Insulin resistance (IR) is commonly observed during aging and is at the root of many of the chronic nontransmissible diseases experienced as people grow older. Many factors may play a role in causing IR, but diet is undoubtedly an important one. Whether it is total caloric intake or specific components of the diet, the factors responsible remain to be confirmed. Of the many dietary influences that may play a role in aging-related decreased insulin sensitivity, advanced glycation end products (AGEs) appear particularly important.
1. Introduction
Insulin resistance (IR) is a pathophysiological condition in which organs—mostly skeletal muscle, adipose tissue, and liver—do not respond at an adequate rate to insulin, and it is considered to be a consequence of the disruption of different molecular pathways affected by insulin in these tissues
[1]. In the general population, sensitivity to insulin-mediated glucose disposal in several tissues varies greatly
[2]. The major consequence of IR, type 2 diabetes, arises when people who are insulin-resistant are unable to maintain the level of hyperinsulinemia required to correct the insulin action deficiency. Clinically, it appears as a defect in insulin-mediated glucose control in tissues, prominently in the above named muscle, fat and liver. Primary characteristics of IR are inhibited lipolysis in adipose tissue, impaired glucose uptake by muscle and inhibited gluconeogenesis in liver
[3]. Therefore, IR also encompasses defects in lipid metabolism, in line with the multifaceted roles of insulin in metabolism regulation
[4]. IR is one of the earliest manifestations of a constellation of human pathologic conditions that include metabolic syndrome, type 2 diabetes, cardiovascular diseases and aging
[5]. Lifestyle modifications, including reduced intake of ultraprocessed foods containing advanced glycation (AGEs) and lipo-oxidation end products (ALEs), body weight loss and increased physical activity, have been shown to increase insulin sensitivity, thereby preventing IR
[6,7][6][7].
Whether total caloric intake with body fat accretion or the presence of specific nutrients or diet-derived insulin-signaling disruptors is mostly responsible for the IR of aging is unclear
[8]. Of the many dietary factors that may play a role in the aging-related lack of or decreased insulin sensitivity, AGEs appear potentially important. Recent clinical data suggest that food-derived AGEs may contribute to IR
[9].
2. Evidence of an Association between Dietary AGEs and IR
2.1. Animal Data
A role of dietary AGEs as a causative agent in IR has been well documented in several studies in different mouse strains by independent teams.
WResearche
rs review some of these studies here and in
Table 1. Reduced AGE intake leads to lower levels of circulating AGEs and to improved insulin sensitivity in the db/db mouse IR model
[50][10]. To demonstrate this, db/db mice were randomly placed for 20 weeks (more than 50% of their usual life span) on a diet with either low AGE content (LAGE) or a 3.4-fold higher content of AGEs (HAGE). LAGE mice showed lower fasting plasma insulin levels and body weight compared with HAGE mice, despite equal caloric intake. LAGE mice had improved responses to both glucose (at 40 min,
p = 0.003) and insulin (at 60 min,
p = 0.007) tolerance tests, which correlated with a doubling of glucose uptake by adipose tissue. LAGE mice had twofold lower serum carboxymethyllysine (CML) and methylglyoxal (MG) concentrations and a better-preserved structure of pancreatic islets compared with HAGE mice
[50][10]. Thus, the effect of dietary AGEs affects multiple tissues (liver, adipose tissue, pancreas), leading overall to impaired metabolism.
Table 1. Selected animal studies showing an association between dietary AGEs and IR.
| Author, Reference |
Animal Model |
Study Design |
Intervention |
Findings |
| Hofmann [50][10] |
Db/Db mice (5 week old) |
Dietary intervention with random assignment into two parallel groups for 20 weeks (n = 20) |
High versus low AGE diets |
Lower body weight, lower serum AGEs, better response to both glucose and insulin tolerance tests and better preservation of pancreatic islets than with the high AGE diet |
| Sandu [51][11] |
C57/BL6 female mice (6 week old) |
Dietary intervention with random assignment into two parallel groups for 6 months |
High fat (35% fat) high AGE diet (HAGE-HF) versus High fat low AGE diet (LAGE-HF) |
None of the LAGE-HF mice became diabetic, while 75% of HAGE-HF did. |
| Cai [52][12] |
C57/BL6 |
Dietary intervention with random assignment into three parallel groups for 18 months |
Pair-fed three diets throughout life: (1) low AGE (MG−) *, (2) MG supplemented low AGE chow (MG+) and regular chow (Reg) |
Older MG+ and Reg fed mice developed IR (higher fasting insulin levels and abnormal intraperitoneally glucose tolerance test) and dementia, which did not happen in MG− mice. |
| Wang [28][13] |
C57/BL6 male mice (12 week old) |
Dietary intervention with random assignment into three parallel groups for 24 weeks |
Three parallel diets: (1) regular chow (n = 10), (2) regular chow + MG (n = 15) or (3) heat-treated chow (n = 15) |
IR (high fasting insulin, HOMA and abnormal intraperitoneal glucose tolerance test) developed in groups 2 and 3, but not 1. Microbiota was also altered in groups 2 and 3 (not in group 1) leading to loss of butyrate-producing bacteria |
| Mastrocola [53][14] |
Db/Db and C57/BL6 mice |
Dietary intervention followed by pharmacological intervention in the C57/BL6 mice |
C57/BL6 mice were randomly assigned to 4 groups for 12 weeks: (1) standard diet, (2) high fat diet (60%trans-fat), (3) standard diet + pyridoxamine for last 8 weeks, (4) high fat diet + pyridoxamine for last 8 weeks |
High levels of AGEs and RAGE and abnormal enzymes of sphingolipid metabolism were found in the liver of Db/Db and group 2 C57/BL6, but not in groups 1, 3 and C57/BL6 |