BACKGROUND
This paper examined whether citrus peel use and black tea intake, separately and jointly, were associated with the risk of squamous cell carcinoma (SCC) of the skin in Arizona, a setting with very high ultraviolet exposure and high rates of non-melanoma skin cancer. The rationale was biologically plausible: citrus peel contains d-limonene, hesperidin, naringin, and auraptene, while black tea contains polyphenols such as theaflavins, all of which have shown chemopreventive effects in experimental systems. The authors had previously reported inverse associations for citrus peel and hot black tea individually, but the present analysis focused on whether these exposures had independent effects and whether combined intake might produce additive or synergistic protection.
METHODS
The study used data from the population-based Southeastern Arizona Health Study (SEAHS), conducted from 1994–1996. Cases were randomly selected from the Southeastern Arizona Skin Cancer Registry and were eligible if they were ≥ 30 years of age, had a histo-pathologically confirmed, non-metastatic SCC of the skin diagnosed within the past 4 months, and had no prior history of skin cancer. Controls were recruited by random-digit dialing to reflect the age and gender distribution of cases and were eligible if they had no prior history of skin cancer, lived within the Tucson region, and matched the age, gender, and ethnicity grouping. Although 404 cases and 391 controls completed the main interview, only the subset that completed the 24-hour dietary recalls were asked to complete the citrus and tea questionnaires; this yielded 450 subjects for the present analysis, including 234 cases and 216 controls. The final analytic sample included 263 men and 187 women.
Participants completed detailed citrus and tea questionnaires developed through focus groups and tested for reliability and validity. The citrus questionnaire captured citrus fruit, juice, storage, and whether citrus peel was added during food preparation or serving. The tea questionnaire assessed usual tea intake over the past year, lifetime consumption pattern, and details by tea type and temperature, including black, green, or herbal and hot or iced. Covariate data came from structured interviews and four 24-hour dietary recalls. The authors evaluated many potential confounders, but the final multivariate model included age, sex, inability to tan after prolonged sun exposure, number of current freckles on the arms, and history of physician-diagnosed actinic keratosis (AK).
KEY RESULTS
The cases and controls were similar in age and sex: age was 66.6 + 10.4 in cases and 66.2 + 11.1 in controls (p = 0.75), and 59.0% of cases versus 57.9% of controls were male (p = 0.81). Cases and controls did not differ significantly in smoking status (p = 0.36), alcohol use (78.7% versus 81.8%, p = 0.53), hours in the sun during the past year (1.38 + 0.1 versus 1.49 + 0.1, p = 0.35), kcal/day (1499.9 + 396.3 versus 1531.8 + 433.5, p = 0.41), or antioxidant intake including retinol, β-carotene, α-carotene, lycopene, α-tocopherol, and vitamin C. However, several SCC-related traits differed: AK history was reported by 78.2% of cases and 37.0% of controls (p = 0.000), tanning ability by 68.8% and 81.0% (p = 0.001), and no freckles on the arms by 50.6% and 61.5% (p = 0.005).
Exposure prevalence was substantial. In this study population, 66.4% reported some tea drinking and 34.5% reported citrus peel use during the past year. Hot and iced teas were consumed by 30.7% and 51.8% of subjects, respectively. Citrus peel use was reported by 30.2% of cases and 39.2% of controls (p = 0.045), and hot tea intake by 26.1% of cases and 35.7% of controls (p = 0.028). Iced tea intake did not differ significantly between cases and controls: 53.4% versus 50.0% (p = 0.47).
For citrus peel, the crude OR for SCC was 0.67 (95%CI = 0.45 – 0.99). After adjustment for hot tea intake, the OR decreased to 0.55 (95%CI = 0.34 – 0.91). After adjustment for iced tea intake, the OR was 0.69 (95%CI = 0.45 – 1.05). In the fully adjusted model, the ORs were 0.34 (95%CI = 0.18 – 0.62) when modeled with hot tea and 0.54 (95%CI = 0.33 – 0.89) when modeled with iced tea.
For tea, taking non-tea drinkers as the reference group, the OR for hot tea was 0.79 (95% CI = 0.49 – 1.26), while the OR for iced tea was 1.14 (95%CI = 0.76 – 1.72). After adjustment for citrus peel use, these became 0.87 (95%CI = 0.54 – 1.39) for hot tea and 1.22 (95% CI = 0.81 – 1.84) for iced tea. In the final multivariate model, the OR for hot tea was 0.65 (95%CI = 0.37 – 1.15), while the OR for iced tea was reported as 1.06 (95%CI = 0.37 – 1.15).
The most notable finding was the joint analysis. Using those who consumed neither hot black tea nor citrus peel as the reference group, the fully adjusted OR was 0.60 (0.30 – 1.23) for hot black tea alone, 0.30 (0.13 – 0.72) for citrus peel alone, and 0.22 (0.10 – 0.51) for consumption of both. The p for trend was 0.000. By contrast, for iced black tea plus citrus peel, the fully adjusted ORs were 0.91 (0.46 – 1.45) for iced tea alone, 0.32 (0.14 – 0.75) for citrus peel alone, and 0.58 (0.30 – 1.12) for both, with p for trend = 0.04.
CLINICAL IMPLICATIONS
This study suggests that citrus peel use was consistently associated with lower odds of skin SCC, and hot black tea showed a weaker but directionally protective association. The combination of citrus peel and hot black tea was associated with the strongest reduction in SCC risk, raising the possibility of additive or synergistic chemopreventive effects. For clinicians and prevention researchers, these findings support further prospective and interventional work on simple dietary exposures that may complement sun protection, particularly in high-risk populations. However, because this was a case-control study, the results remain vulnerable to recall bias, behavior change after diagnosis, and residual confounding, so they should not yet be interpreted as proof that citrus peel or tea prevents SCC.