The riddle of the RMJ
Researchers have already identified the most significant causes of most of the most common types of cancer: smoking causes 90% of lung cancer worldwide, hepatitis virus – most cases of liver cancer, Helicobacter pylori bacteria – stomach cancer, human papillomavirus – almost all cases of cervical cancer, and colon cancer is almost always explained by lack of physical activity, diet and family history.
The exception is breast cancer, the main cause of the development of which still eludes specialists. This disease is the most common form of cancer in women worldwide. However, the risk of its development is unevenly distributed. For a long time, the risk of developing breast cancer in women living in North America and Northern Europe was five times higher than in women living in Africa and Asia. However, for unclear reasons, the risk of developing this disease in African and Asian women has been growing rapidly lately.
Is the diet to blame?Even 20 years ago, experts were inclined to believe that the basis of these patterns is the diet.
According to the hypothesis generally accepted at that time, the risk of developing breast cancer increases because people abandon local food sources and switch to eating fat-rich foods subjected to intensive processing.
This hypothesis seemed logical, since a statistical comparison of fat consumption per capita of a country and the death rate from breast cancer showed pronounced correlations. Moreover, the rats contained in the fat-rich feed were more likely to develop breast tumors.
In a study involving immigrants from Japan living in California, the researchers found that the first generation of immigrants was characterized by a low risk of developing breast cancer, the same as for their parents in Japan. However, in the second and third generation, the risk increased to the level typical for white Americans. Thus, it was demonstrated that genetic factors are not the reason underlying the striking difference in the risk levels of breast cancer in Asian and American women. It also did not contradict the idea that changing the diet from low-calorie Asian food to a fat-rich American diet causes the development of cancer. It all seemed natural, but only up to a certain point.
Dietary studies have shown that fats are not the answer to the questionIn the mid-1980s, the results of large, carefully planned prospective studies began to appear in print, the purpose of which was to study the possible relationship between diet and breast cancer risk.
All the results obtained were uniformly negative and indicated that fats included in the diet of an adult woman have no effect on the risk of developing breast cancer.
It was both a surprise and a disappointment. The study of other parameters of the diet, such as the consumption of fruits and vegetables, brought contradictory results, while it turned out that alcohol consumption is associated with a moderate increase in risk. It was also found that after menopause, women with a larger body weight are at greater risk, which may indicate the role of the total number of calories consumed, rather than the composition of the diet.
There is also a possibility that fat consumption can have a serious impact on the early stages of life, up to intrauterine development, but the validity of this assumption for a person is extremely difficult to verify.
If the diet is not the main cause of breast cancer, then which other aspect of modernizing life can be blamed for this?
Two types of risk factors: mutable and immutableFactors that have a proven role in the formation of the risk of breast development are divided into two categories.
The first one includes factors that are almost impossible to change: the age of the first menstruation, the age at the time of the birth of the first child, family history, the presence of mutations of tumor suppressor genes, such as BRCA1. The second category includes variable factors: the level of physical activity, body weight, the amount of alcohol consumed and work at night.
The role of environmental pollution is controversial and difficult to study. Concern about chemical compounds, especially substances that disrupt the endocrine system, arose after the appearance of data that such compounds can affect the risk of cancer in model organisms (rodents). However, in studies involving humans, contradictory results have been obtained.
Since pregnancy at an earlier age and breastfeeding reduce the risk of developing breast cancer, the incidence of this disease is lower among African women who marry earlier and give birth to more children.
However, to date, the mortality rates from breast cancer among women in sub-Saharan Africa have almost equaled those of developed countries, despite the significantly lower incidence of this disease. This is due to later diagnosis and limited access to treatment.
All this leads to the question: can the differences between known risk factors for developed and developing countries explain the huge difference in the degree of risk? Most likely, they can't. Experts believe that the high risk characteristic of America is explained by less than half of the known risk factors. Moreover, these factors practically do not explain the difference with the risk level characteristic of Asian countries.
A related question also arises: is the high risk characteristic of America and Northern Europe due to a combination of many known impacts, each of which has a small impact on the level of risk, or is it based on the main reason that has not yet been established? And could it be that some of the known risk factors have a common cause that we have not yet been able to identify?
Maybe we're just detecting more cases of cancer?Mammographic screening, conducted since the 1980s, is one of the reasons for the increase in the incidence of breast cancer in developed countries compared to developing countries.
However, this cannot even come close to explaining the huge difference that exists. To date, it is believed that approximately 20% of tumors detected during mammography will never develop beyond the very early stage detected during screening. However, the problem lies in the impossibility of differentiating such benign tumors from malignant ones.
What about artificial lighting?The active use of artificial lighting is one of the key features of life in developed countries.
It is quite possible that the introduction and growing use of electric energy for lighting in the dark hours of the day is involved in the problem of breast cancer.
This can be explained by a violation of circadian rhythms, which changes the levels of hormones that affect the development of breast cancer. For example, electric lighting at night can put the body into "daytime mode", which is characterized by suppression of the production of the hormone melatonin. It is known that this hormone has a pronounced inhibitory effect on the development of human breast tumors implanted in rats.
The theory is easy to formulate, but it is very difficult to carry out its comprehensive verification. The first prerequisites for the emergence of this theory were the results of studies according to which women working on the night shift are at a higher risk of developing breast cancer than women working during the day.
Other prerequisites could be such facts as a lower risk of developing the disease in blind women and a higher risk – in those who sleep an insufficient amount of time and live in regions that are heavily illuminated at night. There is evidence to support each of these assumptions, but all of them are not sufficiently convincing. It is obvious that electric lighting in the evening and at night can disrupt our circadian rhythms, but whether this harms our health in the long term, including increasing the risk of developing breast cancer, is currently unclear.
Anyway, finding answers to all these questions is an extremely important task, since breast cancer, with almost 2 million new cases annually, has become a threat to women living in all corners of the globe.
Evgeniya Ryabtseva
Portal "Eternal youth" http://vechnayamolodost.ru based on the materials of The Conversation: The mystery of breast cancer.
21.04.2015