the levels of GLP-1 were generally 13–19% higher after the HP/LC:MF drink. This suggests that the appetite-suppressing effects of protein may be at least partly due to increases in GLP-1.
However, fat appeared to increase GLP-1 to some extent as well. These findings are supported by several previous studies.
Conversely, no significant changes in the hunger hormone, ghrelin, were detected between liquid meals.
Bottom Line: Protein appeared to increase circulating levels of the appetite-suppressing hormone, GLP-1.
Although the study’s design did not have any serious shortfalls, a few limitations should be mentioned.
First, including both men and women caused significant variations in calorie intake. This might have masked the effects of the liquid meals on calorie intake at lunch.
Additionally, the generalizability of the findings is limited. There is some evidence that liquid calories have weaker effects on appetite than solid calories.
Second, the drinks were not typical breakfast foods. One study indicates that novelty may affect satiety.
Finally, not all types of protein, carbs and fat affect appetite the same.
Bottom Line: The study’s findings may not be generalized to all situations. For example, the liquid form of the drinks may have affected the results.
Summary and Real-Life Application
This study showed that protein reduces appetite more than carbs and fat. On the other hand, fat appeared to be less filling than protein and carbs.
Simply put, if you want to lose weight, adding more protein to your diet may help curb your appetite.