Lung cancer is the leading cause of cancer mortality worldwide. Global statistics show that in 2012 alone, lung cancer was responsible for an estimated 1.6 million deaths or 19.4% of all cancer-related deaths (1). Since survival remains low, the main hope for reducing the burden of this disease lies in prevention. Cigarette smoking is the foremost risk factor for lung cancer, accounting for up to 90% of all cases (2). Large-scale smoking prevention and cessation efforts have been attempted (3–5), and efforts in that direction must be pursued. However, other modifiable risk factors must be identified so that all possible lung cancer prevention strategies can be implemented. The recognized multifactorial etiology of lung cancer suggests that factors other than smoking, such as diet, can influence its occurrence (6).

Results from a large systematic review published by the World Cancer Research Fund (WCRF) in 2007, as well as the 2016 update of this report, suggested a protective role of diets rich in fruit and vegetables (7, 8). These foods are thought to protect against lung cancer due to their content in micronutrients that have chemopreventive properties such as antioxidant activity. Antioxidants found in the highest concentrations in both the human diet and serum samples include vitamin C and the specific carotenoids β-carotene, α-carotene, β-cryptoxanthin, lutein, zeaxanthin, and lycopene (9). In its assessment, the WCRF stated that while the evidence suggests that foods rich in carotenoids probably decrease lung cancer risk, no firm conclusion could yet be reached about the role of individual carotenoids or vitamin C as the evidence for these was too limited or inconsistent, respectively (7).

Although several studies have examined associations between individual carotenoids and vitamin C and lung cancer, including case–control studies (10–17), cohort studies (18–31), pooled analyses (32, 33), and meta-analyses (34–36), results remain equivocal. This may be attributable to a combination of small sample sizes in some studies (10, 12–19, 21–24, 28) and inadequate control of smoking in others, usually because they were not able to control for time since quitting in addition to other dimensions of smoking (25, 28, 30). Most studies focused exclusively on one sex or the other (11, 12, 14–19, 21, 23, 24, 26–28, 31), and if there was effect modification by sex, this would lead to heterogeneity of results. A few studies have suggested that antioxidants may relate differently to lung cancer risk depending on sex (10, 20, 25), smoking intensity (20, 22, 27, 31, 37) and histological subtype of the tumor (13, 16, 26, 36), and heterogeneity of risk among such subgroups may have contributed to inconsistencies in the literature.

This prompted us to examine the relationship between intake of selected antioxidants and the risk of lung cancer in a large population-based case–control study, using updated food composition databases and detailed control for confounding by several different dimensions of smoking. We examined potential effect modification by sex and by smoking intensity on the antioxidant–lung cancer relationship, and examined the association between these micronutrients and four tumor histological subtypes.

In our study, high dietary intakes of β-carotene, α-carotene, β-cryptoxanthin, lycopene, and vitamin C were associated with a decreased risk of lung cancer in main effects analyses. Findings for β-carotene concord with those of a recent meta-analysis in which a pooled relative risk of 0.77 (95% CI: 0.68–0.87) was reported (36). Our results also give credence to a protective role of several other dietary carotenoids on lung cancer and for which evidence was judged to be too limited to draw firm conclusions by the WCRF (7). We found that high intakes of lycopene, a carotenoid which is derived from food sources that tend to differ from those of other carotenoids and vitamin C, lowered the risk of lung cancer. To our knowledge, only one previous study also reported such an association (27). We also found an inverse association between high intakes of vitamin C and lung cancer risk, a finding in line with two studies (33, 35) but at odds with another (31). We did not observe that high intakes of lutein/zeaxanthin significantly lowered the risk of lung cancer, a finding that contrasts with several observational studies (12, 15, 23, 25, 26). A defining characteristic of these studies is that, unlike ours, most were conducted in men exclusively. Nonetheless, there was no indication of effect modification by sex in our study.

Our study is one of the few to have investigated the role of individual antioxidants in separate strata of smoking intensity (20, 22, 27, 31), and the first to simultaneously control for lifetime intensity and time since quitting smoking, two important dimensions of smoking history for lung cancer (48). Although the interactions between micronutrients and smoking intensity were not statistically significant, stratification by cumulative smoking intensity is an improvement over previous studies in which analyses were stratified by smoking status only (i.e., never/former/current smoking) (49), since the former approach is likely to result in more homogenous groups of smokers than the latter. We found that high intakes of β-carotene, α-carotene, β-cryptoxanthin, and lycopene were associated with a reduction in lung cancer risk among male heavy smokers, while vitamin C reduced the risk of lung cancer among female heavy smokers. This suggests that heavy intensity smokers, who suffer from high oxidative stress due to smoking and are at a disproportionate risk of developing lung cancer, can benefit from high intakes of selected antioxidants. Our findings are in line with a number of previous studies. In one study, the associations between antioxidants and lung cancer were stronger among heavy smokers than among never or light smokers, although there was no significant interaction with smoking (31). In another, an inverse association between serum carotenoid levels and lung cancer death was observed among smokers, but not among never/former smokers (50). Our findings for β-carotene are at odds with those from the CARET (51) and the ATBC (52) randomized controlled trials in which β-carotene supplementation in smokers was associated with an increase in lung cancer incidence. However, these associations disappeared when subsequent analyses considered a longer follow-up post-trial (53, 54). Our results regarding never smoking women concord with previous studies (16, 28), although based on small numbers. Furthermore, our data suggest a preventive effect of vitamin C against lung cancer in women who have never smoked, an association which, to our knowledge, has not been reported previously. We also found that women whose cumulative smoking history ranged from 1 to 850 cigarette-years benefited from intakes in the third tertile level of β-carotene and in the second tertile level of β-cryptoxanthin, lycopene, and vitamin C, findings not reported before.

In analyses stratified by histological subtype, we found similar patterns of results by the three most common tumor subtypes, with selected antioxidants being inversely related to risk. We observed that high intakes of β-carotene, α-carotene, β-cryptoxanthin, lycopene, and vitamin C were associated with a reduced risk of squamous cell carcinoma, while high intakes of β-carotene and α-carotene lowered the risk of adenocarcinoma, and both medium and high intakes of β-cryptoxanthin and lycopene reduced the risk of small cell carcinoma. Our results pertaining to risk of squamous cell carcinoma and β-carotene (36), β-cryptoxanthin (26, 32), and vitamin C intakes (20, 26, 33) are comparable to those observed in earlier studies. We found statistically non-significant associations between high intakes of β-carotene, β-cryptoxanthin, lutein/zeaxanthin, lycopene, and vitamin C and an increased risk of large cell carcinoma. To our knowledge, our study is the first to report on dietary antioxidant intakes and risk of large cell carcinoma, a rarer histological subtype (2).

In addition to their antioxidant activity, carotenoids and vitamin C may act to prevent lung carcinogenesis through different mechanisms. For instance, the pro-vitamin A carotenoids such as β-carotene, α-carotene, and β-cryptoxanthin can be metabolized into vitamin A and play a role in cell differentiation (9). β-carotene, β-cryptoxanthin, lutein, zeaxanthin, and lycopene may also inhibit cancerous cell proliferation and contribute to cell-to-cell communication (55–58). In addition, β-carotene and vitamin C have been shown to stimulate apoptosis (57, 59), while α-carotene is thought to inhibit the promotion of carcinogenesis (55, 60). It is therefore conceivable that exposure to carotenoids and vitamin C could play a role in preventing carcinogenesis at different points during the lifetime, which makes them prime candidates for lung cancer prevention.

Fruits and vegetables containing carotenoids and vitamin C are also rich in other nutrients and phytochemicals which could be responsible for their observed protective role against lung cancer. For example, dithiolthiones and isothiocyanates found in cruciferous vegetables and vegetables from the Allium genus increase the activity of enzymes involved in carcinogen detoxification (7, 61, 62). Selenium, an essential element, acts as a co-factor for glutathione peroxidase, an enzyme known to protect membranes from oxidation and which also plays a role in carcinogen detoxification (63, 64). Flavonoids are also found in fruits and vegetables, and in our study population, we previously observed a reduced lung cancer risk with intake of some flavonoid subclasses (65). Our results regarding selected groups of fruits and vegetables high in a diversity of chemopreventive micronutrients support their protective role against lung cancer. This is somewhat at odds with results from a recent case–control study conducted in Spain, which found no statistically significant reduction in lung cancer risk associated with consumption of selected fruit and vegetable groups (66) but concords with findings from large meta-analyses (7, 8, 49, 67). One should therefore keep in mind that although dietary intake of carotenoids and vitamin C may help prevent lung cancer, they could also be surrogates for other protective compounds found in selected fruit and vegetables.

Important strengths of our study include the large sample size which allowed for stratification by smoking intensity, for studying specific sources of antioxidants, for examining the impact of diet on different tumor histological subtypes, and for comprehensive control of potential confounding by smoking. Good coverage of the case population in the area and the use of incident, histologically confirmed primary lung cancer cases are also key strengths of our study. Advantages specific to our FFQ are that it contained an extensive food list and was specifically designed to cover major carotenoid and vitamin C sources in the study population. We also used updated local nutrient databases to carry out the conversion from food to nutrient intake.

This study also has some limitations. It would have been of interest to study associations in light of disease aggressiveness, but this information was not available. While the case and control participation rates were high, it is still possible that participating controls were more health conscious than the general population, thus biasing risk estimates away from the null. As well, proxy respondents provided answers for almost 40% of lung cancer cases which could have resulted in measurement error. However, sensitivity analyses indicated that associations between antioxidants and lung cancer did not differ considerably whether we considered self-respondents only or proxy respondents as well. Unmeasured changes in dietary patterns over time are another possible source of bias, although diet has been shown to be relatively stable over time (44). The study was presented to potential and actual participants as a study on environmental factors and the collection of food intake data was one of many themes covered during the interview. These precautions should have reduced the risk that subjects would self-select into the study or report intakes based on their perception of the role of antioxidants on their health status. Finally, although the diet questionnaire focused on the period 2 years prior to diagnosis, cases might have been influenced by their illness when reporting their dietary habits.

Measurement error in dietary intakes inevitably occurred in the study. The FFQ used here was adapted from widely used validated questionnaires (39, 44), which served to develop other FFQ validated and used in various Canadian and Québec populations (38, 68, 69). However, it was not comprehensively tested for reliability and validity in the present study population. Measurement error also likely stemmed from the grouping of more than one food into a single statement, from the lack of information on the exact quantities of foods consumed and on whether foods were consumed cooked or not, and from the relatively broad categories from which subjects could choose to report their intake frequency. We elected to assign mid-point values to each closed category; true intake values might have been close or far from these. Nutrient values attributed to each statement represented the average nutrient content of a food item selected from the statement and under a certain form such as raw or cooked, a choice based on Canadian and Québec food consumption statistics (70, 71). While each of these limitations may have contributed to exposure misclassification, it was likely non-differential with respect to case–control status, thereby attenuating associations. Non-differential misclassification of an exposure with more than two categories may theoretically lead to bias away from the null, but this requires misclassification between two non-adjacent categories, an extreme situation that is unlikely to be present in our data (72). Furthermore, it is highly unlikely that a trend would be reversed by such non-differential misclassification (73). The possibility also remains that the dietary data collected with the FFQ better reflected more recent intakes than remote ones. It could be hypothesized that this latter exposure would be more etiologically relevant with respect to lung cancer development which occurs over many years. Nonetheless, antioxidants are suspected to play a protective role at different times in carcinogenesis. Indeed, antioxidants can inhibit both initiation and promotion stages of tumorigenesis by protecting cells against oxidative damage (74, 75). Therefore, despite the fact that our FFQ assessed more recent antioxidant intakes, it remains a useful measurement tool to capture an effect at the promotion stage.

Finally, although careful attention was given to the control for smoking by using an algorithm shown to be superior to many others in this specific study population (45), residual confounding due to smoking cannot be excluded. Such confounding, if present, could have produced bias toward the null in the estimated associations between nutrient intake and lung cancer risk. However, such a bias could not explain the associations, albeit non-significant ones, observed among the stratum of non-smoking women. We also tested a range of other potential confounding variables such as income, lifetime alcohol intake, and exposure to asbestos, which ultimately were not included in our models because of a lack of effect on the associations.

Overall, our findings contribute both new knowledge and confirmatory data to the as yet inconclusive body of evidence on the hypothesized associations between high intakes of individual carotenoids and vitamin C, and lung cancer risk. They suggest a protective effect of selected antioxidants against lung cancer in general, for different smoking intensity groups and particularly heavy smokers, and for the squamous cell, adenocarcinoma, and small cell histological tumor subtypes. Even though smoking remains the strongest predictor of lung cancer risk, it appears desirable, in light of these findings, to further promote consumption of fruits and vegetables rich in carotenoids and vitamin C to reduce the lung cancer burden among both smokers and non-smokers.

This study was carried out in accordance with the recommendations of McGill University’s Institutional Review Board, the Ethics Committee of the Research Center of Hôtel-Dieu de Montréal and of all individual ethics committees listed in the Material and Methods section, with written informed consent from all subjects. All subjects gave written informed consent in accordance with the Declaration of Helsinki. The protocol was approved by the McGill University’s institutional Review Board (A12-E06-99) and the Ethics Committee of the Research Center of Hôtel-Dieu de Montréal (HDM-941213-18).

This manuscript was at the core of MS’s Master’s dissertation. MS conceptualized the research question and was responsible for reviewing the literature, cleaning the data, conducting the analyses, interpreting the results, and writing the manuscript. M-EP and M-CR were MS’s dissertation supervisors and provided overall guidance at all stages of the project and manuscript preparation. AK provided constructive feedback on the manuscript from its first version to the final submitted one. JS and M-EP were co-principal investigators of the original study, provided access to the data, and contributed invaluable feedback as part of manuscript revisions.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.