Growing evidence suggests that a predisposition towards obesity and adult chronic diseases can result from a suboptimal early life environment. Maternal exposure to stress or poor diet during fetal development or in the early postnatal period can have long-term health consequences for the offspring. Animal models have been used extensively to better our understanding of the potential developmental origins of adult chronic disease. We previously found that gestational stress or high-fat diet can increase body weight and adiposity as well as impair glucose tolerance in neonatal rats [12
]. In addition, rats exposed to either prenatal stress or maternal high-fat diet gain more weight when weaned to high-fat diet than controls perhaps indicating a deficit in their ability to compensate for the higher energy density of the high-fat chow. These early disturbances appear to be critical in shaping the offspring's metabolism yet the mechanisms involved are unclear.
In these studies we examined the neonatal environment in an attempt to gain a better understanding of possible contributing factors to the observed phenotype. We found that dams maintained on HF diet throughout gestation and lactation spent more time nursing their pups than CHOW fed dams during the first week following parturition. In fact, even in the absence of the dam, we found that within the first postnatal week pups exposed to prenatal stress or maternal high-fat diet will consume more milk in tests of independent ingestion than controls. Analysis of the nutritional composition of milk from HF diet and CHOW-fed dams revealed an increase in total fat content from PND 10-21, and higher gross energy content and protein at PND 21 in milk samples from dams on the HF diet. Together, these differences in maternal behavior, pup ingestive behavior, and early nutritional environment could contribute to lifelong metabolic changes.
The mechanisms underlying metabolic programming are unclear, but it is likely that changes in eating behavior early in life are important for later development. Even accounting for body weight differences, pups exposed to prenatal stress or maternal HF diet increased the amount of milk they will consume in the presence of an unlimited supply. This could have a bearing on how satiety mechanisms are established later in life. Unlike adults, neonatal rats do not respond to the nutritional composition of milk until approximately PND 9 and gastric fill is likely the chief inhibitor of ingestion until this age [36
]. We have previously reported that offspring of STRESS or HF dams have impaired glucose tolerance and gain more weight when challenged with a HF diet post-weaning. These results suggest that gestational stress or maternal HF diet or a combination of the two may be changing the offspring's ingestive behavior as well.
Diet and maternal adiposity have been shown to affect the nutritional composition of the dam's milk [13
]. Certainly large differences between groups in the nutritional composition of the milk provided by the dam could contribute to offspring body weight differences. While there were no differences in milk fat content at PND 3 or 7, from PND 10-PND 21 the milk from HF dams had a significantly higher fat content than that of CHOW dams. It is unlikely, however, that this difference in milk fat content is solely responsible for the pups’ increased body weight. Our previous data has shown the pups’ body weights from HF litters to diverge as early as PND 7 [12
]. Del Prado's 1997 study similarly found offspring body weight increases to precede increases in maternal milk fat content. This group also showed that the dams maintained on a high-energy diet produced more milk at PND 14 [40
] which could be the case with our animals as well. Based on the results of our independent ingestion tests it is possible that given a greater supply of milk the pups from HF or STRESS litters might consume more. Additionally, Schroeder and colleagues have shown that in a genetic model of early-onset obesity (the OLETF rat that lacks a functional CCK1 receptor), differences in suckling efficiency can also be a contributing factor [42
]. Furthermore, a similar study by Doerflinger and Swithers showed that pups from dams exposed to a high-fat diet throughout gestation and lactation initiated independent ingestion of solid food 1 day earlier than those litters given standard chow [43
]. Epidemiological data indicate that infants who start consuming solid foods earlier are at greater risk to develop obesity and other associated metabolic disorders although the underlying mechanisms remain unclear [44
]. Together these results suggest that there are many factors related to milk composition and ingestion, all of which might contribute to body weight differences in neonatal rats.
Finally, in analysis of milk hormone content at PND 21 we found increased insulin concentration in HF-STRESS dam's milk although no differences were found in leptin concentration. Leptin has been shown to be an important trophic factor mediating the development of hypothalamic circuitry [46
]. Another group studying obese and lean rat mothers with a genetic predisposition to obesity (DIO) also found no difference in leptin concentrations between the milk of obese or lean rats [13
]. This group did find insulin levels to be elevated in the milk of both lean and obese DIO rats at PND 7. Here, we report a similar finding in dams maintained on HF diet at PND 21. Previous studies have shown exposure to higher than normal insulin levels can affect the development of hypothalamic circuitry [47
] and that insulin and other circulating factors have been shown to pass through milk and can affect the neonate [48
]. Further studies examining insulin and leptin levels in maternal milk at earlier time points could shed light on this and other potential contributing factors to the pup body weight and food intake differences we have observed.
Importantly, this study has some limitations which should be addressed. First, while ours and other previous studies have shown long-term effects of PS, few developmental differences are reported here. The early-life HF diet exposure results in a much more robust phenotype which is expressed almost immediately. Our previous studies suggest that exposure to PS imparts a susceptibility to later challenges; weaning PS animals to a HF diet results in more weight-gain than control animals that did not have the prenatal stress experience [12
]. Furthermore, future studies may be helpful in understanding maternal-pup interactions and in further analyzing milk composition at earlier time-points. While videotaping ensured we did not disturb the litters, it does not allow for the observation of more detailed behaviors such as the initiation of nursing or potential sex differences. Growing evidence for gender-specific effects of prenatal stress [49
] and early-life diet [51
] necessitate further investigation.