Whole Egg Consumption Impairs Insulin Sensitivity in a Rat Model of Obesity and Type 2 Diabetes

Abstract

Background: The literature regarding the relation between egg consumption and type 2 diabetes (T2D) is inconsistent and there is limited evidence pertaining to the impact of egg consumption on measures of insulin sensitivity.

Objectives: The aim of this study was to investigate the effect of dietary whole egg on metabolic biomarkers of insulin resistance in T2D rats.

Methods: Male Zucker diabetic fatty (ZDF) rats (n = 12; 6 wk of age) and age-matched lean controls (n = 12) were randomly assigned to be fed a casein- or whole egg-based diet. At week 5 of dietary treatment, an insulin tolerance test (ITT) was performed on all rats and blood glucose was measured by glucometer. After 7 wk of dietary treatment, rats were anesthetized and whole blood was collected via a tail vein bleed. Following sedation, the extensor digitorum longus muscle was removed before and after an intraperitoneal insulin injection, and insulin signaling in skeletal muscle was analyzed by Western blot. Serum glucose and insulin were analyzed by ELISA for calculation of the homeostatic model assessment of insulin resistance (HOMA-IR).

Results: Mean ITT blood glucose over the course of 60 min was 32% higher in ZDF rats fed the whole egg-based diet than in ZDF rats fed the casein-based diet. Furthermore, whole egg consumption increased fasting blood glucose by 35% in ZDF rats. 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, HOMA-IR and skeletal muscle insulin signaling, regardless of experimental dietary treatment.

Conclusions: 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.

Keywords: diabetes; egg consumption; insulin resistance; insulin signaling; rat; whole egg.

FIGURE 1

ITT blood glucose in lean and ZDF rats fed a casein- or whole egg–based diet for 5 wk. Data are means ± SEMs; n = 3–6. Data within the same time point without a common letter differ (P < 0.05). Three-factor repeated-measures ANOVA: time, P < 0.001; diet, P = 0.027; genotype, P < 0.001; time × diet, P = 0.662; time × genotype P = 0.031; diet × genotype P = 0.025; Time × diet × genotype, P = 0.572. ITT, insulin tolerance test; ZDF, Zucker diabetic fatty.

FIGURE 2

Skeletal muscle p-IR β^Tyr1150/1151 (A) and representative Western blot images of skeletal muscle p-IR β^Tyr1150/1151 and total protein (B) pre- and post-insulin injection in lean and ZDF rats fed a casein- or whole egg–based diet for 7 wk. Data are expressed relative to pre-insulin p-IR β^Tyr1150/1151 in lean rats fed the casein-based diet. Data are means ± SEMs; n = 5–6. Bars without a common letter differ (P < 0.05). Three-factor mixed ANOVA: insulin, P = 0.029; diet, P = 0.492; genotype, P = 0.874; insulin × diet, P = 0.297; insulin × genotype P = 0.169; diet × genotype P = 0.723; insulin × diet × genotype, P = 837. LC-Pre, lean casein pre-insulin; LC-Post, lean casein post-insulin; ZC-Pre, ZDF casein pre-insulin; ZC-Post, ZDF casein post-insulin; LWE-Pre, lean whole egg pre-insulin; LWE-Post, lean whole egg post-insulin; ZWE-Pre, ZDF whole egg pre-insulin; ZWE-Post, ZDF whole egg post-insulin. p-IR, phosphorylated insulin receptor; ZDF, Zucker diabetic fatty.

FIGURE 3

The ratio of skeletal muscle p-Akt^Ser473:total Akt (A) and representative Western blot images of skeletal muscle p-Akt^Ser473, total Akt, and total protein (B) pre- and post-insulin injection in lean and ZDF rats fed a casein- or whole egg–based diet for 7 wk. Data are expressed relative to the pre-insulin p-Akt^Ser473:total Akt ratio in lean rats fed the casein-based diet. Data are means ± SEMs; n = 5–6. Bars without a common letter differ (P < 0.05). Three-factor mixed ANOVA: insulin, P < 0.001; diet, P = 0.53; genotype, P = 0.157; insulin × diet, P = 0.571; insulin × genotype P = 0.11; diet × genotype P = 0.535; insulin × diet × genotype, P = 0.609. LC-Pre, lean casein pre-insulin; LC-Post, lean casein post-insulin; ZC-Pre, ZDF casein pre-insulin; ZC-Post, ZDF casein post-insulin; LWE-Pre, lean whole egg pre-insulin; LWE-Post, lean whole egg post-insulin; ZWE-Pre, ZDF whole egg pre-insulin; ZWE-Post, ZDF whole egg post-insulin. p-Akt, phosphorylated Akt; ZDF, Zucker diabetic fatty.