Dairy product consumption and development of cancer: an overview of reviews

Abstract

Objectives: To provide a comprehensive systematic overview of current evidence from pooled analyses/meta-analyses and systematic reviews (PMASRs) pertaining to dairy consumption and incident cancer and/or all-cause or cancer-specific mortality.

Design: Overview of reviews.

Setting: Community setting.

Participants: The unit of analysis is PMASRs. A total of 42 PMASRs was included in this overview of reviews.

Interventions/exposures: Any dairy product consumption (eg, milk, yogurt, etc).

Primary and secondary outcomes measures: Primary outcome measure is development of any type of cancer. Secondary outcome measures are all-cause mortality and cancer-specific mortality.

Results: From 9693 citations identified, we included 42 PMASRs (52 study reports) published between 1991 and 2017. Thirty-one (74%) of these was pooled analyses/meta analyses, and only 11 (26%) were systematic reviews and meta-analyses. There was a wide variability in the type of study designs included within the other PMASRs, thus contributing to variable and, in instances, divergent estimates of cancer risk for several cancer subtypes. For example, only one systematic review and meta-analysis exclusively included prospective study designs. Most PMASRs were of low to moderate quality based on the Assessing the Methodological Quality of Systematic Reviews (AMSTAR) scores. The median AMSTAR score was 5 (IQR 2-7). Our overview identified conflicting evidence from PMASRs on association between dairy consumption and incident cancers or mortality. Heterogeneity in summary estimates reflected the inclusion of variable study designs and overall low methodological quality of individual PMASRs.

Conclusions: The association between dairy consumption and cancer risk has been explored in PMASRs with a variety of study designs and of low to moderate quality. To fully characterise valid associations between dairy consumption and risk of cancer and/or mortality rigorously conducted, PMASRs including only high-quality prospective study designs are required.

Trial registration number: CRD42017078463.

Keywords: cancer; dairy; dairy products; overview of reviews.

Figure 1

Preferred Reporting Items for Systematic Reviews and Meta-Analyses flow diagram depicting study selection process.

Figure 2

Bar chart depicting the Assessing the Methodological Quality of Systematic Reviews (AMSTAR) scores of included pooled analyses/meta-analyses and systematic reviews (PMASRs). The y-axis represents the percentage of included PMASRs, and the x-axis represents the AMSTAR scores (low, moderate or high).

Figure 3

Radar plot depicting the nature of association between dairy consumption and risk of cancer (as reported in included pooled  analyses / meta- analyses and systematic reviews (PMASRs)). The numbers on the plot represent the total number of meta-analyses from included PMASRs reporting a specific cancer association. The black, red, and green lines represent number of meta-analyses reporting no association with, increased risk or decreased risk of specific type of cancer, respectively.

Figure 4

Radar plots depicting the associations between various dairy products consumption and risk of gastrointestinal cancer. The numbers on the circles of each plot represent the total number of meta-analyses from included pooled  analyses / meta- analyses and systematic reviews reporting a specific cancer association. The black and green lines represent number of meta-analyses reporting no association with or decreased risk of gastrointestinal cancers, respectively.

Figure 5

Radar plots depicting the nature of association between various dairy products consumption and risk of hormone-dependent cancers. The numbers on the circles of each plot represent the total number of meta-analyses from included pooled analyses/meta- analyses and systematic reviews reporting a specific cancer association. The black, red and green lines represent number of meta-analyses reporting no association with, increased risk or decreased risk of hormone-dependent cancers, respectively.