Breast cancer, lactation, and genetics

Abstract

A review of mortality rates from breast cancer in several nations indicates that most Caucasian populations have higher rates than do most non‐Caucasian. The only environmental factor shown to affect breast cancer rates consistently is parity, and its mechanisms of actuation are unknown. The higher risks of breast cancer among relatives of patients are reviewed, and a multifactorial genetic control is assumed in which the effects of genetic and environmental factors may be in part additive. While genetic effects are evident in both breast cancer and lactation, it is not clear whether they depend on two distinct sets of genetic factors or on a single set. Assuming that the higher frequencies of breast cancer in Caucasian populations are in some way associated with abnormal genetic factors in hypolactation, the hypothesis is suggested that their rate of elimination by natural selection might have become relaxed or diminished in remote prehistoric times only in Caucasian populations through their widespread development of domestic animal milk as an artificial infant food. Relaxed selection of genetic factors for hypolactation might have caused them to accumulate among Caucasian populations in sufficient frequencies to produce appreciably higher rates of breast cancer in these populations today, even where breast‐feeding is the rule.