4.1. Postprandial Satiety
Results from this study showed that when PGX in the form of granules or softgel was co-consumed with a standard meal, the postprandial satiety (AUC) was higher, compared to WD co-consumed with the same meal. Several factors may have contributed to the higher satiety response of the PGX meals; these include the mixing and hydration of PGX with fluids in the stomach, the rate of dissolution of PGX, and the final viscosity of PGX before and after stomach emptying.
Understanding the mechanism of how a fibre, such as PGX, affects satiety is complex, particularly in mixed meal studies [34
] as in the present research. The mechanism and timeframe for dissolution of water-soluble polysaccharides, and the rate and magnitude of viscosity developed, are key factors influencing the effectiveness of soluble fibre on increasing satiety [20
]. Different soluble viscous fibres have different hydration rates which impact the ability and time that it takes for maximum viscosity to be reached, particularly when they are consumed in the unhydrated form. Some guar gums take up to five hours to reach 60% of their maximum viscosity, which is beyond the time relevant to stomach emptying of 4.5 h [20
]. Subjects in this research consumed 500 mL water with the PGX meals and the viscosity results presented suggests that the meals containing PGX could become viscous in the stomach. In the meal containing the non-viscous WD, any resulting viscosity in the stomach would only be provided by the breakfast cereal in the standard meal. Research investigating the development of viscosity from polysaccharides [20
] showed PGX forms a viscous matrix in sufficient time to influence feelings of fullness. In a study examining the effect of konjac glucomannan addition, on in vitro
dairy protein digestion, it was reported that the konjac glucomannan addition led to high apparent viscosity aggregates [36
] suggesting that the konjac glucomannan in PGX could also contribute to higher viscosity aggregates during digestion. Furthermore, in the same study of adding hydrocolloids to dairy proteins in vitro
], the addition of alginate was hypothesised to have slowed down the rate of protein digestion and therefore delayed gastric emptying; both of these findings support the postprandial satiety effect of PGX.
Peptide YY (PYY) is a physiological gut-derived satiety hormone, which slows gastric emptying, regulates appetite, and is released in response to a meal [37
]. PYY has been shown to increase in the blood in the first 15 min following PGX consumption and peaks at 1–2 h [38
]. The effects of increased PYY may therefore have contributed to the satiety effect of PGX found in the present study. Another mechanism for elevated PYY levels after PGX consumption may be via PGX microbial fermentation, as products of fermentation may stimulate the release of PYY [30
When considering the satiety effects of the different forms of PGX, granules versus softgel, the nature of the food with which they are consumed must also be considered. The mouthfeel, flavour, and odour of the food may influence the pleasantness and palatability of the two forms of PGX and this in turn may have influenced the satiety responses [40
]. In the present study, the satiety effect of PGX did not differ from that of PGX softgel, however since the PGX in the softgels demonstrated higher viscosity, this form of PGX may have greater potential to provide increased satiety under different meal conditions. It was observed by the researchers that there was little change to the structure of the PGX granules during the meal consumption time (<12 min) indicting a lack of full hydration. This was possibly due to the fibre being dispersed in the breakfast cereal flakes and the milk being cold (5 °C) limiting fibre hydration and swelling. Alternately, the breakfast cereal may have been hydrating more rapidly and thus competed for the water in the meal. Subjects in the present research consumed 500 mL of water with the standard meal, which assisted in the hydration of the PGX and formation of a viscous matrix in sufficient time to influence feelings of fullness. The time taken for the PGX to fully hydrate and reach maximum viscosity, however appears to be longer than 120 min and therefore it is suggested that full hydration of the PGX may produce different effects from the two PGX forms, granules and softgels, on satiety. PGX softgels have been found to be effective at providing extended satiety effects in previous research where subsequent food intake was measured [32
]. A longer postprandial period for satiety evaluation after PGX consumption, greater than 120 min, may therefore result in higher satiety of the softgel compared to the granular form due to the combined effects of higher viscosity development from the softgel form slowing down digestion [36
] and fermentation [39
4.2. Postprandial Glycaemia
The lower peak blood glucose “spike” for PGX compared to WD may be in part due to the higher viscosity of PGX; slowing gastric emptying and reducing the rate of starch digestion and glucose absorption in the small intestine [36
]. The higher blood glucose level for PGX at 120 min compared to the WD meal however, may have been due to the delayed release of glucose from starch digestion. The management and prevention of impaired glucose tolerance and type 2 diabetes requires the avoidance of postprandial “spikes” and a more gradual drop of blood glucose over time [42
], thus the postprandial response for PGX indicates its potential benefits in relation to type 2 diabetes prevention and management.
The glycaemia results from the present study are supported by those of Brand-Miller et al.
] who reported that consumption of PGX with a bread meal was effective in reducing the postprandial glycaemic response and that consuming PGX with water, as part of a meal, reduced the GI of the meal. Additional support for the present findings on PGX can be found in studies of other viscous fibres, such as β-glucan, which improved postprandial glycaemia due to the fibre’s ability to increase the viscosity within the gut [43
]. The ability of PGX to maintain plasma glucose concentrations after a standard meal within a relatively narrow range, compared to WD, suggests that this highly viscous soluble fibre may have a clinically significant role in long term blood glucose control which warrants further research, particularly in at-risk subjects.
4.3. Trained Panel Results
The subjects in the present study were trained in an effort to carefully control factors other than the treatments which may influence satiety. Measurements relating to feelings of fullness are highly subjective and depend on a number of physiological, psychological, and social factors, which makes assessment of satiety challenging due to lack of precision. During training, all subjects consumed 45 g of breakfast cereal which resulted in a degree of fullness as recorded on the LMS and indicated the 45 g meal was sufficient to alleviate feelings of hunger in all of the subjects. In previous research by Solah et al.
], training of the subjects improved the discriminatory power to detect differences in the satiety effect between PGX and inulin, and reduced between subject variability. Training may have contributed to the low standard error of mean (SEM) in this study (Figure 3
). The low SEM for postprandial satiety for the PGX meal versus the WD meal is reflected in low SEM for postprandial glycaemia values.
The LMS was considered to provide better discrimination of satiety sensations compared to a VAS for the trained panel used in this research. Furthermore, as a result of training, there is an expectation that the trained panel can detect differences in the feelings of fullness after consuming meals of different volumes or compositions. During training, “feelings of hunger” are considered separate to “feelings of fullness” so the “mean fullness score” was not adjusted using the fasting hunger score. The first reason for not adjusting the fullness score was related to the purpose of the training. For example, to adjust a fullness score of 3.5 for the WD meal to a minus value, using the fasting score, implies subjects were not feeling a degree of fullness at 15 min. Secondly, fasting triglycerides and satiety are more variable than postprandial triglycerides and satiety results [45
]. Previous research has shown that fasting triglycerides levels have a high intra-individual day-to-day variability of 15% to 30% [45
]. Although training was undertaken to carefully control factors other than the treatment which may influence satiety, the effect of difference related to gender and weight [48
] may not be alleviated by training.