Date of Award
Doctor of Philosophy
Food Science and Human Nutrition
Vitamin D deficiency has been reported to affect 30% of the American population, owing to poor dietary intake and insufficient sun exposure. Pregnant and lactating women, the elderly, and individuals with increased skin melanin pigmentation are at especially high risk for developing poor vitamin D status. The classical, hormonal actions of vitamin D related to mineral metabolism and skeletal health are well-established. More recently, however, evidence suggests that vitamin D deficiency increases the risk for chronic diseases such as obesity, diabetes, cancer and autoimmune disorders. Dietary eggs represent a natural whole food source of vitamin D3 as well as 25-hydroxycholecalciferol (25D), the vitamin D metabolite that represents an individual’s status. The objective of the studies described in this dissertation were to determine if a whole egg-based diet could maintain vitamin D status in 1) Zucker Diabetic Fatty (ZDF) rats, a well-characterized animal model of type 2 diabetes (T2D), 2) Streptozotocin-induced type 1 diabetic (T1D) rats, and 3) in Sprague Dawley rats with dextran sulfate sodium-induced colitis.
In the first study described in this dissertation, male ZDF rats (n = 12) and their lean counterparts (n = 12) were randomized to one of two dietary treatment groups, a casein- or whole egg-based diet, for 8 weeks. Both diets contained 25 μg cholecalciferol/kg diet, provided by the vitamin mix. The whole egg-based diet contained an additional 12.6 μg cholecalciferol/kg diet; thus, the whole egg-based diet provided a total of 37.6 μg cholecalciferol/kg diet. Both diets provided protein at 20% (w/w) and contained the same lipid content by the addition of corn oil to the casein-based diet to match the lipid contribution by the addition of whole egg. Whole egg consumption attenuated both hyperglycemia and hypertriglyceridemia, as well as reduced weight gain in ZDF rats compared to casein-fed diabetic rats. Circulating 25D was lower in casein-fed ZDF rats compared to lean controls; however, ZDF rats fed whole egg exhibited the same circulating 25D concentrations as casein-fed lean rats. These data suggest that dietary whole egg can attenuate metabolic anomalies, as well as maintain normal circulating 25D concentrations in T2D rats.
This second study described in this dissertation compared whole egg consumption to supplemental cholecalciferol with respect to vitamin D balance, body weight gain, and body composition in T2D rats. Male ZDF rats (n = 24) and their lean controls (n = 24) were randomly assigned to one of 3 dietary treatment groups: a casein-based diet (CAS), a dried whole egg-based diet (WE), or a casein-based diet containing supplemental cholecalciferol (CAS+D) at the same level of cholecalciferol provided by the dried whole egg-based diet (37.6 μg/kg diet). All diets provided protein at 20% (w/w) and contained the same lipid contribution by the addition of whole egg. Rats were fed their respective diets for 8 weeks. Weight gain and percent body fat were reduced by approximately 20% and 11%, respectively, in ZDF rats fed WE compared to ZDF rats fed CAS or CAS+D. ZDF rats fed CAS had 21% lower serum 25D concentrations than lean rats fed CAS. In ZDF rats, WE consumption increased serum 25D concentrations 130% compared to CAS, whereas consumption of CAS+D increased serum 25(OH)D concentrations 35% compared to CAS. Our data suggest that dietary consumption of whole egg is more effective than supplemental cholecalciferol in maintaining circulating 25D concentrations in T2D rats. Furthermore, whole egg consumption reduced weight gain in obese T2D rats, without effect on body weight in a lean phenotype. These data may support new dietary recommendations targeting obesity and prevention of vitamin D insufficiency in T2D.
The objective of the third study was to investigate the impact of whole egg consumption in T1D rats. Male Sprague Dawley rats were randomly assigned to either a casein-based (n = 12) or a whole egg-based diet (n = 6) for 32 days. Both diets provided protein at 20% (w/w) and contained the same total lipid content by the addition of corn oil to the casein-based diet to match the lipid contribution by the addition of whole egg. The vitamin mix in both diets provided 25 μg cholecalciferol/kg diet. The whole egg-based diet contained an additional 12.6 μg cholecalciferol/kg diet; thus, the whole egg-based diet provided a total of 37.6 μg cholecalciferol/kg diet. On day 26, all rats in the whole egg-based diet group and half of the rats on the casein-based diet received a streptozotocin injection to induce T1D for the final week of the experimental period. Whole egg consumption attenuated polyuria, proteinuria and renal hypertrophy in T1D rats. These data suggest that dietary intervention with whole egg may offer renal protection in T1D. Understanding the mechanism underlying the nephroprotective effect of dietary whole egg will be a focus of future work.
The goal of the fourth and final study presented in this dissertation was to investigate the impact of whole egg consumption in dextran sulfate sodium (DSS)-induced colitis. In an initial dose response study, male Sprague Dawley rats (N= 24) were maintained on a casein-based diet for 5 weeks and randomly assigned to 0, 3, 4 or 5% DSS-treated drinking water for the final 7 days of the study. Serum 25D concentrations exhibited a dose-response decrease with respect to increasing DSS concentrations. In a follow-up study, Sprague Dawley rats (N=36) were randomly assigned to a casein-, whole egg- or a casein-based diet containing supplemental cholecalciferol at the same level of cholecalciferol provided by the dried whole egg-based diet (37.6 μg/kg diet). All diets provided protein at 20% (w/w) and contained the same lipid contribution by the addition of whole egg. Rats were fed their respective diets for 8 weeks. For the final 7 days of the study, half of the rats in each group were given 3.5% DSS-treated drinking water, based on the results of initial dose response study. Serum 25D concentrations were the same between rats fed the casein-based diet and casein-based diet containing supplemental cholecalciferol. Rats fed a whole egg-based diet, however, exhibited increased serum 25D concentrations that were significantly higher than rats in either the other dietary intervention groups, regardless of colitis status. These data suggest that whole egg consumption may be more effective than supplemental cholecalciferol at increasing circulating 25D concentrations in experimental colitis.
The studies described in this dissertation indicate that whole egg consumption is an effective means of increasing serum 25D concentrations in animal models of diabetes and inflammatory bowel disease. Future dose response studies are needed to identify the specific quantity of egg consumption that is efficacious with respect to influencing vitamin D status and disease complications.
Pritchard, Samantha, "Impact of whole egg consumption and maintenance of vitamin D homeostasis in disease" (2018). Graduate Theses and Dissertations. 17292.