Wednesday September 26 2007
There is some evidence that oily fish reduces the risk of heart disease
Omega-3 oils found in oily fish such as trout and sardines could protect children against diabetes, newspapers reported on September 26, 2007. They say that research in 1,770 children has found that those who ate a diet rich in these fatty acids reduced their chances of developing type I diabetes by up to half.
The stories are based on a study that observed children who had a genetic predisposition to developing diabetes. Over a period of time, researchers examined how the childrens’ fatty acid intake affected the development of islet cell autoimmunity (ICA), which, when progressed, typically preceeds the development of type 1 diabetes.
This study does seem to suggest that there may be a link between omega-3 intake and the development of diabetes in children who are at risk. However, the study and news reports should not be interpreted to mean that eating oily fish can improve symptoms or cure diabetes in those who have already developed the condition.
People should follow advice from the FSA for the maximum amount of oily fish that can be eaten in a week.
Where did the story come from?
Dr Jill Norris and colleagues from the University of Colorado and University of Florida, USA conducted this research. The study was funded by grants from the National Institutes of Health and Diabetes Endocrine Research Centre, Clinical Investigation and Bioinformatics Core. The study was published in the peer-reviewed, The Journal of the American Medical Association.
What kind of scientific study was this?
This was a cohort study where the researchers followed children, who had an increased risk of developing diabetes by having high risk genes or a first-degree relative with diabetes. The looked at whether the children developed Islet Cell Autoimmunity (ICA) where the body’s immune system attacks insulin producing cells in the pancreas, and which often leads to the development of diabetes. The researchers wanted to see how the consumption of omega-3 and omega-6 fatty acids affected their risk of developing the condition.
The researchers observed 1,770 children between January 1994 and November 2006. Children could be enrolled at any point in the 12-year study period and average age at last follow-up was 6.2 years.
The childrens’ diet was assessed from the age of 2 onwards by an annual food-frequency questionnaire. Within it, the parents were asked to recall what their child had eaten over the past year. The questionnaire asked about the consumption of foods containing omega-3 and omega-6 fatty acids; such as tuna, salmon, mackerel, etc; and the parents were asked to estimate how often the child had eaten these foods. The researchers then calculated the total amount of fatty acids consumed.
The children were also examined at nine, 15 and 24 months and then annually thereafter for evidence of ICA. The risk of developing this immune response according to fatty acid intake was then calculated. Additionally, the researchers took into account data from the questionnaire about factors that could also have an effect, such as genetic risk, socio-demographic factors, total energy intake and the child's age when introduced to cereals.
What were the results of the study?
The researchers found that 58 of the 1,770 children developed Islet Cell Autoimmunity (ICA).
The researchers defined the criteria for ICA to be a condition where at least 1 of 3 possible autoantibodies (cells which are produced by the body that attack the individual's own cells)were detected on two consecutive occasions.
After taking into account other potentially contributing factors, they found that every additional 0.8 grams per day of omega-3 fatty acid the children consumed was associated with a 55% decreased risk of developing ICA.
When they limited the analysis to only the 45 children who had two or more of the autoantibodies, or those children who had actually been diagnosed with type 1 diabetes, they found that the decrease in risk was even greater.
We are given no details of how the “total omega-3 fatty acid intake” was quantified. Omega-6 and other fatty acids tested were not found to be associated with a reduced risk of ICA.
What interpretations did the researchers draw from these results?
The researchers conclude that higher intake of omega-3 fatty acids is associated with reduced risk of developing ICA in those with a genetically increased risk of developing type 1 diabetes. They suggest that this may be due to omega-3 promoting the production of certain anti-inflammatory substances in the body.
If their hypothesis is confirmed, they say that “dietary supplementation with omega-3 fatty acids could become a mainstay for early intervention to safely prevent the development of type I diabetes”.
What does the NHS Knowledge Service make of this study?
This study does seem to suggest that there may be a link between omega-3 intake and the development of diabetes in children who are at risk. However, the following points must be considered when interpreting these findings:
- This study has relied upon very rough estimates of fatty acid intake: asking parents to remember the amount of certain foods the child has consumed during the whole of the past year. Details are not given of the questions asked about oily fish intake. There are likely to be some inaccuracies in the food frequency data.
- The study only included children who were at increased risk of type I diabetes through having a first-degree relative with diabetes or high risk genes. Some potential risk factors for the development of islet cell autoimmunity, such as infections, have not been considered.
- It is also important to realise when reading the news reports, that this condition is different from the growing epidemic of type II diabetes (often attributed to obesity).
- The children in this study were recruited at various ages and points in time and so would have all received varied lengths of follow-up. This could lead to inaccuracy in the data collected; for example, some would have been observed for a much longer time than others and so changes in antibody status may have been more likely to have been picked up.
This study suggests an area for further research into the possible relationship between omega-3 oils and type I diabetic risk. Importantly, the study and news reports should not be interpreted to mean that eating oily fish can improve symptoms or cure diabetes in those who have already developed the condition.
The food standards agency has given a recommended upper limit for eating oily fish because “some oily fish contain chemicals such as dioxins and PCBs, which accumulate over time in the body and could have adverse health effects if consumed over long periods at high levels”
Their recommendation (to reduce the risk of heart disease) is that people should eat at least two portions of fish a week, and that one should be oily. The recommended maximum levels to avoid the possible risks from dioxins are as follows:
- Men and boys, and women past child bearing age, can eat up to four portions of oily fish a week.
- Women of child bearing age, including pregnant and breastfeeding women, and girls, can eat up to two portions of oily fish a week.
Sir Muir Gray adds...
An interesting finding, but this does not change the FSA advice; fish is good for you but, like all foods, moderation is sensible and an overdose of any single type of food is to be avoided.