The umami flavour generated by monosodium glutamate (MSG) has been proposed as the marker for the presence of protein in foods. As protein is the most closely regulated macronutrient in the diet, the present study addressed whether acute protein deprivation, habitual protein intake or a combination of the two influenced liking for the taste of MSG. 24 low-restraint male participants (mean age: 22; BMI: 23) consumed either their habitual breakfast (baseline), a low protein breakfast (breakfast meal with low protein milk and milkshake) or a high protein breakfast (breakfast meal with high protein milk and milkshake) on three different days, and then evaluated the acceptability of umami (MSG), salty (NaCl) or sweet (Acesulphame K) tastes at low or high concentrations in a soup context at lunchtime. Participants also completed a habitual protein intake questionnaire (39-item protein Food Frequency Questionnaire). Liking for all tastes was higher on the low than on the high protein day, and NaCl and Acesulphame K were liked less on both protein manipulation days when compared to the no added flavour control. Habitual protein intake was not related to liking for MSG stimuli alone but habitual high protein consumers rated a high concentration of MSG as more pleasant than any other taste when in protein deficit. Overall, these findings suggest that liking for high MSG concentrations may be moderated by nutritional need in high protein consumers.

Background: Monosodium glutamate (MSG) has been shown to increase satiety when combined with protein. Inosine 5′-monophosphate acts synergistically with MSG when tasted, is present in high-protein sources, and may potentially further enhance satiety.

Objective: We assessed effects of a combination of monosodium glutamate and inosine 5′-monophosphate (MSG/IMP) provided either alone or in a high-energy, high-carbohydrate and -protein soup on appetite during ingestion and postingestive satiety.

Design: Fixed portions (450 g) of a low-energy control and high-energy, high-carbohydrate and -protein soup preload with added monosodium glutamate and inosine 5′-monophosphate (MSG/IMP+) or without added monosodium glutamate and inosine 5′-monophosphate (MSG/IMP−) were consumed on 4 nonconsecutive days, and changes in appetite during soup intake and at a subsequent ad libitum lunch were assessed in 26 low-restraint volunteers by using a within-participant design.

Results: MSG/IMP+ conditions significantly reduced subsequent intake more than the MSG/IMP− condition did irrespective of energy. The high-carbohydrate and -protein condition also reduced intake independently of MSG/IMP. Energy compensation was greater in the MSG/IMP+ carbohydrate and protein conditions than MSG/IMP− condition. The addition of the MSG/IMP+ also increased the soup pleasantness and caused an immediate increase in appetite when the soup was first tasted.

Conclusion: The addition of MSG/IMP to a low-energy preload had a biphasic effect on appetite by stimulating appetite during ingestion and enhancing postingestive satiety. This trial was registered at www.controlled-trials.com/isrctn as ISRCTN14567895.

Previous research suggests that monosodium glutamate (MSG) may have a biphasic effect on appetite, increasing appetite within a meal with its flavour-enhancing effect, but enhancing subsequent satiety due to its proposed role as a predictor of protein content. The present study explored this by assessing the impact of a 450 g soup preload differing in MSG concentration (1 % MSG added (MSG+) or no MSG (MSG-)) and nutrient content (low-energy control or high-energy carbohydrate or high-energy protein) on rated appetite and ad libitum intake of a test meal in thirty-five low-restraint male volunteers using a within-participant design. Protein-rich preloads significantly reduced intake at the test meal and resulted in more accurate energy compensation than did carbohydrate-rich preloads. This energy compensation was stronger in the MSG+ protein conditions when compared with MSG+ carbohydrate conditions. No clear differences in rated appetite were seen in MSG or the macronutrient conditions alone during preload ingestion or 45 min after intake. Overall, these findings indicate that MSG may act to further improve energy compensation when provided in a protein-rich context.

The fifth basic taste, ‘umami’, is the flavour function elicited by amino acids like monosodium glutamate (MSG) in foods. This taste is recognized for its flavour enhancing properties but little is known about its effects on appetite and intake. Thus the experiments in this thesis aimed to understand how umami influences pleasantness, appetite stimulation, satiation and satiety using MSG, with some additional focus on its associated ribonucleotide inosine 5’-monophosphate (IMP).

Chapter 2 established a bland, low glutamate control soup which was used throughout all subsequent experiments to test the effects of MSG on palatability using commercially-relevant concentrations. Chapters 3 and 4 assessed the influence of increasing palatability on rated appetite and intake of this soup with either added MSG (Chapter 3) or added sucrose (Chapter 4). No increase in hunger or intake was found after the more palatable conditions. Chapter 5 explored the relationship between MSG taste and protein regulation, assessing acute and habitual protein intake with findings indicating that high protein consumers liked high MSG concentrations more after an acute protein deprivation than sweet, salty or control flavours. Chapter 6 examined the time course of rated MSG satiety alone and in combination with either protein or carbohydrate in a preload soup and found enhanced rated satiety in MSG protein conditions. This design was extended in Chapter 7 to include an intake test after a pre-specified time of consuming the preload soup. The results indicated better compensation after MSG protein conditions but no differences in intake were found across carbohydrate or control conditions. Chapter 8 assessed MSG and IMP with or without added protein using the same design as Chapter 7 and found reductions in intake in MSG/IMP conditions. This suggests that the flavour of umami plays an important role in the regulation of appetite and intake.