Introduction
There is limited understanding regarding the etiology of childhood brain tumors (CBTs), which are the most common solid tumors among children [
1]. Evidence from animal studies has led to a hypothesis that the central nervous system is susceptible to carcinogenesis during the prenatal period [
2]. Maternal exposures during pregnancy might play a crucial role in the risk of CBTs, as reported in two recent meta-analyses [
3,
4].
In 2022, the prevalence of tobacco use among females aged 15 years and older was 7.4% [
5]. A cumulative count of 83 constituents found in tobacco and tobacco smoke, including polycyclic aromatic hydrocarbons (PAHs) and tobacco-specific N-nitrosamines (TSNAs), have been categorized as carcinogens by the International Agency for Research on Cancer (IARC) [
6]. The IARC has classified parental smoking as a causal factor for childhood leukaemia and childhood hepatoblastoma [
7]. Furthermore, the presence of carcinogens in tobacco smoke might exert a more pronounced impact on fetuses and young children due to their underdeveloped blood-brain barrier [
2]. Therefore, maternal smoking during pregnancy might be a potential cause of CBTs. Findings from prior studies investigating the association between maternal smoking during pregnancy and the risk of CBTs have shown inconclusive results [
8‐
41]. In a meta-analysis published in 2014, no significant association was found between maternal smoking during pregnancy and risk of CBTs (odds ratio (OR) 0.96, 95% confidence interval (CI) 0.86–1.07) [
42]. While, the latest meta-analysis reported that maternal smoking > 10 cigarettes per day during pregnancy (effect sizes 1.18, 95% CI 1.00–1.40) were associated with CBTs risk in cohort studies [
4]. However, the four included cohort studies involve a duplicated population, leading to inaccurate results [
43,
44]. In comparison to previous meta-analyses on this subject, the present study included more original studies with relatively high quality and avoided duplicated population. In addition, our current study also explored the correlation between maternal smoking during pregnancy and the risk of CBTs, while categorizing it by tumor category, quantity of cigarettes smoked, age at diagnosis, and the type of exposure (active/passive smoking).
In 2020, an estimated 4.1% of new cases of cancer worldwide were attributable to alcohol consumption [
45]. Alcohol has been reported to be associated with various types of cancer, including liver cancer, colorectal cancer, and upper digestive tract tumors [
46]. The exact mechanisms by which alcohol exerts carcinogenic effects are not fully understood. Possible mechanisms include the genotoxic effects of acetaldehyde, which can cause DNA damage [
46,
47]. Alcohol can also cross the blood-brain barrier [
48], which may be a risk factor for the central nervous system and warrant further investigation. Most studies suggest no significant association between maternal alcohol consumption and the risk of CBTs [
12,
20,
29,
49‐
51]. While there are still some studies suggesting an increased risk, especially for beer consumption [
11,
15,
52]. In this meta-analysis, we investigated the relationship between alcohol consumption during pregnancy and the risk of CBTs. Additionally, we conducted subgroup analyses based on the types of alcohol consumed and subtypes of brain tumors.
Coffee and tea are the most popular beverages worldwide. It has been reported that the consumption of coffee and tea is associated with various metabolic diseases, cardiovascular conditions, cancers, and so forth [
53,
54]. Both coffee and tea contain caffeine [
55]. The CARE Study Group has proved that caffeine is rapidly absorbed and readily passes the placental barrier [
56]. Accumulating evidence from epidemiological studies showed that consumption of caffeine during pregnancy is associated with adverse gestational outcomes. In addition, caffeine exposure during pregnancy may induce epigenetic changes in the developing fetus [
57]. Several studies have explored the association between maternal coffee and tea consumption during pregnancy and the risk of CBTs [
29,
38,
50,
58‐
61]. However, the results are inconsistent. Evidence from the study conducted by Plichart et al. suggests that maternal consumption of coffee and tea during pregnancy might elevate the risk of CBTs [
38]. Greenop et al. found that maternal consumption two or more cups of coffee a day during pregnancy is associated with an increased risk of CBTs [
60]. On the other hand, Pogoda et al. reported no associations between brain tumor risk and maternal consumption of caffeine, but the results suggested a borderline increased risk tendency [
61]. While three others found no significant associations with coffee, tea, or caffeinated beverages [
29,
58,
59]. In the present study, we meta-analyzed these data to further explore such relationship.
The present study aimed to investigate the potential association between maternal smoking, alcohol and caffeinated beverages consumption during pregnancy and the risk of CBTs.
Discussion
Smoking, alcohol consumption, and consumption of caffeinated beverages have become common lifestyles for people. In recent decades, studies have explored the relationship between maternal exposure to these factors during pregnancy and the risk of childhood brain tumors, the most common solid tumor in children. This study aimed to compile data to provide clues and evidence for the prevention of childhood brain tumors.
Maternal smoking during pregnancy and the risk of CBTs
Findings from prior studies investigating the association between maternal smoking during pregnancy and the risk of CBTs have shown inconclusive results. The results of the current meta-analysis indicated a borderline statistically significant increased risk of CBTs associated with maternal smoking during pregnancy (OR 1.04, 95% CI 0.99–1.09), which is inconsistent with previous meta-analyses [
4,
42] and the results from the conference in 2022 [
69]. Furthermore, the meta-analyzed results of cohort studies also showed increased risk of CBTs (OR 1.12, 95% CI 0.98–1.28). However, the three prospective studies which largely avoided recall bias all lacked data on potential confounding factors that could impact the risk of CBTs [
24,
26,
27]. Findings derived from the large Swedish cohort study indicate that while maternal smoking during pregnancy has a limited overall effect on risk of CBTs, it may increase the risk of astrocytomas [
27]. When we conducted subgroup analyses for active and passive smoking during pregnancy separately, we found that passive smoking (OR 1.12, 95% CI 1.03–1.20), rather than active smoking (OR 1.00, 95% CI 0.93–1.07), led to an increased risk of CBTs. Some studies demonstrated that passive smoking, but not active smoking, is associated with increased risk of some cancers [
70,
71]. While, some studies reported that both active smoking and passive smoking increased cancer risk [
72,
73]. However, these findings do not imply encouragement for active smoking during pregnancy. Such results may be influenced by confounding factors, although it cannot be ruled out that women might have a higher tolerance for active smoking.
In this meta-analysis, for studies that did not explicitly specify passive smoking, maternal exposure to paternal smoking during pregnancy was defined as passive smoking. Furthermore, a statistically significant association was identified in cases of glioma (OR 1.14, 95% CI 1.05–1.25). Additionally, in this study, no dose-response relationship was found between the number of cigarettes smoked by mothers during pregnancy and the risk of brain tumor incidence. These results suggest that during pregnancy, reducing the amount or frequency of smoking may not decrease the risk of childhood brain tumors. Instead, quitting smoking is necessary. In the present study, we also noticed a consistent pattern suggesting a link between maternal smoking during pregnancy and the risk of CBTs, particularly in younger age groups at the time of diagnosis. In addition, mothers who smoked during pregnancy are more likely to smoke after delivery. Therefore, it can also be further speculated that maternal smoking during pregnancy may have a greater impact on the child than after delivery.
Maternal consumption of alcohol during pregnancy and risk of CBTs
Our meta-analysis did not find any statistically significant association between maternal alcohol consumption during pregnancy and the incidence of CBTs (OR 1.04, 95% CI 0.83–1.32). Interestingly, when we conducted subgroup analysis on different types of alcohol consumption, we observed a trend indicating a potential decreased risk of CBTs with wine consumption (OR 0.87, 95% CI 0.72–1.05), although this finding did not reach statistical significance. Unlike other alcoholic beverages, low-to-moderate wine consumption can reduce the incidence of cardiovascular diseases, type 2 diabetes, and lower the risk of certain tumors [
74,
75]. However, there is still insufficient evidence at present to definitively classify consumption of wine as part of a healthy lifestyle. Howe et al. and Bunin et al. found that maternal beer consumption during pregnancy is associated with increased risk of CBTs [
11,
15]. However, the results of the present meta-analysis suggested no statistically significant association (OR 1.13, 95% CI 0.89–1.44). Furthermore, neither glioma risk (OR 1.00, 95% CI 0.73–1.39) nor embryonal tumor risk (OR 1.12, 95% CI 0.84–1.49) was significantly associated with maternal consumption of alcohol during pregnancy.
While our meta-analysis suggests that there is no significant association between maternal alcohol consumption and the risk of CBTs, it is important to interpret these conclusions cautiously due to the fact that all the studies included in our analysis were case-control studies. Additionally, the number of studies included in this meta-analysis is small, highlighting the need for larger and less biased studies in the future to validate these findings. Specifically, prospective cohort studies would be valuable in providing more robust evidence regarding the potential link between maternal alcohol consumption during pregnancy and the risk of CBTs. Furthermore, it is important to note that while some current research results suggest that moderate alcohol consumption may reduce the risk of CBTs, it does not change the overall understanding of alcohol’s impact on public health. The World Health Organization still considers alcohol to increase the risk of cancer, regardless of the amount consumed [
76,
77]. There is strong evidence linking alcohol consumption to an increased risk of breast, liver, oral, and colorectal cancer in adults [
78,
79]. Therefore, it is still advisable to avoid alcohol consumption during pregnancy since it is related with cognitive defects and fetal alcohol spectrum disorders [
80].
Maternal caffeinated beverages consumption during pregnancy and risk of CBTs
Due to the limited number of studies investigating the relationship between maternal consumption of caffeinated beverages during pregnancy and the risk of CBTs, as well as the inclusion of studies utilizing overlapping population data that needed to be excluded [
38,
50], only five case-control studies were involved in the present meta-analysis [
29,
58‐
61]. Among these studies, two of them reported the intake of coffee and tea [
29,
60]. As both coffee and tea contain caffeine, in these studies, coffee and tea were categorized as caffeinated beverages [
55]. The remaining three studies classified caffeine as the exposure factor but did not specifically report the information of coffee and tea consumption [
58,
59,
61]. Our results indicate that maternal caffeinated beverages consumption during pregnancy may increase the risk of CBTs (OR 1.16, 95% CI 1.07–1.26). Subgroup analysis of tumor category showed a similar trend in gliomas (OR 1.15, 95% CI 1.04–1.27), which is consistent with the conclusions of two previous meta-analyses on the relationship between coffee and tea intake and the risk of adult gliomas [
81,
82]. No significant association was found between tea consumption during pregnancy and the risk of CBTs (OR 1.06, 95% CI 0.90–1.24). Differences in manufacturing processes and different types of coffee and tea may play different roles in the progression of cancer [
83]. Individuals may also change their preference for coffee types, and different conclusions may be drawn due to regional differences in coffee preferences. However, currently, there is a lack of research on the risk of CBTs associated with maternal consumption of different types of coffee.
Until now, no explicit explanations have been given to explain the association between maternal caffeinated beverages consumption and increased risk of CBTs. Both coffee and tea contain caffeine. Caffeine and its related substances could inhibit DNA topoisomerase II (topo II), which plays an important role in cell growth and division [
84]. Topo II inhibition may result in chromosomal aberrations and translocations, speculated to contribute to the pathogenesis of infant tumors. Ross et al. reported a positive association between maternal intake of Topo II inhibitors during pregnancy and the development of infant tumors [
85]. On one hand, numerous studies suggest that caffeine consumption might act as a protective factor against various cancers [
86‐
88]. On the other hand, several observational studies and most Mendelian Randomization studies did not provide sufficient evidence for a causal role of coffee or caffeine on these health outcomes [
89‐
91].
Bias, limitations and strengths
The following aspects might contribute to bias to the involved original studies: (1) Most of the involved studies were case-control studies which cannot avoid recall bias. It is difficult for parents to correctly remember their lifestyle 10 years (or more) before the studies. In addition, case mothers were more likely to over-report their exposure because they might be more inclined to consider smoking and consumption of beverages (alcohol, coffee, or tea) as a risk factors. (2) Mothers might under-report their exposure to smoking and beverages (alcohol, coffee, or tea) during pregnancy because they may not want to admit or be accused of harming the child. (3) About 20–50% of female smokers attempt to quit smoking during pregnancy, but half of them will fail. Women who fail to quit smoking typically go through a cycle of trying to decrease or quit, then relapsing, and making renewed attempts to quit. Therefore, in this situation, it is difficult for the studies to collect precise information about smoking [
92,
93]. In addition, mothers who smoked during pregnancy are more likely to smoke also before conception and after delivery. However, the present study did not explore the association between maternal smoking before conception, after delivery and risk of CBTs. (4) Women classified as nonsmokers might have been exposed to passive smoking, potentially diminishing the effect of maternal smoking during pregnancy. (5) There is a possibility that children with CBTs, exposed to parental smoking, may be more active and may more frequently go to the hospital for physical examination, which might bring selection bias to the studies.
This study has some limitations: (1) The majority of the studies involved in the current meta-analysis were case-control studies, demonstrating an association rather than causality. (2) Some involved studies reported the data that could be used for subgroup analysis, while some other studies did not report such data. Thus, the results of subgroup analyses may not represent all the populations of the involved studies. (3) The number of studies regarding maternal alcohol and caffeinated beverages consumption, as well as the sample sizes in many subgroup analyses, is still insufficient. (4) Mothers exposed to maternal smoking and consumption of beverages during pregnancy are more likely to be exposed to these factors both before conception and after delivery. However, the current study did not investigate the correlation between exposure to these factors before conception, post-delivery, and the risk of CBTs. Therefore, these findings cannot precisely represent the exposure of mothers during pregnancy.
The strengths of this study include: (1) The present study is the largest meta-analysis to date that investigated the association between maternal smoking, alcohol, and caffeinated beverages consumption during pregnancy and risk of CBTs. In this study, we performed a comprehensive literature search. We reviewed the references of relevant literature to avoid any omissions. In addition, quality control was conducted on the literature. (2) This meta-analysis avoided the inclusion of duplicate populations when combining effect sizes. (3) We conducted multiple subgroup analyses to further investigate the relationship between exposure factors and the disease.
Conclusions
In conclusion, the current meta-analysis revealed an association between passive smoking during pregnancy, rather than active smoking during pregnancy, and an increased risk of CBTs. Furthermore, maternal smoking during pregnancy is associated with an elevated risk of childhood glioma. In addition, a trend was noticed showing an elevated risk of CBTs in younger age groups exposed to maternal smoking during pregnancy. Moreover, maternal caffeinated beverages consumption is associated with an increased risk of CBTs, especially glioma. The results of the present meta-analysis suggest no significant association between maternal alcohol consumption and the risk of CBTs. Because of the limitations of the present study, more large well-designed prospective cohort studies and Mendelian Randomization studies with large sample size are warranted to provide a higher level of evidence.
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