Mediterranean diet 'curbs diabetes'

A study has shown that “the Mediterranean diet, with abundant quantities of virgin olive oil, provides strong protection against diabetes”, The Independent reported today. It said that the diet includes high quantities of fresh vegetables, oily fish and olive oil, and may cut risk by 83 percent.

The Daily Express also covered the story and said that the diet can slash your risk of diabetes even if you are young and healthy. Other news sources also mentioned that the diet can help ward off heart disease, asthma in children and help people live a longer life.

These reports are based on a large Spanish study that involved more than 13,000 university graduates of normal weight and who did not suffer from diabetes. These recruits had an average age of 38, and they responded to questionnaires about their dietary habits. Only 33 cases of diabetes occurred over the follow up period (average 4.4 years) and although the reduction in rates of diabetes quoted is statistically significant, the way that the figures have been analysed is not ideal. The rates quoted may not apply to older or overweight individuals or those from other countries.

Where did the story come from?

Dr Miguel Martínez-González the University of Navarra, and colleagues from other hospitals in Pamplona, Spain carried out the research. The study was funded by grants awarded by the Spanish Ministry of Health and the Navarra Regional Government and published in the (peer-reviewed): The British Medical Journal.

What kind of scientific study was this?

This prospective cohort study enrolled 13,380 healthy Spanish university graduates without diabetes and followed them for an average of 4.4 years. The researchers first defined the Mediterranean dietary pattern using a recognised scoring system and then assessed the relationship between adherence, that is, how well the participants stuck to this dietary pattern, and onset of new cases of diabetes.

The researchers sent an invitation to take part to all former students of the University of Navarra, registered nurses from Spanish provinces, and graduates from other universities. The study began in December 1999 when the first participants were enrolled and continued until November 2007 as successive waves of graduates were invited to take part in the study. In total, 18,700 participants enrolled in the study. However, after excluding anyone who took part in the study for less than two years or anyone who hadn’t completed the two-year follow up questionnaire, the researchers were left with 13,753 (86.1%). A further exclusion of anyone who already had diabetes or heart disease at the start of the study reduced that number to 13,380.

All the remaining recruits completed a medical assessment and a food frequency questionnaire with 136 items. The questionnaire asked about the frequency that different food types were eaten in the previous year. The food types included vegetables, fruits, cereals, legumes such as peas and beans, fish, meats, fast food and dairy products. It also included questions on the use of fats and oil, cooking methods, and dietary supplements.

The participants scored points on how many times they ate typical portions of the different types of food on a usual day. They had a choice of nine answers that ranged from never or almost never to six or more times a day. A computer programme calculated the estimated daily intake.

A second scoring system was used to assess adherence, i.e. how well the students stuck to the dietary pattern. This index included nine components to define the Mediterranean diet: a high ratio of monounsaturated:saturated fatty acids, moderate intake of alcohol, high intake of legumes, high intake of grains, high intake of fruit and nuts, high intake of vegetables, low intake of meat and meat products, moderate intake of milk and dairy products and high intake of fish.

The index assigned a score of nought or one for each of the nine components depending on the participants daily consumption of them. Scores ranged from nought to nine with people who adhered to all the characteristics of the Mediterranean dietary pattern scoring a nine.

Questionnaires were then sent out every subsequent two years asking about diet, lifestyle risk factors and any medical conditions that had developed. New cases of diabetes reported by the participants in the follow-up questionnaires were confirmed through medical reports and by an additional detailed questionnaire posted to those who reported a new diagnosis of diabetes by a doctor. This additional questionnaire asked for details such as the highest level of blood glucose, whether they were taking insulin and date of diagnosis.

The researchers took into account a variety of factors such as sex, age, years of university education, total energy intake, bmi (body mass index), physical activity, sedentary habits, smoking, family history of diabetes, and personal history of high blood pressure.

What were the results of the study?

The researchers reported that those participants who adhered closely to a Mediterranean dietary pattern had a lower risk of diabetes. The risk of developing new diabetes when sex and age were taken into account was 83% lower for those with the highest adherence (score seven to nine) and 59% lower for those with moderate adherence (score three to six) when compared to those with the lowest adherence index (score less than three). The researchers also said that adding the other factors, such as bmi, physical activity or smoking, into their model did not significantly change the association.

What interpretations did the researchers draw from these results?

The researchers conclude, “adherence to a Mediterranean diet is associated with a reduced risk of diabetes”.

What does the NHS Knowledge Service make of this study?

The authors acknowledge several limitations to their study:

  • The total number of diabetes cases that occurred over the course of this study was small (33 cases confirmed from 58,918 person years of follow up). When the researchers designed the study they had expected more than 150 cases based on the estimate that there would be about three new cases per 1000 person years of follow up. This meant that they had to revise the way they divided the adherence index into three categories instead of the four originally planned. This increased the statistical power of the study. If this hadn’t been done and the researchers had followed their original plan, it’s possible that the study may not have shown a statistically significant result.
  • The researchers say that they were not surprised by these low rates of diabetes, as the population studied were mostly young and mainly healthy, and already maintained a basic Mediterranean type of diet. This is confirmed by the low bmi ranging between an average of 23 to 23.8 across all three adherence groups. This may limit the applicability of these findings in non-Spanish countries or older age groups where average bmi levels may be higher.
  • To its merit, the study used a comprehensive food frequency questionnaire to assess the entire diet at the beginning of the study. Despite the possibility that there may be errors in the participants measurements of their food intake, this provides a reliable way of estimating food intake that is superior to those relying on recall alone.
  • The authors took into account a number of factors that may have affected the relationship between diet and outcome and this had little effect on their overall results. They say that it is unlikely that there are other unmeasured or unrecorded factors that could have “confounded” or affected the results in a misleading way. It is surprising that the results were not affected when the researchers took into account factors such as obesity or sedentary behaviour, as these are also recognised risk factors for developing diabetes. It’s possible this reflects the low numbers of cases of diabetes and the generally healthy state of this young population.

This study suggests another benefit to those already confirmed for the Mediterranean dietary pattern. A higher intake of olive oil, plant based foods such as fruits, vegetables, peas and beans with high fibre foods and a low intake of meats seem linked with a low incidence of type 2 diabetes. However, it’s important to realise that this study was in young adults of normal weight and anyone changing their diet to a more Mediterranean style diet should be careful not to increase calorie intake by increasing their intake of olive oil. Overall weight still provides the strongest link to the chance of developing diabetes. All interventions in this area are only likely to delay onset of diabetes; this study was not about treatments.

NHS Attribution