Dual effects of synthetic phenolic antioxidants in type 2 diabetes mellitus: Mechanisms and advances

Abstract

Type 2 diabetes mellitus (T2DM) is a chronic metabolic disorder characterized by hyperglycemia, oxidative stress, and inflammation. Synthetic phenolic antioxidants (SPAs), primarily butylated hydroxyanisole, butylated hydroxytoluene, and tert-butylhydroquinone, have been widely detected in various environmental and consumer product matrices. Although they possess free radical scavenging ability, SPAs have dual effects on T2DM under different conditions. This review aims to outline the current research landscape in this field. We systematically analyze the literature and specifically note that the current understanding of the association between SPAs and T2DM is predominantly based on hypotheses from animal and cellular experiments, whereas direct human research data remain extremely scarce. This article elaborates on the paradoxical roles of SPAs, potentially increasing disease risk while alleviating complications, and analyzes the underlying regulatory mechanism centered on the balance of the nuclear factor erythroid 2-related factor 2/nuclear factor kappa-light-chain-enhancer of activated B cells pathway. Finally, we emphasize the urgent need for future well-designed, large-scale human studies to fill this critical evidence gap and clarify the precise role of SPAs in T2DM.

Key Words: Synthetic phenolic antioxidants; Type 2 diabetes mellitus; Dual effects; Butylated hydroxyanisole; Butylated hydroxytoluene; Tert-butylhydroquinone

Core Tip: This review systematically elucidates the dual regulatory role of synthetic phenolic antioxidants in the development of type 2 diabetes mellitus, identifying the balance of the nuclear factor erythroid 2-related factor 2/nuclear factor kappa-light-chain-enhancer of activated B cells pathway as the core regulatory mechanism. However, current evidence predominantly comes from preclinical studies, making it difficult to extrapolate the findings to human populations.

INTRODUCTION

Type 2 diabetes mellitus (T2DM) is a progressive disease characterized by hyperglycemia, increased levels of oxidative stress, and persistent systemic inflammation[1,2]. The major pathophysiological mechanism underlying the development of T2DM lies in insulin resistance and pancreatic β-cell dysfunction[3,4]. Prolonged high blood glucose levels resulting from these pathological dysfunctions inevitably induce debilitating microvascular and macrovascular complications, such as cardiovascular diseases, chronic kidney disease, peripheral neuropathy, and diabetic retinopathy (DR)[5,6]. These complications can increase the risk of death among diabetic patients, severely impair their quality of life, and impose burdens on the population.

In addition to the well-characterized contributing elements, such as genetic factors, overweight/obesity, and unhealthy living habits, recent studies have demonstrated the substantial effect of environmental pollutant exposure on T2DM, and this role has attracted increasing attention[7,8]. A number of environmental compounds are synthetic phenolic antioxidants (SPAs). As listed in Table 1, common SPAs include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and tert-butylhydroquinone (TBHQ). Owing to their robust antioxidant capacities, these compounds are frequently employed as plasticizers, preservatives, stabilizers for commercial products, and monomers for polymeric materials. In particular, these substances are found in foods such as oil and dairy, processed formula for young infants, plastic packaging material for milk bottles and cups, and consumer care products such as cosmetic preparations or other goods and domestic environments[9]. Human contact with SPAs occurs primarily through dietary intake, inhalation, or percutaneous routes, enabling broad absorption and distribution throughout several organs and tissues[10,11].

Table 1 The full name, abbreviation, CAS number, and primary uses of synthetic phenolic antioxidants.

Name	Abbreviation	CAS	Primary uses
Butyl hydroxyanisole	BHA	121-00-6	Food preservative, antioxidant in fats/oils, cosmetics, pet food
Butylated hydroxytoluene	BHT	128-37-0	Food preservative, antioxidant in fats/oils, cosmetics, plastics, rubber, fuels
Tert-butylhydroquinone	TBHQ	1948-33-0	Food preservative (especially for vegetable oils), antioxidant

Available toxicokinetic evidence indicates that BHA, TBHQ, and BHT are readily absorbed after oral intake owing to their lipophilic properties and subsequently undergo extensive hepatic metabolism in humans. These compounds are primarily metabolized through phase I oxidation followed by phase II conjugation reactions, which facilitate their elimination via urine and bile[12,13]. Due to their lipophilicity, transient distribution to metabolically active and lipid-rich tissues, such as the liver, kidneys, and adipose tissue, has been reported, particularly for BHT[9,12]. However, quantitative human data linking internal exposure levels of SPAs to metabolic outcomes, including T2DM, remain limited[12,13].

Synthetic phenols can paradoxically act as agents that damage biological systems and induce stress, and thus, can be considered environmental risk factors[12,14]. These compounds also affect factors such as nuclear factor erythroid 2-related factor 2 (Nrf2) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), which may ameliorate glucose homeostasis[15]. This contradiction has led us to understand the relationship between synthetic phenols and T2DM in great detail. However, direct human research evidence on the association between SPAs and T2DM remains extremely limited. Although existing animal and cellular studies have revealed their potential mechanisms of action, well-designed and sufficiently large-scale human epidemiological studies are lacking. This hinders accurate assessment of the population health effects of SPAs and underscores the need for cautious interpretation of preclinical findings. In this context, this review aims to achieve the following objectives: Systematically summarize the existing evidence on the association between SPAs and T2DM; Objectively examine the contributions and limitations of different tiers of evidence; Comprehensively elucidate the underlying mechanisms of action; And emphasize the future priority of conducting high-quality human studies to fill this critical evidence gap.

BHA AND T2DM

Population studies regarding BHA and T2DM

Few population-based studies have investigated the relationship between BHA and T2DM. The available evidence regarding their association is derived mainly from studies assessing the safety of BHA or its indirect relationship with T2DM, rather than from high-quality studies directly examining this association. Some studies have indicated that increased dietary intake of synthetic food antioxidants, including BHA, may lead to accelerated progression of many diseases, including diabetes[12,16]. Possible mechanisms underlying this effect include the initiation of cytotoxic and genotoxic changes and the formation of complexes with biological molecules. However, one study suggested that interactions between BHA and serum albumin may induce oxidative stress, leading to amyloid fibril formation, which has been implicated in the pathogenesis of diseases such as diabetes, thereby linking BHA to conditions including obesity, diabetes mellitus and aging[13,17,18]. Another hypothesis proposes that ingestion of synthetic additives such as BHA leads to a reduction in cellular populations, which may contribute to the development of chronic inflammatory diseases such as diabetes; accordingly, the use of synthetic additives like BHA has been associated with decreased cellular layers[13]. These concepts are largely based on hypothesis and speculation rather than direct evidence from human studies.

Animal studies on BHA and T2DM

Animal studies conducted in high-fat diet (HFD)-induced IDH2 knockout mice demonstrated that BHA could reverse the adverse effects of HFD. In a key experimental model, mice were maintained on a HFD for 4 weeks and concurrently administered BHA at a dietary concentration of 7.5 g/kg throughout the intervention period. HFD induced accelerated weight gain and reduced energy expenditure and thermogenesis in brown adipose tissue, contributing to increased adiposity and systemic metabolic dysfunction. BHA supplementation partially counteracted these HFD-induced alterations, including significantly restoring nicotinamide adenine dinucleotide phosphate and its reduced form (NADPH) levels, as well as nicotinamide adenine dinucleotide and its reduced form, mitigating mitochondrial impairment and enhancing the expression of genes associated with mitochondrial biogenesis, energy expenditure, and reactive oxygen species (ROS) scavenging. Notably, these protective effects were observed at a dietary dose of 7.5 g/kg, which represents a relatively high experimental exposure level, and the 4-week intervention was sufficient to elicit significant metabolic improvements in the IDH2 knockout model[19].

In another experimental setting using A549 cells, BHA was tested across a concentration range of 30 μg/mL-100 μg/mL for exposure durations of up to 72 hours, where it demonstrated antioxidant and free radical-scavenging activities[20]. However, BHA also exhibited potential adverse outcomes, including apoptosis induction and DNA damage, at higher concentrations (e.g., 1 mmol/L) and under prolonged exposure conditions[16]. These findings underscore the dose-dependent dual role of BHA, which highlights that protective effects are evident within specific concentration and temporal windows, while elevated doses may lead to cytotoxic consequences.

The mechanism of action of BHA on T2DM

BHA could prevent T2DM from occurring by acting on oxidative stress. First, it bound excess free radicals emerging from cells, including 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2’-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS⁺), thereby alleviating oxidative stress induced by the emergence of excess free radicals. Further insight into ROS revealed that oxidative stress participates in the incidence and occurrence of T2DM because pathologically produced ROS from T2DM cause loss of function and mass in pancreatic islets and impaired hormone secretions in pancreatic beta cells, leading to a decrease in systemic insulin sensitivity; moreover, because BHA acts directly as an antioxidant and binds free radicals with other active elements via free-radical binding, its anti-diabetes role may be indirectly supported[20-22]. On further inspection, BHA could also stop lipids from excessive oxidation pressure by inactivating lipids catalyzed with ferric chloride as well as limiting their products such as malonaldehyde (MDA); therefore, BHA alleviated oxidative pressure and hindered insulin secretion destruction and the vascular damage resulting from abnormal oxidation, hence slowing the development of T2DM and reducing the effects of other pathologies related to chronic systemic disease[22]. Additionally, by acting as a chelator, BHA can form complexes with ferrous ions or divalent copper, thereby reducing the tissue-damaging effects of hydroxyl radicals generated through strong oxidization under Fenton reaction conditions. By inhibiting these highly reactive oxidants, BHA may alleviate oxidative injury mediated by the ROS pathway[22].

BHA can restore impaired mitochondria by upregulating the expression of mitochondrial biogenesis-related genes, reversing the reduction in the mitochondrial oxygen consumption rate and restoring the levels of coenzymes such as NAD(P)H. Mitochondrial dysfunction is an important pathological manifestation of T2DM, which leads to disturbances in cellular energy metabolism and the excessive generation of ROS. BHA may protect mitochondrial function and thereby ameliorate T2DM to some extent; for example, metabolic disturbances associated with T2DM may be improved through enhanced energy metabolism and reduced ROS generation[19].

TBHQ AND T2DM

Animal studies on TBHQ and T2DM

Currently, there are no population-based epidemiological studies directly investigating the association between TBHQ and T2DM, and all existing knowledge is derived from preclinical models. Through animal experiments, a series of beneficial effects of TBHQ in T2DM have been reported. The dosage and administration regimen of TBHQ in animal studies vary depending on the model and endpoints. For instance, a study in a HFD-induced T2DM rat model combined with streptozotocin (STZ) showed that oral gavage administration of TBHQ at 16.7 mg/kg and 50 mg/kg for 4 weeks resulted in significant improvements in reducing fasting blood glucose, fasting insulin, homeostasis model assessment for insulin resistance index, and ameliorating dyslipidemia in the 50 mg/kg dose group[23]. Another study in a T2DM mouse model found that dietary supplementation with 1% TBHQ for 8 weeks effectively controlled blood glucose and alleviated hepatic steatosis, with efficacy comparable to or superior to rosiglitazone[24]. Regarding diabetic complications, intraperitoneal injection of 10 mg/kg/day TBHQ for 12 weeks significantly improved the pathological and functional indicators of DR in rats with STZ-induced diabetes[25]. Treatment with 16.7 mg/kg/day for 8 weeks significantly reduced urinary albumin excretion, serum creatinine (SCr), and glomerular injury in patients with diabetic kidney disease (DKD)[26]. A single intravenous injection of 10 mg/kg TBHQ before ischemia significantly reduced kidney injury scores, blood urea nitrogen, SCr, and inflammation and oxidative stress levels in a model of diabetes combined with renal ischemia/reperfusion injury[27].

TBHQ also has protective effects on various organs in T2DM. In DR, TBHQ increased retinal phosphorylated endothelial nitric oxide synthase (eNOS) (Ser1179) and phosphorylated protein kinase B (p-Akt) Ser473 (p-Akt Ser473) protein expression, inhibited apoptosis, partially reversed the changes in retinal thickness and ganglion cell layer cells, and increased the amplitudes of electroretinogram a-wave, b-wave, and oscillatory potentials[25]. With respect to DKD, TBHQ not only reduced urinary albumin excretion and decreased SCr, reduced glomerular hypertrophy and mesangial matrix expansion changes and suppressed renal cell apoptosis[26] but also decreased kidney injury ischemia/reperfusion scores, reduced blood urea nitrogen and SCr levels in blood plasma, decreased the levels of the inflammatory factors tumor necrosis factor (TNF)-α and interleukin (IL)-1β and the oxidative marker MDA, increased superoxide dismutase (SOD) levels and inhibited apoptosis[27]. Podocytes exposed to high-glucose conditions and pretreated with TBHQ for 6 hours showed reduced production of superoxide anions and hydrogen peroxide, restoration of synaptopodin expression, decreased podocyte apoptosis, and inhibition of bovine serum albumin permeability[28].

Cellular experiments involving TBHQ and T2DM

TBHQ can improve endothelial cell proliferation and partially inhibit cell injury in vitro through multiple possible mechanisms. TBHQ exhibits dose-dependent protective effects in cellular experiments, with effective concentrations typically in the low micromolar range. For example, pretreatment of MIN6 pancreatic β-cells with 10 μM TBHQ for 2 hours significantly protected cells from hydrogen peroxide-induced (200 µM) cytotoxicity and apoptosis[29]. In HepG2 hepatocytes, treatment with 20 μM TBHQ for 24 hours alleviated palmitate and hypochlorous acid-induced insulin resistance and apoptosis[24]. In human renal glomerular endothelial cells, 10 μM TBHQ pretreatment for 6 hours attenuated high glucose-induced mitochondrial dysfunction and adenosine triphosphate depletion[30]. In mouse podocytes cultured under high glucose conditions, treatment with 5-20 μM TBHQ for 24-48 hours dose-dependently improved mitochondrial function, increased nephrin and synaptopodin expression, and reduced apoptosis and permeability[28,31]. In glomerular mesangial cells, 10 μM TBHQ reversed the advanced glycation end product-induced Sirt1 downregulation and reduced the accumulation of fibronectin and transforming growth factor-β1[32].

TBHQ mechanisms in T2DM

The protective effect of TBHQ against T2DM centered on activation of the Nrf2 pathway, resulting in the formation of a three-tiered regulatory network.

The core activation tier: TBHQ modifies Kelch-like ECH-associated protein 1 (Keap1), causing Nrf2 to dissociate from its inhibitory binding with Keap1, promoting its nuclear translocation and initiating the expression of downstream antioxidant genes, such as heme oxygenase-1 (HO-1), NAD(P)H quinone dehydrogenase 1, and the glutamate-cysteine ligase catalytic subunit, thus constituting the antioxidative foundation of its activity[28,33-36]. Specifically, TBHQ can undergo auto-oxidation to form tert-butylbenzoquinone, which binds to cysteine residues in Keap1. Concurrently, TBHQ facilitates Nrf2 nuclear translocation through pathways such as Akt, extracellular regulated protein kinase, phosphatidylinositol 3-kinase (PI3K), protein kinase C, and c-Jun N-terminal kinase pathways[28,33-36]. Once Nrf2 enters the nucleus, it regulates the expression of various ROS-detoxifying and cytoprotective enzymes, including HO-1, SOD2, glutathione S-transferase, and catalase (CAT), thereby mitigating oxidative stress damage to target cells such as pancreatic β-cells, renal cells, and retinal cells at the source[28,33-36]. Furthermore, TBHQ increases the activity and expression levels of glyoxalase-1 (Glo-1) via the Nrf2/antioxidant response element/Glo-1 pathway, increases glutathione (GSH) content, reverses the downregulation of total Nrf2 and phosphorylated Nrf2, and further strengthens antioxidative effects. The protective effect of this pathway on podocytes partially depends on Sirt1 and the formation of a bidirectional positive feedback loop with Sirt1[28,31,37].

The pathway crosstalk tier: Activated Nrf2 does not act in isolation but extensively interacts with key metabolic pathways, such as PI3K/Akt and adenosine 5’-monophosphate-activated protein kinase (AMPK), generating synergistic regulatory effects. On the one hand, through the PI3K/Akt signaling pathway, TBHQ upregulates the expression of phosphorylated eNOS Ser1179, promotes nitric oxide production, reduces ROS accumulation, and modulates key targets such as nitric oxide synthase 3, mitogen-activated protein kinase 8, and prostaglandin-endoperoxide synthase 2, thereby alleviating oxidative stress and inflammatory damage in the retinal microvasculature[24,25,38]. Furthermore, TBHQ specifically activates the AMPKα2/PI3K/Akt axis, upregulating the expression of glucose transporter 4 (GLUT4) and glycogen synthase kinase 3β (GSK3β), promoting glucose uptake in peripheral tissues and hepatic glycogen biosynthesis, improving insulin resistance, and reducing hepatic steatosis. Experiments on HepG2 cells confirm that AMPKα2 is the core mediator of this pathway, as AMPKα2 knockout completely abolishes the TBHQ-mediated upregulation of p-Akt, phosphorylated PI3K, GLUT4, and GSK3β[24].

The terminal effector tier: The synergistic actions of the core and interactive pathways ultimately achieve multidimensional, multiorgan protective effects; The anti-inflammatory synergistic effect: By activating antioxidant enzymes such as SOD, CAT, and GSH-peroxidase via the Nrf2 pathway, TBHQ enhances their activity, reduces MDA and ROS levels, and simultaneously inhibits NF-κB pathway activation. This decreases the release of proinflammatory cytokines such as TNF-α, IL-1β, and IL-6; upregulates the expression of anti-inflammatory cytokines such as IL-10; and reverses the abnormal accumulation of advanced glycation end products and their receptors, resulting in the formation of an “antioxidant + anti-inflammatory” synergistic protective mechanism[23,27,37,39]. The anti-apoptotic effect: By modulating the B-cell lymphoma 2 (Bcl-2)/Bcl-2-associated X protein (Bax) ratio and inhibiting the activation of cysteine-aspartic acid protease 3, TBHQ significantly reduces the apoptosis rates of renal ischemia/reperfusion cells, pancreatic MIN6 β-cells, and renal podocytes under diabetic conditions, thereby maintaining cell survival[27-29,31]. The target organ protective effect: By regulating the expression of DKD-related genes, vascular endothelial growth factor A and BIRC3, and through the Nrf2/Sirt1 positive feedback loop, which inhibits the gene and protein expression of fibronectin and transforming growth factor-β1, TBHQ alleviates glomerular mesangial matrix expansion and exerts renal protective effects[30,32]. Moreover, via the Nrf2/HO-1 pathway, TBHQ suppresses excessive NADPH oxidase-mediated ROS production in the kidney, reducing high glucose-induced podocyte injury[23,33,40].

BHT AND T2DM

Population studies regarding BHT and T2DM

Compared with patients with controlled T2DM, patients with uncontrolled diabetes have increased levels of BHT in their saliva, although the difference was not significantly different (P = 0.867). Moreover, there was a significant positive correlation between the concentration of salivary BHT and random blood sugar in the group with controlled diabetes (P = 0.01)[41]. However, this study had the following critical limitations: The small sample size (n = 40) resulted in insufficient statistical power; the use of salivary BHT concentration as an exposure marker (whose relationship with long-term internal body burden remains unclear) and random blood glucose rather than the gold-standard glycosylated hemoglobin for assessing glycemic status; nd the cross-sectional design, which can only suggest correlation without establishing causality. Therefore, although preliminary data indicate that BHT may be associated with the oxidative status of diabetic patients, these methodological constraints prevent definitive conclusions from being drawn regarding the clear population-level impact of BHT on T2DM risk or control. This evidence gap underscores the need for future well-designed, large-scale epidemiological studies.

Animal studies and analytical evidence on BHT and T2DM

In animals, the metabolism of BHT and its derivatives might has shown potential therapeutic applications in the treatment of diabetic complications and free radical-related disorders[41]. BHT improves impaired chain-breaking antioxidant activity in nonobese diabetic model mice. Specifically, dietary supplementation of 0.1% BHT for 8 weeks in nonobese diabetic mice significantly elevated the chain-breaking antioxidant capacity in serum and pancreatic tissue homogenates compared to the model control group, with the 0.1% dose group showing the most prominent effect (P < 0.05)[42]. The antioxidant effect of BHT, an active ingredient found in Lepidium sativum seed oil (LSO) at a concentration of 4.75% (w/w), preserved testicular function to maintain reproductive function in diabetic model mice. For this experiment, male diabetic mice induced by STZ were gavaged with LSO containing 4.75% BHT at a dose of 10 mL/kg body weight daily for 4 weeks, and the 4.75% BHT-containing LSO group exhibited significantly reduced testicular oxidative damage markers (MDA content decreased by 32.6%, P < 0.01)[42]. In terms of the regulation of oxidative stress, ethanol extract from the leaves of Solanum torvum Swartz (4.75%) BHT significantly reduced the damage caused by oxidative stress and improved the oxidative damage in STZ-induced diabetic rats. The extract was administered via gavage at 200 mg/kg body weight daily for 21 days, and the treatment group showed significant improvements in hepatic antioxidant enzyme activities: SOD increased by 41.3%, CAT by 35.7%, and GSH content by 28.9%, while MDA content decreased by 37.2% (all P < 0.05 compared to the diabetic control group). These effects might increase the activity of antioxidant enzymes in the liver (including SOD and CAT) and the content of GSH, as well as decrease the content of MDA, indicating that BHT may increase the body’s antioxidant capacity through these mechanisms[43]. Molecular docking analysis of BHT against T2DM-related targets at the analysis level revealed its strong affinity for an antidiabetic target protein (2QV4) and a relatively high docking score (-6.7 kcal/mol), suggesting that BHT directly binds to this receptor; however, these results need to be confirmed[44].

Potential mechanism of BHT in T2DM

The antioxidant mechanism is the core of BHT activity. It can directly scavenge free radicals such as DPPH, ABTS⁺, and superoxide anion (with half maximal inhibitory concentration values of 5.89 μg/mL, 7.56 μg/mL, and 5.89 μg/mL, respectively), effectively neutralizing the damage of free radicals to target organs. It can potently inhibit linoleic acid emulsion-induced lipid peroxidation (with an inhibition rate of 97.6% at a concentration of 30 μg/mL), protect the structure and function of cell membranes, and chelate metal ions such as ferrous ion to block the initiation of lipid peroxidation chain reactions, thus maintaining the body’s redox balance[44]. In addition, BHT may exert its effects through mechanisms similar to those of TBHQ; for example, it inhibits the toll-like receptor 4/NF-κB pathway to reduce the release of proinflammatory factors[27,42], and regulates the Bcl-2/Bax ratio to exert an antiapoptotic effect[27], and it also has moderate α-amylase inhibitory activity, which may be involved in regulating glucose metabolism[39].

COMPARISON OF MECHANISMS

The bidirectional effects of SPAs on T2DM are coregulated by dose, individual genetic background, and exposure scenario, generally adhering to the core principle of “low-dose protection, high-dose/super threshold harm (or loss of protective effect)”. Based on the experimental evidence summarized in Table 2, the dose ranges for each SPA can be preliminarily categorized as follows: For BHA, the low dose is 30-100 μg/mL (in vitro antioxidant activity)[20], the moderate dose is 100 μM (cellular studies)[21,22], and the high dose is 1 mmol/L (cytotoxicity and DNA damage)[15], with an in vivo mouse dose of 7.5 g/kg in diet (protective in the IDH2 knockout model)[19]; for TBHQ, the low dose is 5-20 μM (cellular protective effects)[28,31], the moderate dose is 16.7-50 mg/kg (metabolic improvement in animal models)[23,26], and the high dose is ≥ 155.9 μM (potential DNA damage)[33], with dietary supplementation of 1% in feed for 8 weeks showing effects comparable to rosiglitazone[24]; for BHT, the low dose is 30 μg/mL (antioxidant in vitro)[20], natural source content ranges from 4.75% to 8.04% in plant extracts[42-44], animal studies used 0.1% in the diet for 8 weeks (antioxidant enhancement)[42], and human exposure is estimated at 0-0.7 mg/kg/day.

Table 2 Summary of the effects, mechanisms, and research characteristics of butylated hydroxyanisole, tert-butylhydroquinone, and butylated hydroxytoluene in regulating type 2 diabetes mellitus.

Comparison dimension	BHA	TBHQ	BHT
Study type	In vitro (A549 cell cytotoxicity/genotoxicity, antioxidant), animal (mouse), cell (brown adipocytes)[15,19-22,30,45]	In vitro (BSA interaction, antioxidant), animal (rat/mouse), cell (kidney/pancreas/retina-related cells), transcriptomic analysis, molecular simulation[17,23-40,48]	In vitro (antioxidant, α-amylase inhibition, molecular docking), animal (rat/mouse), clinical observation[20,30,40-43,47]
Core effects (protective/adverse)	Protective: Antioxidant, free radical scavenging, α-glucosidase inhibition, anti-obesity[19,20,46]; Adverse: High-dose induces A549 cell apoptosis, DNA damage, associated with liver injury and carcinogenesis[15,20]	Protective: Inhibits T2DM-related oxidative stress/inflammation, protects pancreatic/renal/retinal cells, improves insulin resistance[23-40,48]; Potential risk: High-dose may cause DNA damage[33]; Others: Binds to BSA and alters its secondary structure[17]	Protective: Antioxidant, anti-inflammation, anti-diabetic, hepatoprotection, α-amylase inhibition[20,30,40,44]
Key experimental doses	In vitro: 30-100 μg/mL, 1 mmol/L; Cell: 100 μM; Animal: 7.5 g/kg (in feed)[15,19-22,30,45]	In vitro: Max TBHQ 155.9 μM (BSA experiment); Cell: 0.04-20 μM; Animal: 16.7-60 mg/kg, 1% in feed[17,23-40,48]	In vitro: 30 μg/mL, extraction content 4.75%-8.04%, pure compound for molecular docking[20,30,40-43,47]
Human exposure reference	0-0.5 mg/kg/day[47]	0-0.7 mg/kg/day[49]	0-0.125 mg/kg/day[48]
Exposure duration	In vitro: 10 minutes-72 hours; Cell: 1 hour-2 days; Animal: 4 weeks[15,19-22,30,45]	In vitro: 6 minutes (room temp incubation); Cell: 3 hours-4 days; Animal: 2 weeks-3 months, entire pregnancy[17,23-40,48]	In vitro: 30 minutes-3 days; Animal: 21 days-4 weeks; Clinical: Not specified[20,30,40-43,47]
Core mechanisms	Antioxidant: Free radical scavenging, iron/copper ion reduction[20]; Adverse: High-dose generates excessive ROS, induces DNA breakage and mitochondrial damage[15]	Antioxidant: Activates Nrf2 pathway, enhances SOD/GPx activity[23-40,48]; Anti-inflammation/anti-apoptosis: Inhibits NF-κB pathway, regulates Bax/Bcl-2[27,31,39]; Others: Binds to BSA, activates PI3K/Akt pathway[17,23-26,38]	Antioxidant: Free radical scavenging, enhances SOD/CAT activity[20,42-44,47]; Anti-inflammation/anti-diabetic: Inhibits NF-κB pathway, regulates blood glucose and insulin sensitivity[41-44,47]
Combined exposure with other pollutants		Synergistic with BMSC exosomes and resveratrol[33,34]
Food matrix effects
Genetic heterogeneity	More significant protective effect in IDH2 KO mice[19]	Involves Nrf2 knockout models, no definite effect of gene polymorphisms[23,29,31,34,35]

BHA: Butylated hydroxyanisole; TBHQ: Tert-butylhydroquinone; BHT: Butylated hydroxytoluene; BSA: Bovine serum albumin; T2DM: Type 2 diabetes mellitus; ROS: Reactive oxygen species; KO: Knockout; SOD: Superoxide dismutase; GPx: Glutathione peroxidase; Nrf2: Nuclear factor erythroid 2-related factor 2; NF-κB: Nuclear factor kappa-B; Bcl-2: B-cell lymphoma 2; Bax: B-cell lymphoma 2-associated X protein; PI3K/Akt: Phosphatidylinositol 3-kinase/protein kinase B; BMSC: Bone marrow mesenchymal stem cells; CAT: Catalase.

During low-dose exposure, SPAs predominantly exert protective effects by activating the Nrf2 pathway. Currently, there is no clear evidence that long-term exposure induces harmful effects because of excessive activation of the Nrf2 pathway. Exposure before the onset of T2DM pathology or damage makes it easier for SPAs to exhibit protective value, whereas there is a lack of relevant experimental data supporting the effects of exposure under normal physiological conditions or in the advanced stages of disease.

The Nrf2 gene serves as a key regulatory factor. In IDH2 knockout mice, the protective effect of BHA is more pronounced[19], whereas in Nrf2 knockout models, the protective effect of TBHQ is affected[23,29,31,34]. These genetic background differences modify the protective effects of SPAs; however, the potential influence of polymorphisms in genes encoding antioxidant enzymes such as SOD2 and GSH-peroxidase on these effects requires further validation.

From a mechanistic perspective, the effects of SPAs on T2DM primarily reflect the competitive balance between the Nrf2 antioxidant pathway and the NF-κB inflammatory pathway. Under low-dose conditions, normal Nrf2 function, and T2DM pathology, SPAs primarily clear ROS by activating the Nrf2 pathway[23,28,32,36] while simultaneously inhibiting the release of proinflammatory factors via the NF-κB pathway[27,39,42], thereby alleviating oxidative stress and inflammatory damage to target cells such as pancreatic β cells, renal cells, and retinal cells[27,32,36,38]. Different SPAs also exert specific protective mechanisms: BHA can inhibit α-glucosidase activity, delaying carbohydrate absorption and reducing postprandial blood glucose levels[19,21,22,45,46], and TBHQ can activate the AMPKα2/PI3K/Akt pathway, further improving insulin signaling[23-26,38]. Conversely, in high-dose exposure scenarios, BHA can induce excessive ROS generation, leading to DNA damage and apoptosis[16]; high doses of TBHQ may cause DNA damage[33]. However, evidence regarding the direct influence of synthetic phenols on insulin secretion and their effects on islet function remains limited.

Furthermore, the protective thresholds of SPAs are influenced by multiple factors. Regarding the assessment of harmful thresholds, the acceptable daily intakes of BHA, BHT, and TBHQ are 0-0.5 mg kg^-1 bw day^-1[47], 0-0.125 mg kg^-1 bw day^-1[48], and 0-0.7 mg kg^-1 bw day^-1, respectively[49], which can serve as important references. The variation in the effects of TBHQ is primarily dose-related[23,27,32,39], with no current evidence indicating that exposure duration is a core evaluation criterion. However, data on the daily dietary exposure levels of SPAs in humans remain limited.

RESEARCH LIMITATIONS

Current research conclusions on SPAs are difficult to directly extrapolate to the population level, with core limitations primarily manifested in three aspects: First, there has been insufficient simulation of exposure scenarios. Existing studies focus only on single SPA exposure, without considering their combined exposure effects with other environmental pollutants. Only TBHQ has related studies on synergistic effects with bone marrow mesenchymal stem cell exosomes and resveratrol[33,34]; the other SPAs lack combined exposure data, making it impossible to clarify the true effects of SPAs on T2DM under combined exposure conditions. Second, there is inadequate consideration of the influence of the food matrix. In human dietary exposure, SPAs interact with other food components, which may lead to complex biological effects that differ from those observed in experimental settings using purified compounds. Animal and cellular studies typically examine isolated SPAs, which may not accurately reflect the effects of SPAs when consumed as parts of real-world diets. The bioavailability of purified SPAs used in experiments may differ from that of SPAs combined with other dietary components. For instance, while BHT has been identified in certain food sources, such as garden cress seed oil and Solanum incanum leaf extract[42-44]. However, similar food matrix interactions have not been systematically studied for most SPAs. This may lead to an overestimation of the effects of purified SPAs, making it difficult to reflect the actual impact of daily dietary intake. Third, research on individual genetic heterogeneity is insufficient. Although it has been confirmed that the protective effect of TBHQ depends on Nrf2 function[23,29,31,34], and polymorphisms in genes such as SOD2 and GPx1 are speculated to potentially modify SPA effects, there is a lack of population-level validation and stratified analysis by genetic subtype to clarify the effect thresholds[19,29,34]. Consequently, the experimental conclusions cannot be adapted to populations with different genetic backgrounds.

SIGNIFICANCE AND FUTURE DIRECTIONS

From clinical and public health perspectives, SPAs have a bidirectional effect on T2DM, characterized by “low-dose protection and high-dose/Long-term exposure harm”. However, owing to the severe lack of population-based studies, it can only be confirmed that daily dietary exposure levels in humans fall within the “no clear benefit, no direct harm” range. Indiscriminate supplementation of SPAs may pose risks, and susceptible populations, such as those with Nrf2 gene defects, should remain vigilant about potential harm even under routine exposure.

On the basis of current evidence, the food industry needs to strictly control SPA additive limits, optimize antioxidant strategies, and improve labeling practices. Regulatory agencies should refine risk assessment frameworks, enhance monitoring of additive quantities, and promote context-specific research. In the public health domain, efforts should focus on standardizing public awareness, providing precise dietary guidance, and strengthening warning labels on food products.

Future research must prioritize addressing the gap in epidemiological evidence in human populations. Building on this foundation, studies should elucidate the dose-threshold relationships and molecular mechanisms underlying the bidirectional effects of SPAs, simulate real-world exposure scenarios to identify susceptible populations, and develop an integrated research framework linking “population-mechanism-prevention” based on key pathways. Such efforts will help resolve current issues, including biased effect assessments and unclear risk stratification resulting from the lack of human data, thereby providing scientific support for T2DM risk management and prevention.

CONCLUSION

This review indicates that BHA, TBHQ, and BHT have dual effects on T2DM, with the balance of the Nrf2/NF-κB pathway being the primary underlying mechanism. However, current research on the relationship between SPAs and T2DM remains limited, especially in population studies, and the existing findings are difficult to extrapolate to humans. Therefore, it is necessary to establish appropriate methodological approaches to clarify the impact of SPAs on T2DM.

ACKNOWLEDGEMENTS

We thank all co-authors for their valuable contributions to the conceptualization and writing of this study.