In this meta-analysis, we found that the intake of dairy products, including total dairy, milk, cheese, yogurt, or low-fat milk, as well as calcium were not significantly associated with lung cancer risk. The association remained unchanged when stratified by study design, gender, geographic area, quality and smoking status. Our results are consistent with the findings from the largest cohort study (NIH-AARP Diet and Health Study)8, and this null association was also found in many other types of cancer, such as gastric6, bladder7 and pancreatic cancers5. Our study involved a large sample size, covered a long time span from 1989 to 2012, and thus provided a conclusive answer to the association issue between dairy consumption and lung cancer risk.

Dairy products are nutrient-rich foods, which contain carbohydrates, fatty acids, proteins, vitamins, minerals and small bioactive molecules. However, the effects of dairy products on human health are just as complex as their constituents, and the role of dairy products in cancer development remains controversial. On one hand, some studies showed that dairy might be beneficial for cancer prevention as it contains various minerals and vitamins36. The most remarkable example is the protective effect of vitamin D and calcium intake on colorectal cancer37,38. However, in our meta-analysis, calcium intake appears not to be associated with a lower risk of lung cancer. In addition, we analyzed the conflicting results of several studies that reported the association of vitamin D intake and lung cancer risk39,40,41. The pooled RR for highest versus lowest supplements of vitamin D was 0.97 (95% CI: 0.80–1.18), with no heterogeneity observed (I2 = 0.00%, P- heterogeneity = 0.43), indicating that vitamin D supplements are not likely to be helpful for lung cancer prevention. The consistency of the results from different types of dairy products, calcium and vitamin D intake, supported the null association to some degree.

On the other hand, some others argue that high content of lactose, D-galactose in milk might promote oxidative stress, which will induce aging, chronic inflammation and other injuries to the organism. Consistent with this view, Ji et al recently performed a large cohort study that demonstrated that individuals with lactose intolerance, who consumed less milk or dairy products had a lower risk of lung cancer42. Additionally, increases in levels of saturated fat, exogenous DNA agents43, several bioactive components like IGF-1 (insulin-like growth factor-1) and hormone metabolites in milk are also suggested to contribute to the development of many types of cancer44. In contrast, a previous meta-analysis, showed that circulating levels of IGF-1 in plasma, was not associated with lung cancer45. In fact, studies on the influence of specific components in dairy products on lung cancer performed in vitro or in animal models are quite few, which might be explained by the fact that their relationship is not close, since negative results often remain to be unpublished.

As a systematic quantitative method, meta-analysis is useful to reveal those possible associations that are not so obvious in individual studies. It also allows more sub-group analyses to be stratified by confounders, thus it enables a reliable conclusion. However, several shortcomings should also be noted in our analysis. First, to increase the sample size, both case-control and cohort studies were included in the meta-analysis, as a known fact, retrospective studies are susceptible to recall and selection bias. Indeed, there are some differences in the results of case-control studies and cohort studies, however, the differences were not statistically significant, except that the result of cheese intake in cohort studies was inversely significant (RR: 0.63, 95% CI: 0.46–0.87), which might be explained by the small number of studies included.

Secondly, since different exposure ranges and dietary assessment were used in original studies, we did not perform dose-responses effect analyses, and the lack of association also reduced the need for performing this analysis. The limitations described above contributed to high heterogeneity across studies, other factors like baseline characteristics of the population, ascertainment of lung cancer cases and different adjusted confounders also influenced the results. Specifically, high heterogeneity was found in our meta-analysis, especially in lower-quality or case-control studies, in according with our expectations. Meta-regression analyses were used to explore the sources of heterogeneity, however, significant factors were not easily located. In particular, lung cancer is a highly heterogeneous disease with various genetic backgrounds, which may have contributed to increased discrepancy between studies.

In addition, in our sub-group analyses, because of different dietary patterns between Western and Eastern countries, the results from Asia were relatively few, which weakened the statistical power. However, we did not observe much differences in the results between Asian and Western countries. Another problem was the confounder of smoking, as shown in Table 1, as there were indeed significant differences between the results of the studies whether they adjusted for smoking or not in the categories of milk and cheese intakes, implying the confounding effect of smoking on the results. Yet, the data stratified by smoking status were not commonly provided in the original studies, unless the results were a bit abnormal, which increased the artificial bias and limited our sub-group analyses. Thus, we only roughly combined the results of both total dairy products and milk consumption in different smoking groups, and found that the results were non-significantly positive in smokers and non-smokers.

Lastly, both the incidence and mortality of lung cancer were included in our study, and interestingly, subgroup analyses stratified by outcome showed that there were significant differences between the two groups in the categories of dairy and cheese intake (as shown in Table 1). Dairy and cheese intake was inversely associated with lung cancer mortality, this might be explained by the possibility that some components in dairy products are beneficial for the survival of lung cancer. In fact, some studies reported that intake of vitamin D was associated with improved survival in early stage lung cancer patients46,47. However, these mortality studies are limited, and the pooled RR was not significant in the category of milk when more studies were included. As such, additional studies are needed to investigate the association between dairy product intake and lung cancer mortality and survival.

Hence, based on both epidemiological data and biological evidence, there is no apparent association between the intake of dairy products or calcium and the risk of lung cancer. Future efforts should be focused on seeking other nutritional risk factors for lung cancer. Furthermore, there were indeed some concerns regarding the influence of dairy products on the incidence of cancer, especially prostate and ovarian cancers. To avoid the harmful effects of dairy products, more advanced technology in milk processing is urgently needed, which might reduce lactose content, bioactive components and other detrimental ingredients found in dairy products.