Meal Patterns & Circadian Rhythmicity

The high-fat diets did not disrupt the temporal sequence of feeding (diurnal-nocturnal partitioning). With all diets, diurnal meals were consistently smaller and less frequent than nocturnal ones, which was to be expected, as rats are well-known for their nocturnal feeding activity (Strubbe & Woods, 2004). Although nocturnal feeding frequency decreased over the course of the study, in that rats took progressively fewer meals, the rate of feeding at each meal increased, while meal duration remained unchanged. This suggests that, as if to compensate for feeding less often, rats increased the amount they ate at each visit to the hopper, rather than increasing the amount of time spent eating, but not enough to prevent an overall reduction in food intake. The constancy of meal lengths also corresponds to unchanging glucose concentrations across the study in all groups, as blood glucose levels correlate with hunger, determining the start and termination of meals (Strubbe & Woods, 2004). Studies of rat models of both diet-induced and monogenic forms of obesity mirror findings in humans that obesity correlates directly with meal size, but inversely with meal frequency (Hariri & Thibault, 2010; Hinney et al., 2010). Specifically, consumption of high-fat diets over a five-month period have been shown to lead to hyperphagia in male outbred rats through an increase in meal size, not number (Farley et al., 2003). Although this pattern was shown in the high-fat-fed groups in the current study, its manifestation in the control group as well suggest that, at least in terms of influencing feeding behaviour, the diet compositions were not sufficiently different, such that, at the given concentrations, sucrose and fatty acids have similar effects. It is also possible that in only two months, group differences had not had time to develop.

Although the high-fat diets appeared to have little measurable impact on most aspects of feeding activity, importantly, they were distinguished by their effects on satiety. This is to be expected, as FAs are known to be metabolised differently depending on their degree of saturation, which differentially affects satiety in humans (Blundell et al., 1993; Blundell & Macdiarmid, 1997; Fernandez-Quintela et al., 2007). The enhancement of high-PUFA feeding in daytime satiety over the course of the study substantiates findings on PUFA-induced satiety in humans (Lawton et al., 2000); for example, when consumed for three months, omega-3 FAs will suppress appetite in humans, and the mechanism of this action is thought to

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occur through the associated elevation of satiety signal levels (Scharrer, 1999). However, the novelty of the current result means that there are no previous findings to explain the significance of enhanced satiety occurring during the day, rather than the night. The circadian effects of high-SFA diets have been examined, on the other hand, but findings are inconsistent. In some studies, high-SFA diets have been shown to increase food intake in rodents during the diurnal phase (Kohsaka et al., 2007; Kaneko et al., 2009), but to be less effective in producing satiety at the end of a feeding cycle (Geliebter, 1979; Walls et al., 1992). This is thought to affect metabolism, leading to obesity (Arble et al., 2009). Thus, it is unclear how the PUFA-induced satiety can coexist with the observed increase in adiposity, without being balanced, for example, by a reduced satiety at night. This is, in fact, what was observed (Figs. 3.12B, D & F), but again, only slightly below that of SFA-fed animals, possibly due, once again, to the SFA content in the PUFA diet. In other studies, rodents consuming diets very high in SFAs (67% energy from fat) consume fewer meals in the day than controls, consistent with increased IMI and satiety ratio (Hariri & Thibault, 2010). This, too, was observed here, but only subtly. What is perhaps most striking overall when considering the three diets together is that, although the shapes of the IMI and satiety ratio curves (Figs. 3.12A-D), and therefore, the progress of change in satiety differs, they all converge at the end of the study, supporting, once again, the idea of adaptation to the high-fat diets.

The rat consumes 90% of its energy intake in several discrete meals throughout the nocturnal phase (Armstrong, 1980; LeMagnen, 1981), with peaks occurring towards the beginning and end of this period (Siegal, 1961). This pattern alters in rats fed diets very high in fat (86% of energy provided as lard), as meals tend to shift towards the latter part of the dark phase (Tempel et al., 1985). Food ingested throughout the first half of the nocturnal phase may be utilised to fulfil any immediate energy requirements, as well as for lipogenesis, whereas feeding at the end of the nocturnal phase may be more anticipatory of the diurnal phase to follow, and therefore, serves to build energy reserves. Indeed, the rats’ stomach contains large amounts of undigested food at the end of the nocturnal phase (Armstrong et al., 1978). Therefore, a high energy meal may be consumed at the end of the nocturnal phase in preparation for the 12-hour diurnal period when little feeding activity occurs (Tempel et al., 1989). This shift was not observed here, but this could be due to the

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more moderate fat content. Moreover, the characteristic peaks described above, towards ‘lights on’ and ‘lights off’, were not obvious in the control group either, again casting doubt on the validity of the control diet and suggesting that normal feeding periodicity may not actually have been observed in this study. Finally, the temporary clustering of meals in the middle hours of the night in the high-fat-fed groups’ mid-way through the study may have been instrumental in stimulating accumulation of fat mass. Early nocturnal meal skipping has recently been shown to alter the peripheral clock and increase lipogenesis in mice (Yoshida et al., 2012).