Results and Discussion
We found that ascorbic acid synthesis (GLO enzyme activity) was occurring in the liver of fetal pigs from 60 to 111 d of gestation (Figure 1), however, its rate of synthesis seemed to decline (P < 0.01) quite dramatically as pregnancy progressed. This decline was more noticeable, however, between the 60 and 100 day period whereupon it plateaued. Although we did not measure the GLO enzyme activity prior to the 60-day period, it is probable that fetal liver GLO activity may have been even higher. Fetal ascorbic acid requirements would be expected to be high during early fetal development because of the need for collagen and other cellular constituents that require the vitamin for their synthesis.
In contrast to the decline in fetal ascorbic acid synthesis, the amount of ascorbic acid in liver tissue increased (P < 0.01) during pregnancy (Figure 2). This suggests that the sow provided increasing amount of ascorbic acid to fetal pigs as gestation progressed and the need for the fetus to synthesize the vitamin declined. Other unpublished data demonstrated that sow ascorbic acid synthesis had a tendency to increase as pregnancy progressed. These combined results suggest that with sows synthesizeing an increasing amount of ascorbic acid during gestation they apparently transferred it to the fetus. Consequently, during early pregnancy the fetus was dependent upon its own synthesis of the vitamin, but as gestation progressed the dependence seemed to be that ascorbic acid was transferred to the fetus by the sow.
From birth to 3 day of age, the pig liver GLO activity declined further by approximate 66% where it was maintained at a relatively constant activity until weaning (Figure 1). After weaning (day 14), the pig's liver GLO activity then seemed to increase by three fold within a 2-week period.
While pigs were nursing the sow, liver ascorbic acid concentration declined, but there was an even more marked decline during the first week postweaning whereupon it then began to increase.
Ascorbic acid concentration in pig serum followed a similar trend as liver ascorbic acid (Figure 3). Serum ascorbic acid concentration declined (P < 0.01) from 3 to 14 day of age but the greater decline (approximately three fold) occurred after pigs were weaned. The decline during the nursing period may have been at least partially attributed to the decline in sow milk ascorbic acid concentrations as lactation progressed. These combined results suggest that the pigs ability to synthesize vitamin C increased after the source of the vitamin had been eliminated (i.e., milk), which resulted in the decline of ascorbic acid in the serum and liver postweaning.
The combined milk data from the sows of these two experiments showed that colostrum and milk ascorbic acid concentrations declined (P < 0.01) from 0 to 24 day postpartum (Figure 4). This demonstrates that colostrum and milk are the principal sources of ascorbic acid for the pig. Consequently, the high concentration of the vitamin in these milk supplies may have resulted in the suppression of liver GLO activity in young pigs during the nursing period.
Pig birth order did not appear to affect the pig's ability to synthesize ascorbic acid, but it did effect the ascorbic acid concentration in the liver at birth. Liver ascorbic acid concentration was therefore higher (P< 0.01) when pigs were born early compared with that born later (Figure 5). Previous research (Brown, 1984) had indicated that the farrowing process was stressful on both the sow and her progeny and that ascorbic acid declined in the young pigs during uterine contractions. Our data confirm this response and indicates that pigs that are born later in the birthing sequence may have had more stress imposed upon them during the birth process and therefore more liver ascorbic acid may have been metabolized.
Lutalyse injection into sows on day 112 gestation seemed to have no affect on their progeny's ability to synthesize ascorbic acid. Pigs, however that were born 2 to 3-days early had lower liver ascorbic acid concentrations (P < 0.05) when compared with pigs born at the normal delivery age (Figure 6). It is possible that less maternal ascorbic acid had been transferred to the fetuses during the latter days of gestation, which resulted in this lower tissue ascorbic acid concentration.
At weaning the pig liver ability to synthesize vitamin C was relatively low at the three (10, 17, or 24) weaning ages (Figure 7). However, within 2 weeks after pigs were weaned, the GLO enzyme activity in each of the three pig groups increased (P < 0.01) by approximately three fold. Pigs that were weaned at the older age (i.e. 24 d) seemed to have a higher rate of liver enzyme activity during the initial weeks postweaning than those weaned earlier. These results suggest that pig liver GLO enzyme activity or ascorbic acid synthesis seemed to be affected more by weaning age than by chronological age. The data further suggests that when the pig was nursing the sow the synthesis of the vitamin was low and received an ample supply from the dams milk.
As our previous data showed pig serum ascorbic acid concentrations declined markedly during the initial week postweaning in all three pig weaning groups (Figure 8). This decline probably reflected the loss of ascorbic acid from the milk supply and the initial lack of ascorbic acid synthesis by the young pig immediately upon weaning. After the initial week postweaning serum ascorbic acid remained low but there was an indication that its ability to synthesize ascorbic acid increased. Supplementing ascorbic acid to the diets of weaned pig may therefore have the greatest benefit during the initial period postweaning. Studies are currently underway to evaluate this prospect and initially suggests that vitamin C in the phase 1 diet may result in a growth response to supplemental vitamin C.