Association between tea types and number of teeth: a cross-sectional study of the Chinese Longitudinal Healthy Longevity Survey

The data used for this study were obtained from the seventh wave of the Chinese Longitudinal Healthy Longevity Survey (CLHLS) dataset in 2018. The CLHLS conducted face-to-face interviews with centenarians (people aged 100 or above) randomly chosen across 23 Chinese provinces. For each centenarian, two local residents (same village, street, town, or city district) were also interviewed [23]. Predefined age and sex were randomly determined based on the assigned code numbers for matched comparisons of both sexes in each age group [7]. Interviews and basic physical assessments were conducted at participants' homes. Specifically, the survey covered diverse aspects of their lives, including demographics, family structure, diet, chronic diseases, healthcare, smoking and alcohol consumption, early nutrition, and health-related conditions. Refusal rates among the Chinese elderly were minimal due to their enjoyment of interaction and pride in their longevity [23]. For disabled individuals, proxy respondents (typically spouses or other close family members) were interviewed [23]. The survey included 15,874 older adults in 23 out of 31 Chinese provinces; among them, 67.4% were aged ≥80 years. The CLHLS study was approved by the Institutional Review Board of Duke University (Pro00062871) and the Biomedical Ethics Committee of Peking University (IRB00001052–13,074). The study protocol was approved by the Ethics Committee of Osaka Dental University (approval number: 111229; August 23, 2022), which waived the requirement for additional informed consent. Figure 1 presents the flowchart for patient selection. After excluding ineligible participants, 6,387 participants were included in the final analysis. Generally, we excluded outliers and missing values for the tooth number variable, the tea variable, and other covariates.

To collect information regarding tooth count, the survey asked respondents, “how many natural teeth does the respondent have (excluding false teeth)?” and “Do you have false teeth?” False teeth included both fixed and removable dentures in this study [24]. Based on previous reports, we categorized the number of teeth into two groups: more than 20 or less and more than 10 or less [5, 6].

The items regarding the tea type were as follows: “What type of tea do you mainly drink at present?” and “What type of tea did you mainly drink around age 60?”. The responses regarding the frequency of tea drinking included the following: almost every day, not every day but at least once per week, not every week but at least once per month, not every month but occasionally, rarely or never. We selected people aged 60 years who were drinking the same type of tea almost every day. This was because the study focused on people aged >65 years who had consumed the same tea type for at least 5 years. The tea types included green tea, black tea, oolong tea, white tea, yellow tea, dark tea, compressed tea, scented tea, and others. Based on the sample size of the tea drinking population, we selected green tea, oolong tea, black tea, and scented tea.

Age was divided into five groups: 65–69, 70–74, 75–79, 80–84, and ≥85 years. The participant characteristics included sex (male/female), ethnicity (Han/minority), residence (city/rural), and geographical region (northern China/eastern China/central China/south-western China). Educational level was divided into three categories according to years of education: 0, 1–6, and >6 years. Health level was classified as good, average, and poor. Household income was divided into five 20-percentile groups. Smoking habits were divided into three groups according to the number of years smoking as follows: never smoked, 0–14 years, and ≥15 years. Drinking habits were classified as never, current drinker, and former drinker. Marital status was assessed by providing a yes/no response to the following options: divorced, widowed, married but not living with a spouse, and never married. Brushing frequency was determined based on the daily frequency: less than once and once or more. Further, the presence of hypertension, diabetes, and cardiovascular disease was assessed based on a yes/no response. Body mass index (BMI) was classified based on the World Health Organization classification as follows: <18.5 kg/m², 18.5–24.9 kg/m², and ≥25 kg/m² [25]. The frequency of sugar consumption was assessed based on the following responses: almost every day, at least once per week or month, rarely or occasionally. Activity of daily living (ADL) grades were also classified as disabled and non-disabled based on a previous study [26].