Impact of whole egg consumption on nutrient homeostasis in metabolic disease

Metabolic diseases, such as obesity and type 2 diabetes (T2D), are characterized by aberrant nutrient metabolism, such as disrupted metabolism of vitamin D and methyl groups. Whole eggs are a source of several nutrients, including vitamin D, B vitamins and choline, which may assist in the maintenance of micronutrient balance. Additionally, the high-quality protein content of whole eggs may contribute to satiety and body weight management. However, the relation between egg consumption and measures such as insulin sensitivity, glycemic control and cardiovascular risk factors in individuals with obesity, T2D and other metabolic abnormalities remains inconsistent. The objectives of the studies described in this dissertation were to evaluate the impact of whole egg consumption on 1) vitamin D homeostasis and body weight gain in T2D rats, 2) metabolic biomarkers of insulin resistance in T2D rats and 3) homocysteine metabolism in a rat model of hyperhomocysteinemia. The first study described in this dissertation compared dietary whole egg to supplemental cholecalciferol with respect to vitamin D balance, weight gain, and body composition in T2D rats. Male Zucker diabetic fatty (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 were matched for lipid quantity to account for the additional lipid contributed by the 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 25-hydroxyvitamin D [25(OH)D] concentrations than lean rats fed CAS. In ZDF rats, WE consumption increased serum 25(OH)D concentrations 130% compared to CAS, whereas consumption of CAS+D increased serum 25(OH)D concentrations by 35% compared to CAS. Our data suggest that dietary consumption of whole egg is more effective than supplemental cholecalciferol in maintaining vitamin D status in T2D rats. Furthermore, whole egg consumption reduced weight gain and body fat percentage in obese T2D rats, without an effect on body weight parameters in the lean phenotype. These data may support new dietary recommendations targeting body weight management and prevention of vitamin D insufficiency in T2D. The objective of the second study was to perform a follow-up, dose-response study to determine the minimal amount of dietary whole egg effective at maintaining vitamin D balance and attenuating the obese phenotype in T2D rats. A secondary objective of this study was to determine the effect of varying concentrations of whole egg on serum trimethylamine N-oxide (TMAO), a candidate cardiovascular disease risk factor. Male ZDF rats (n= 40) and their lean controls (n=40) were randomly assigned to a diet containing 20% casein (CAS), 20% egg protein (20% EGG), 10% egg protein (10% EGG), 5% egg protein (5% EGG) or 2.5% egg protein (2.5% EGG) for 8 weeks. All diets contained 20% total protein (w/w). The 20% EGG diet maintained vitamin D balance in ZDF rats, whereas the 10, 5 and 2.5% EGG diets did not prevent vitamin D insufficiency. Body weight gain was reduced by 29% and 31% in ZDF rats consuming 20% EGG and 10% EGG diets, and by 16% and 12% in ZDF rats consuming 5% EGG and 2.5% EGG diets compared to the CAS diet. All EGG diets increased serum TMAO, regardless of genotype, although a greater increase was observed in the ZDF genotype. These data demonstrate that whole egg consumption attenuates weight gain in a dose-dependent manner in T2D rats. Additionally, egg consumption increases circulating TMAO concentrations in both lean and T2D rats, with a heightened TMAO response in T2D rats. In the third study described in this dissertation, the effect of dietary whole egg on metabolic biomarkers of insulin resistance was evaluated in T2D rats. Male ZDF rats (n=12) and their lean controls (n=12) were randomly assigned to a casein- or whole egg-based diet. At week 5 of dietary treatment, mean blood glucose over the course of an insulin tolerance test was 32% higher in ZDF rats fed the whole egg-based diet compared to ZDF rats fed the casein-based diet. After 7 weeks of dietary treatment, whole egg consumption increased fasting blood glucose by 35% in ZDF rats. Furthermore, insulin-stimulated phosphorylation of key proteins in the insulin signaling pathway did not differ in skeletal muscle of ZDF rats fed casein- and whole egg-based diets. In lean rats, no differences were observed in insulin tolerance or skeletal muscle insulin signaling, regardless of experimental dietary treatment. These data suggest that whole body insulin sensitivity may be impaired by whole egg consumption in T2D rats, although no changes were observed in skeletal muscle insulin signaling that could explain this finding. The objective of the final study presented in this dissertation was to determine the effect of whole eggs and egg components (i.e. egg protein and choline) with respect to homocysteine balance in a folate-restricted rat model characterized by moderate hyperhomocysteinemia. Furthermore, this study sought to determine the differential effects of whole eggs, egg protein or supplemental choline on the hepatic expression and activity of betaine-homocysteine S-methyltransferase (BHMT) and cystathionine -synthase (CBS), key enzymes in homocysteine metabolism. Male Sprague Dawley rats (N=48) were randomly assigned to a casein-based diet (n=12), a casein-based diet supplemented with choline (1.3%, w/w; n=12), an egg protein-based diet (n=12), or a whole egg-based diet (n=12). At week 2, half of the rats in each of the 4 dietary groups were provided a folate-restricted (FR; 0 g folic acid/kg) diet and half continued on the folate-sufficient (FS; 0.2 g folic acid/kg) diet for an additional 6 weeks. All diets contained 20% (w/w) total protein. Folate-restricted casein-fed rats exhibited a 53% increase in circulating homocysteine concentrations compared to FS rats fed a casein-based diet. In contrast, consumption of egg protein prevented hyperhomocysteinemia in FR rats compared to FR rats fed the casein-based diet. Hepatic BHMT activity was increased by 45% and 40%, respectively, by the egg protein-based and whole egg-based diets compared to the casein-based diets. These data demonstrate that dietary intervention with egg protein prevented elevated circulating homocysteine concentrations in a rat model of hyperhomocysteinemia, due in part to upregulation of hepatic BHMT. These data may support the inclusion of egg protein for dietary recommendations targeting hyperhomocysteinemia prevention