DIET AND CANCER: SPECIFIC ASPECTS OF DIET – FAT

The intake of fat (and particularly animal fats) has been put forward as an important causative factor in breast cancer. The story starts with comparisons made between the fat intake of individuals in particular countries and deaths from breast cancer in those countries. Detailed statistics are not necessary to demonstrate that countries with a high fat intake in the diet are also those where breast cancer is common. Other studies show that the association is mainly with saturated animal tat in the diet. At first glance, this is strong evidence, and indeed it is important and cannot be ignored but it does not prove that there is a link between dietary fat and breast cancer. The information on fat consumption in these countries is not very precise. It tends to be what is known as ‘disappearance data’. The fats enter the food distribution system and disappear from it. That means a lot of them will have been eaten but that some will have been cut off and thrown away. More importantly, this link is an association and not necessarily a causation. Going back to the argument that we considered when we talked about the work of epidemiologists it will be recognized that this distinction is a very important one indeed. There are many differences between the countries on that list. In general, the rich countries have a high fat intake with lots of meat and the poor countries have a lower fat intake because their diet is generally less rich. In fact the link between gross national product and breast cancer is almost as strong as the link to animal fats. It may be any one of the factors that contribute to the wealth of these countries which is important in the development of breast cancer. Something in the difference between these countries is responsible for the different incidence of breast cancer but we don’t know what it is on the basis of the evidence given so far. It means that we have to explore further to find an answer.
What about the cohort studies? The reader will recall that in this approach groups of people are identified and the dietary flat history is taken. They are then followed over a long period of rime to see who gets breast cancer and to find out if there is any connection with their dietary fat intake as measured at the beginning of the study. Three large studies of this kind have been published during the 1980s. In the United States, the Seventh Day Adventists were followed for twenty-one years. Again in the United States, 89,538 nurses aged between thirty-four and fury-nine were followed up after giving a dietary history. In a smaller US study over five thousand women were followed up. These were immensely difficult studies to perform and requited a big logistic effort. Again it has to be said that the results were essentially negative. Ho link was found between dietary fat and breast cancer in the cohort studies.
Ideally, the whole question could be resolved by an intervention study in which some women would agree to reduce their dietary fat substantially while others would not, and we could then sec if there was a reduction in the breast cancer incidence in those who had reduced their fat. It has been argued that this should be done, but no such study has been undertaken. Preliminary work has suggested that it might be possible but that it would be a hugely expensive and very time-consuming effort, with large numbers of people having to make substantial changes in their diet. Until the kind of information that we would get from an intervention study is available we will not have a certain answer, but, in our opinion, the balance of the evidence at present is rather against a strong link between dietary fat and breast cancer.
The studies of differences between countries in fat intake and incidence of breast cancer seem to point in one direction, although not conclusively. The case-control studies and cohort studies seem to point in the opposite direction. How can we explain this?
One response to this is to say that the situation is not really confused. A perfectly consistent explanation for all the observations would be that there is no causative link between dietary fat and breast cancer. The case-control studies and the cohort studies by the analytical epidemiologists are negative and this is in keeping with this view. The comparisons between countries do not prove a causative link, they just show that there is an association and there may be another explanation that we have not yet uncovered.
*57\194\4*

DIET AND CANCER: SPECIFIC ASPECTS OF DIET – FATThe intake of fat (and particularly animal fats) has been put forward as an important causative factor in breast cancer. The story starts with comparisons made between the fat intake of individuals in particular countries and deaths from breast cancer in those countries. Detailed statistics are not necessary to demonstrate that countries with a high fat intake in the diet are also those where breast cancer is common. Other studies show that the association is mainly with saturated animal tat in the diet. At first glance, this is strong evidence, and indeed it is important and cannot be ignored but it does not prove that there is a link between dietary fat and breast cancer. The information on fat consumption in these countries is not very precise. It tends to be what is known as ‘disappearance data’. The fats enter the food distribution system and disappear from it. That means a lot of them will have been eaten but that some will have been cut off and thrown away. More importantly, this link is an association and not necessarily a causation. Going back to the argument that we considered when we talked about the work of epidemiologists it will be recognized that this distinction is a very important one indeed. There are many differences between the countries on that list. In general, the rich countries have a high fat intake with lots of meat and the poor countries have a lower fat intake because their diet is generally less rich. In fact the link between gross national product and breast cancer is almost as strong as the link to animal fats. It may be any one of the factors that contribute to the wealth of these countries which is important in the development of breast cancer. Something in the difference between these countries is responsible for the different incidence of breast cancer but we don’t know what it is on the basis of the evidence given so far. It means that we have to explore further to find an answer.What about the cohort studies? The reader will recall that in this approach groups of people are identified and the dietary flat history is taken. They are then followed over a long period of rime to see who gets breast cancer and to find out if there is any connection with their dietary fat intake as measured at the beginning of the study. Three large studies of this kind have been published during the 1980s. In the United States, the Seventh Day Adventists were followed for twenty-one years. Again in the United States, 89,538 nurses aged between thirty-four and fury-nine were followed up after giving a dietary history. In a smaller US study over five thousand women were followed up. These were immensely difficult studies to perform and requited a big logistic effort. Again it has to be said that the results were essentially negative. Ho link was found between dietary fat and breast cancer in the cohort studies.Ideally, the whole question could be resolved by an intervention study in which some women would agree to reduce their dietary fat substantially while others would not, and we could then sec if there was a reduction in the breast cancer incidence in those who had reduced their fat. It has been argued that this should be done, but no such study has been undertaken. Preliminary work has suggested that it might be possible but that it would be a hugely expensive and very time-consuming effort, with large numbers of people having to make substantial changes in their diet. Until the kind of information that we would get from an intervention study is available we will not have a certain answer, but, in our opinion, the balance of the evidence at present is rather against a strong link between dietary fat and breast cancer.The studies of differences between countries in fat intake and incidence of breast cancer seem to point in one direction, although not conclusively. The case-control studies and cohort studies seem to point in the opposite direction. How can we explain this?One response to this is to say that the situation is not really confused. A perfectly consistent explanation for all the observations would be that there is no causative link between dietary fat and breast cancer. The case-control studies and the cohort studies by the analytical epidemiologists are negative and this is in keeping with this view. The comparisons between countries do not prove a causative link, they just show that there is an association and there may be another explanation that we have not yet uncovered.*57\194\4*

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