Associations between food-specific IgG and health outcomes in an asymptomatic physical examination cohort

Background Although the association of food-specific IgG with the development and progression of specific diseases was shown by many studies, it is also present in the population without clinical symptoms. However, the association between food-specific IgG and physical examination outcomes in healthy people has not been studied yet. Methods An asymptomatic physical examination cohort (APEC) was selected according to the inclusion and exclusion criteria, the physical examination data were compared between IgG positive and IgG negative groups, and their odds ratios (ORs) and 95% confidence intervals (CIs) were calculated using multivariable logistic regression. Results The data of 28,292 subjects were included in the analysis. The overall IgG positive rate was up to 52.30%, mostly with mild to moderate IgG positivity. The multivariable Logistic regression showed the prevalence of hypertriglyceridemia, abnormal fasting blood glucose and overweight was lower in the IgG (+) positive group (OR 0.87, 95% CI 0.83–0.92; OR 0.93, 95% CI 0.87–0.99; OR 0.92, 95% CI 0.87–0.96) but there was a higher prevalence of thyroid disease (OR 1.09, 95% CI 1.04–1.15). Conclusion Food-specific IgG positivity was widespread in the APEC and was associated with lower prevalence of hypertriglyceridemia, abnormal fasting blood glucose and overweight. The underlying physiological mechanism merits further study.


Introduction
At present, the effects of food-specific IgG on human health remain controversial. Numerous studies have suggested that food-specific IgG is involved in the development and progression of specific diseases, such as inflammatory bowel disease [1], irritable bowel syndrome [2], migraine [3,4] and mental disease [5][6][7]. The symptoms of these disease can be relieved by food-specific IgG-based diet recommendations [8][9][10][11][12][13]. IgG antibody can form an immune complex with allergens in foods and thus induce mild inflammatory reactions in the body [14], which are manifested as various systemic symptoms and diseases. According to other studies, however, foodspecific IgG actually provides natural protection against food allergy [15,16]. The most direct evidence comes from an oral immunotherapy (OIT) study showing that OIT for peanuts can induce a remarkable increase in the plasma levels of food-specific IgG [16][17][18], which indicates the involvement of IgG in mitigating the symptoms and inducing food tolerance of allergic patients.
Previous clinical studies on food-specific IgG typically focused on patients with a specific disease. Studies have found a high prevalence of serum IgG antibodies against Open Access *Correspondence: zongtaochen@126.com Health Management Center, First Affiliated Hospital of Army Medical University (Third Military Medical University), Chongqing 400038, China specific food allergens in patients with IBD. Accordingly, IgG antibodies may potentially indicate disease status and be utilized to guide dietary recommendations for patients [19][20][21]. Additionally, migraine patients with positive food specific IgG antibodies had worse migraine, anxiety, and gastrointestinal symptoms [22]. Moreover, significantly higher serum food antigen-specific IgG positivity rates were found in patients with depressive disorder [23]. Food-specific IgG is also present in healthy subject [5], but this important has not received much attention. If food-specific IgG is associated with the development and progression of diseases, it is also important to analyze if it has correlates in the physical examination results of healthy subjects, which we investigated in this study.

Study design and participants
The study data was based on a healthy APEC who firstly received the test of 14 food-specific IgGs at the Health Management Department of the First Affiliated Hospital of Army Medical University during 2010-2020 and met the following criteria: age ≥ 18 years, no previous food intolerance test, no food intake restriction, and no food allergy symptoms or resultant medical care requirement. Datasets without such information as age, sex, height and body weight were excluded. Other physical examination data were also collected, including blood pressure, waist circumference, hematological examination indicators (e.g., total cholesterol (TC), triglycerides (TG), low density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), fasting blood glucose (FBG), uric acid (UA), plasma-albumin (ALB), liver function, hemoglobin and other examination items (e.g., thyroid ultrasonography, pulmonary radiography, abdominal ultrasonography, bone densitometry, gastroscopy, enteroscopy, and prostatic ultrasonography for men, as well as breast ultrasonography, uterus and appendage ultrasonography, as well as cervical cancer screening for women).
After screening, a total of 28,292 physical examination subjects were included in the statistical analysis (32,876 subjects were preliminarily screened, 3744 of whom were excluded due to a lack information on sex, height and body weight, 840 were excluded due to previous food-specific antibody tests), and the various examinations they completed are shown in Fig. 1. The subjects were divided into two groups according to age (< 40 and ≥ 40 years). In these age groups, the probands were divided into IgG positive and IgG negative groups, and the above examination and test results were statistically analyzed (Table 1). To further investigate the relationship of food-specific IgG with the statistically different health outcomes, we established a model based on the available data. Specifically, we combined the influential factors of health outcomes reported previously, included factors such as age, sex, food-specific IgG, body weight, blood pressure, blood lipids, blood glucose, uric acid and fatty liver, and then calculated their OR and 95%CI using multivariable logistic regression. This study was approved by the Ethics Committee of the First Affiliated Hospital of Army Medical University (Approval No. KY2021046), and as all data were retrospectively analyzed in an anonymous form, the written informed consent from subjects was waived.

Statistical analysis
The quantitative data of normal distribution were presented as means ± standard deviation (x ± S) and analyzed for pair-wise and multiple comparisons using independent samples t-test and analysis of variance (ANOVA), respectively. Data with a skewed distribution were expressed as medians and analyzed using the Kruskal-Wallis H test. Qualitative data were analyzed using the χ 2 test. The OR and 95% CI were calculated using multivariable logistic regression to determine the relationship between food-specific IgG and different health outcomes. All statistical analyses in this study were conducted using SPSS software (v. 23.0; IBM Corp., USA). A P-value < 0.05 was considered to indicate statistical significance.

Analysis of physical examination data among different subgroups
In the ≥ 40 year-old group, IgG (+) group had a lower prevalence of overweight, central obesity, dyslipidemia, hypertriglyceridemia and hypercholesterolemia than the IgG ( −) group. There was no significant difference in the prevalence of high density lipoproteinemia between the two groups (24.83% vs. 24.78%, x 2 = 0.01,   Abdominal color Doppler ultrasonography showed that the prevalence of fatty liver was lower in the IgG (+) group (34.52% vs. 39.38%, x 2 = 50.53, P < 0.001), but the prevalence of cholecystolithiasis, gallbladder polyps and renal calculi were not significantly different between the groups. Compared with the IgG (-) group, the IgG (+) group had a higher prevalence of thyroid disease (43.45 vs. 39.67%, x 2 = 20.93, P < 0.001). The results of female breast color Doppler ultrasonography, gynecological ultrasonography and cervical cancer screening, as well as male prostatic nodule screening, showed that there was no significant difference between the two groups. The trend and distribution of the above indicators was similar in the < 40 year-old group (Table 1).

Further analysis of the relationships of food-specific IgG with TG, FBG and BMI
The above analysis showed that food-specific IgG positive state was associated with a lower prevalence of abnormal FBG, hypertriglyceridemia and overweight. We further investigated the relationships of the IgG titers (−, +, ++, +++) and the number of IgG positive foods (−, SIPO and MUPO) with plasma FBG, TG and BMI. There were statistical differences in the distribution of TG, FBG and BMI (H = 372.30, P < 0.001; H = 216.67, P < 0.001; H = 243.33, P < 0.001) among the subgroups with different IgG titers, as well as in the SIPO and MUPO groups (H = 372.30, P < 0.001; H = 172.39, P < 0.001; H = 312.58, P < 0.001) (Fig. 3).

Discussion
Different from previous studies focusing on patients with a specific disease, this study investigated an APEC with no apparent clinical symptoms and no special dietary guidelines. By analyzing food-specific IgG and physical examination results of the APEC, we deduced the Table 2 Regression analysis of food-specific IgG and various healthy outcomes Adjusted for age, sex, BMI (except BMI-specific models), waistline (except waistline-specific models), blood pressure (except BP-specific models), blood lipid (except blood lipid specific models), FBG (except FBG-specific models). Fatty liver and uric acid were also introduced as covariates in the model with liver function as outcomes. Complete covariate data available for 10,637 person examinations CI, confidence interval; BP, blood pressure; HDL-C, high density lipoprotein cholesterol; SBP, Systolic blood pressure; DBP, Diastolic blood pressure; FBG, fasting bloodglucose

Health outcomes
Odds ratio (95% CI)) p value Adjusted odds ratio p value (95% CI)) distribution profile of food-specific IgG in the APEC and a possible correlation of food-specific IgG-mediated immune response with a number of health outcomes. The positivity rate of food-specific IgG in the blood of the probands was up to 52.30%, generally with mild to moderate elevation of antibody titers, which was lower than that of ulcerative colitis patients (57.5-70.1%), Crohn disease patients (90.72%) [12,21,24], and that of food-specific IgGs associated with clinical allergic symptoms [25]. Such differences indicate that food-specific IgG with a higher positivity rate and a greater titer may be associated with specific pathological states, but it is not clear whether the disease results in an increase of the IgG positivity rate or the food-specific IgG causes the pathological state. In this study, we investigated 14 foods most frequently eaten by the Chinese population, and found a significantly higher IgG positivity rate in women than in men, which perhaps is somewhat associated with a greater prevalence of autoimmune disease (AI) in women than in men, as shown in previous studies [24,26,27].
The IgG positivity rate for some foods (e.g., crab, mushrooms and shrimp) was higher in older age, while a previous study found a negative correlation of IgG levels against all foods with age [28]. Another study showed that in Northeast China, a higher food-specific IgG positivity rate was observed for crabs, shrimp, eggs, milk and fish [29], which was greatly different from our study, which indicated that the food-specific IgG positivity rate was higher for eggs, crab, milk, corn and tomato. This suggests that the local dietary habits may influence the type of food-specific IgG prevalent in a specific area, which was avoided effectively because a large sample of the same area was used in this study.
It is believed that food-specific IgG is involved in the body's complex immune response [29]. Some foods cannot be completely digested by the human body due to a lack of corresponding enzymes, and they are recognized as foreign substances in the gastrointestinal mucosal lymphatic tissues, leading to the formation of immune complexes with specific IgG antibodies in the body. Subsequently, macromolecular complexes are phagocytized by monocytes, while micromolecular complexes are excreted by the kidneys. It remains unknown whether such an IgG-mediated immune response has broad effects on the physical examination results of healthy probands. This study showed no difference in the prevalence of abnormal renal function between the two groups. In the IgG (-) group, renal disfunction seemed to have a lower trend and the prevalence of fatty liver was much lower, but there was no association between these characteristics with the IgG positive state after adjusting for confounding factors reported in the literature. Furthermore, there were significant differences in the prevalence of health outcomes (including abnormal blood pressure, hyperuricemia) between the two groups ( Table 1), but the IgG positive state had no statistically significant influence on these health outcomes after adjusting for age, sex and other reported confounding factors.
Previous studies revealed that food-specific IgG may affect the body's nutritional status [12], but no difference in ALB was found between the two groups in this study. Although the prevalence of anemia was higher in the IgG (+) group, there was no association between foodspecific IgG and anemia after excluding factors such as age and sex. Food-specific IgG positive state was related with a lower risk of hypertriglyceridemia, abnormal FBG and overweight, suggesting that it may be an independent protecting factor for these indicators and it was more closely associated at a higher titer and in MUPO probands (Fig. 3). To our best knowledge, this phenomenon was firstly observed in this APEC in humans, but it is similar to the results of an in vivo animal experiment by Batista et al. [30], who found a decrease in body weight, blood glucose and blood lipids, which was followed by a decline of plasma IgE levels, a gradual elevation of IgG levels, and a slight decrease of metabolic indicators (as compared with at the allergic state) in mice treated with sensibiligens after continuing the food antigen stimulation. These similar metabolic characteristics suggest that food-specific IgG is somewhat associated with food allergy.
Acute food allergy and chronic food allergy are correlated with a set of co-factors [31]. However, the diagnosis of food allergy is a clinical challenge and requires history inquiry and food exclusion in most cases [32,33]. Moreover, the diagnosis is very difficult to make in patients without apparent symptoms. In our study, IgG positive subjects might initially be in a non-obvious food allergy state after contacting some foods, and no restrictions are imposed on food intake in daily life due to an absence of significant symptoms. Similarly, there was long-time repeated food antigen stimulation, which induced immunotolerance. Thus, food-specific IgG with corresponding metabolic changes was detected in the body of these subjects. This is in agreement with the view that food-specific IgG confers natural protection against food allergy [34]. IgG can send negative signals after binding to the FcgRIIb receptor of mast cells, and can also sterically block the binding of IgE to mast cells, thereby suppressing the allergic reaction [15]. These effects of IgG have been confirmed in clinical studies. For example, Savilahti et al. found that milk tolerance in allergic children was associated with blood IgG levels [35]. In addition, maternal food-specific IgG may be correlated with the food tolerance of offspring [36].
Finally, our study also has some potential limitations. First, we did not have specific information about the diet of the subjects, which was associated with overweight [37]. Second, we lacked data related to family history of allergy, so these variables could not be included in multivariate logistic regression. Finally, this study was observational and retrospective in design, which prevented us from being able to draw conclusions about causal relationships between food-specific IgG and health outcomes. Therefore, larger prospective studies are required to establish these relationships.

Conclusions
Food-specific IgG was widely present in APEC, predominantly with mild to moderate elevation. We showed that food-specific IgG is associated with a normal body weight and metabolic indicators (mainly TG and FBG). Therefore, dietary guidelines based on food-specific IgG may be helpful for people with overweight or metabolic disorders. The underlying physiological mechanism merits further study.