We evaluated publication bias by Egger test and Beeg test. The results of the Egger test (P > .05), and the Begg test (P > .05) provided statistical evidence for funnel plot symmetry in the overall results, suggesting the absence of publication bias (Table 2).
The published epidemiologic data do not provide consistent evidence for an association between the consumption of milk or milk products and breast cancer risk. However, among these data many factors must be considered. Moreover, assessment of dietary factor in relation to cancer risk is very difficult and affected by many potential biases. Several methods used in epidemiologic studies, such as food frequency questionnaires and diet records or food diaries, have shown only moderate reliability, and some misclassification of intake is also unavoidable.[18,19] Therefore, different dietary assessment methods may produce different results. It is generally believed that most dietary factors have relatively small effects on cancer risk and the inevitable misclassification of dietary variables increases the difficulty of detection in risk associated with consumption of milk products.
In addition, average intake varies should be considered between different populations such as a level of consumption that is defined as “low” in one population might be considered “high” in another population. For instance, in a Japanese study, daily intake of milk or milk products was identified as highest exposure category. While, in a study conducted in the United States, the reference category of the lowest exposure level was reported consuming ≤1 serving/d and the highest level was reported >3 servings/d. In some cases, the investigators did not report the level of consumption within each quantile, which made it impossible to compare effects at similar levels of consumption across studies. Besides the challenges that are common to many studies of nutritional epidemiology, there are specific challenges related to evaluating milk products. A main hypothesis suggesting that dairy products may reduce breast cancer risk is based on vitamin D content of these products. In the United State, most manufacturers add vitamin D to many products, including milk and margarine. These differences suggest that studies from countries with different regulations and practices regarding vitamin D fortification are not strictly comparable. If vitamin D is the component of milk products that influences breast cancer risk, comparisons should take into account not only the milk products but also the level of vitamin D in mike products.
Generally, the epidemiology studies reviewed do not provide consistent evidence for an association between milk product consumption and breast cancer risk. In addition, some studies also suggest that certain types of fat, growth factors, or environmental contaminants found in milk could increase risk of breast cancer.
Another important question is the cow received bovine growth hormone thus potentially increasing insulin-like growth factors-1 levels in the milk which could in turn stimulate malignant cells to grow more rapidly. The data should analyze whether the milk was derived from cow's receiving bovine growth hormone. We reviewed all the source of milk in those studies, but those studies did not mention sources of milk, and did not analyze the composition of milk. Therefore, additional studies that identify the association with certain hormonal and environmental factors may help to further understand the relationship of milk products and breast cancer risk. In conclusion, although several interesting hypotheses link milk product and breast cancer, the available epidemiologic evidence does not support a strong association between the milk/milk products and breast cancer risk.
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