The extremely high LPL mRNA levels in the mixed oil-fed adult group were unexpected; previous work by Semenkovich et al. (1989) compared LPL mRNA levels in epididymal fat pads taken from rats aged 24 d with those from adult rats and found similar levels of expression at the two ages. However, a pronounced increase in rate of cell growth (cell size and number) occurs in epididymal tissue after 21 d (Cryer and Jones 1978). The finding that LPL expression was so much higher in response to the same test meal in mixed oil-fed adult rats compared to those fed the fish oil, despite the groups having similar plasma triglyceride levels, may suggest that the mixed oil-fed rats are under metabolic controls that keep plasma triglyceride levels at normal levels. Adipose tissue LPL mRNA and activity did not follow similar patterns during this study, but they were measured in different tissues due to the small size of epididymal fat pads in the young rats. Adipose tissue LPL activity and mRNA have previously been shown to vary between anatomical sites within one animal (Cryer et al. 1978) and in humans (Ranganathan et al. 1995).
There were no significant differences in plasma insulin and nonesterified fatty acid levels in response to changes in early diet. The only difference in plasma glucose concentrations was a significantly greater glucose level in young compared to adult rats, in the fish oil fed group. Previous studies have shown lower plasma insulin levels following the consumption of a high fish oil diet in rats (Baltzell et al. 1991), but as they did not pair feed the rats, the differences seen could have been due to changes in actual quantities of food intake.
Cholesterol levels were high in the young rats, suggesting that the levels had not dropped during the 2 wk after the high-fat diet exposure during suckling. Reiser and Sidelman (1972) suggested that early exposure of rats to a high-cholesterol diet initiated mechanisms that maintained serum cholesterol at lower concentrations later in life. The high cholesterol levels in the young rats and those on in the fish oil-fed groups supports previous findings (D’Aquino et al. 1991, L’Abbé et al. 1991).
Catalase activities previously have been reported to increase in response to fish-oil feeding (Yamazaki et al. 1987). In this study, the elevated catalase activities seen in the young animals fed fish oil continued into adulthood. This suggests that these rats remained under oxidative stress despite the cessation of fish-oil feeding after 5 wk of age. We also found raised superoxide dismutase activity in the fish oil-fed young, in contrast to previous studies on (n-3) PUFA that have shown either reduced activity (L’Abbé et al. 1991) or no significant difference (D’Aquino et al. 1991). However, the absence of a dietary effect on glutathione reductase activity confirms the findings of L’Abbé et al. (1991) who found no effect of fish-oil feeding.
In conclusion, the majority of differences shown in this study were between the two age groups rather than between the two diet groups at each age. This suggests that changes in the postprandial handling of a standard test meal are more affected by age than by early dietary fatty acid composition. However, early diet did affect LPL expression and catalase activity later in life, which may indicate important biological imprinting mechanisms for these fatty acids, especially in the areas of the control of gene expression and in the handling of oxidative stress. Oxidative load, in particular, has been associated with premature aging and has been suggested by Van Assche et al. (1998) as a possible link between dietary insufficiency in fetal life and adverse consequences in later life.
Future studies are required to elucidate the mechanisms of these effects. If, through changes in dietary availability or formulation, potentially beneficial alterations in gene expression can be shown in rodents and other animal models, there would be grounds on which to explore the effects of modifying diet in pregnant women and possibly formula composition for infants.