The primary aim of this thesis was to investigate the characteristics of individuals defined by their contrasting habitual selection of high fat and low fat foods. Previous
studies have applied the term phenotype to this classification.
The first study aimed to further characterise the eating patterns of lean high fat (HF-L) and low fat (LF) consumers. It was shown that HF-L consumed much higher amounts of fat (and energy) during the latter part of the evening (Ch 5). HF-L have to deal with this metabolic load late on in the day and during sleep. This may be the reason for a somewhat inferior sleep quality (reflected in a higher heart rate).
The second study aimed to investigate i) whether HF-L and LF have different patterns of physiological satiety signals (and therefore a different modulation of appetite control)
and ii) to examine the impact of short-term changes to the habitual diet on hunger and satiety. HF-L and LF were both found to be less hungry following consumption of the meal most dissimilar to their habitual diet; perhaps indicating an up-regulation of satiety signals related to nutrients least often consumed (Ch. 6). No significant difference on
food preference tests was found between the two groups, indicating that the development of the dietary `habit' may be independent of current preference, at least for
normal weight individuals. HF-L were shown to significantly increase energy and fat intake following a high fat preload (Ch. 7), indicating a tendency toward overconsumption under certain conditions (independent of hedonic response), and
therefore representing a risk factor for obesity. In HF consumers high fat foods seemed to further disinhibit appetite leading to enhanced consumption.
The aim of the third study (Ch. 8) was to define a cluster of characteristics that might ultimately diagnose susceptibility and resistance to dietary induced obesity. A third high fat-overweight phenotype (HF-O) was introduced in order to further investigate how body weight might be influenced by a high fat diet. The results of the free-living study found that both HF-O and HF-L appear to make similar habitual dietary choices in terms of energy and macronnutrient intakes, yet have (by definition) very different body compositions. One issue raised here concerns the validity of the instrument used to define dietary intakes. For example, when using the FFQ, a selection of larger portion sizes would give rise to an underestimation of energy intake from this tool. This may be the case with HF-O who were found to score more highly on the TFEQ disinhibition factor.
Studies in Ch. 9 and Ch. 10 indicated that HF-O actually consume greater amounts of food than HF-L and LF in a test meal situation, and display heightened hedonic responses to certain high fat foods. It may be assumed that the pleasure yielded by food influences the expression of food preferences and that these factors are important in
influencing food choices (confirmed by descriptive reports in Ch. 10). If food is perceived as more pleasant then this would stimulate more eating through an increased sensation of hunger and a consequent weakening of satiety.
A form of qualitative analysis was applied in Ch. 10 to explore motives to eat outside of laboratory conditions. Interestingly, attitudes and intentions were poor predictors of their actual behaviour, suggesting that food choice is not under socio-cognitive control. Therefore a more unconscious or psychobiological explanation of human food selection, emphasising the interaction between biology and behaviour, may be appropriate.
The final study (Ch. 11) specifically targeted motives for eating based on the distinction between liking and wanting.
What characterises individual's susceptibility to weight gain? HF-O receive pleasure from eating but in comparison to their lean counterparts this might be termed a `super'
sensitive hedonic response to food and particularly to high fat (savoury) foods. They show a directed preference for high fat foods coupled with increased levels of disinhibition. This susceptibility to overeating along with the hedonic response to food may be responsible for the increase in meal size demonstrated in experimental
situations. Small but consistent overeating is probably one of the main reasons why some individuals are overweight. It appears to be the case that psychological and physiological dispositions of the subject will effect preferences and responses to the qualities of foods (Stubbs et al, 2002), and that the interaction between behavioural and metabolic responses influence weight gain (Pagliassotti et al, 1997).