The study participants were residents of the cities of Mima and Miyoshi and the towns of Tsurugi and Higashi-Miyoshi, in western Tokushima Prefecture. They underwent regular specific health checkups at various health care centers. These specific health checkups are offered each year to individuals aged 40–74 to prevent or improve metabolic syndrome.

In preparation for the study, the authors provided an explanatory leaflet to these health care centers, as well as verbal explanations regarding the survey to individuals undergoing specific health checkups. In addition to undergoing various clinical tests and answering the questions that form part of the specific health checkups, those who had agreed to participate in the survey had their urinary albumin measured.

In total, 1,693 people took part in the survey from June 2021 to January 2022. There were no participants with missing data, and a total of 1,660 participants—after excluding those with macroalbuminuria—were analyzed. The characteristics of the analyzed participants are shown in Table 1.

To protect the participants’ personal information, we eliminated their names and dates of birth from all their data. This study was conducted in accordance with the Declaration of Helsinki and the national ethical guidelines. Informed consent was obtained from each participant. This study protocol was approved by the Ethics Committee of Tokushima University Hospital (approval number: 3944).

Clinical tests performed during specific health checkups included the following: height (m), weight (kg), systolic and diastolic blood pressure (mmHg), fasting blood glucose (tested after fasting for at least ten hours; mg/dl), HbA1c (%), LDL cholesterol (mg/dl), HDL cholesterol (mg/dl), triglycerides (mg/dl), serum aspartate aminotransferase (AST; U/l), serum alanine aminotransferase (ALT; U/l), and serum gamma-glutamyl transferase (GGT; U/l).

The questionnaire provided during the specific health checkups included the following items: smoking and drinking habits, medication history (antihypertensive drugs, diabetes drugs, dyslipidemia drugs), and tendency to partake in habitual exercise.

BMI was calculated using the formula BMI (kg/m2) = Weight (kg)/ Height (m)× Height (m), and any values exceeding 25 kg/m2 were seen as indicating obesity. BMI of less than 25 kg/m2 was considered not obesity.

Regarding smoking, the answer was set as two options, “Yes” or “No,” to the question, “Are you currently a habitual smoker?” (Previous habitual smokers who had refrained from smoking for the past month or more were instructed to answer “No”). Participants who answered “Yes” were placed in the category of “Currently smoking: Yes.” Regarding drinking, three options (“Every day,” “Occasionally,” and “Sometimes”) were set as the answers to the question: “How often do you drink alcohol beverages per week?” For the question asking about the drinking quantity—“How much alcohol beverages do you drink per day?”—four options were set (“1 drink/day,” “1–2 drinks/day,” “2–3 drinks/day,” and “3 or more drinks/day”). One drink is equivalent to approximately 20 g of ethanol consumption. Regarding “Currently drinking,” participants who met both criteria, in other words “drinking every day” and “drinking quantity exceeding 1 drinks/day,” were placed in the category of “Moderate drinking: No.”

Two options, “Yes” and “No”, were set for the question pertaining habitual exercise habits: “Have you been engaging in a sweat-inducing workouts lasting more than 30 minutes per session, more than twice per week for more than a year?” Participants who answered “Yes” were placed in the category of “Habitual exercise: Yes.”

Hypertension was defined as follows: systolic blood pressure ≥ 140 mmHg, diastolic blood pressure ≥ 90 mmHg, or currently taking antihypertensive medications [11]. In all other cases, it was assumed to be normal. Diabetes was defined as HbA1c ≥ 6.5%, fasting blood glucose ≥ 126 mg/dl, or currently taking antihyperglycemic medications or insulin injections [12]. In all other cases, it was assumed to be normal and borderline. Dyslipidemia was defined as LDL cholesterol ≥ 140 mg/dl, HDL cholesterol < 40 mg/dl, Triglycerides ≥ 150 mg/dl, or currently taking dyslipidemia medication [11]. In all other cases, it was assumed to be normal. Mild liver disorder was defined as follows, based on standard specific health checkups and the health guidance program formulated by the Japanese Health Ministry in 2018: AST ≥ 31 IU/L, ALT ≥ 31 IU/L, or GGT ≥ 51 IU/L [16]. In Japan, this is the level of liver damage which signals the need for health intervention and so on. In all other cases, it was assumed to be normal.

Albuminuria, according to a midstream spot urine test, was measured using immune nephelometry. Based on the Japanese Society of Nephrology’s guidelines, we divided albuminuria into three categories: normoalbuminuria (< 30 mg/gCr), moderately increased albuminuria (≥ 30 mg/gCr and < 300 mg/gCr) (formerly called albuminuria), and macroalbuminuria (≥ 300 mg/gCr) [17].

First, the participants’ background characteristics were compared between the normoalbuminuric group and the moderately increased albuminuria group. Continuous variables were shown using the median and standard deviation, while categorical variables were shown using proportion (%). To confirm the differences between the two groups, the Student’s t-test and the chi-squared test were used.

Second, to calculate the adjusted odds ratio (AOR) and 95% confidence intervals (CIs) for the moderately increased albuminuria with elevated serum transaminase, the multivariate logistic regression model was applied. The model was adjusted for mild liver disorder as well as for sex, age, BMI, currently smoking, moderate drinking, habitual exercise, diabetes, and hypertension. Previous population-based epidemiological studies found an association between moderately increased albuminuria and diabetes, hypertension and obesity. (Model 1)

Third, to calculate the AOR and 95% CIs for the association of moderately increased albuminuria with the combination of elevated AST or low AST/ALT ratio, and elevated GGT, the multivariate logistic regression model was applied. In Model 2, a combination of elevated AST and elevated GGT was added in place of the mild liver disorder of Model 1. In Model 3, a combination of low AST/ALT ratio and elevated GGT was added in place of the mild liver disorder of Model 1.

Statistical tests were based on two-side probabilities, and a p value of less than 0.05 was considered significant. All statistical analyses were performed using IBM SPSS Statistics version 28.0 for Windows.