Wednesday October 7 2009
Mediterranean diet: high in fruit and vegetables, low in red meat
“The Mediterranean diet, already thought to protect against heart disease and cancer, may also help to prevent depression,” BBC News has reported. The article said that a four-year study in over 10,000 healthy adults in Spain found that people were 30% less likely to develop depression if they ate a diet high in vegetables, fruit and cereals, and low in red meat.
This study has strengths in that it regularly collected detailed information from a large number of people over a four-year period. It also has some limitations, including the fact that all the data was gathered by questionnaire and so is susceptible to errors introduced by the participants themselves. There are also numerous factors that could affect both a person’s dietary habits and their predisposition to depression.
Overall, more study is needed before a link between this type of diet and depression is established, but the signs are positive and it is worthy of further research. There is already good evidence that this type of diet has numerous established and possible health benefits.
Where did the story come from?
The research was carried out by Dr Almudena Sanchez-Villegas and colleagues from the University of Las Palmas de Gran Canaria and other Spanish institutions. It was published in the Archives of General Psychiatry.
Funding was received from the Instituto de Salud Carlos III (a Spanish government agency), the Fondo de Investigaciones Sanitarias and the Navarra Regional Government project.
What kind of scientific study was this?
This cohort study examined whether there is a link between the Mediterranean diet and the risk of depression. This diet is associated with other benefits including improved cardiovascular health, and typically consists of a high intake of vegetable, fruit, nuts, fish and monounsaturated fats with a low intake of meat and saturated fats.
The researchers recruited 10,094 healthy university graduates who were members of the SUN Project; a multi-purpose Spanish cohort composed of graduates of the University of Navarra, registered professionals from various Spanish provinces and other graduates. The project is ongoing and has been recruiting since 1999. All information is collected by questionnaires that are mailed out every two years. The overall follow-up of study members is said to be 90%.
Potential participants were sent a 136-item food questionnaire. People who responded were given a score of one to nine for their adherence to the Mediterranean diet. This score was calculated based on their ratio of monounsaturated fat to saturated fat intake, alcohol intake, vegetables, cereals, fish, fruit and nuts consumption, and how much meat and dairy they ate. The participants also gave medical, health and lifestyle information, including their levels of physical activity.
Depression was defined as any diagnosis of depression made by a doctor, or use of antidepressants at any point during follow-up (all participants were free of depression and antidepressants at the start of the study).
What were the results of the study?
The average follow-up time was 4.4 years. During this time, there were 480 new diagnoses of depression (4.8% of the sample). A greater adherence to the Mediterranean diet decreased the risk of being diagnosed with depression.
The participants were grouped according to their adherence to the diet. Compared to the lowest adherence score (zero to two points), the next adherence category up (three points) had a significantly reduced risk (26%) of depression; the third category (four points) a 34% reduced risk and the fourth category (five points) a 51% reduced risk. The final category, which had the highest adherence to the Mediterranean diet (six to nine points) had a 42% decreased risk.
These risk scores were adjusted for sex, age, smoking status, BMI, physical activity, total daily energy intake and employment status. When they excluded from their analysis people who reported antidepressant use but did not report that a doctor had diagnosed depression, the risk reductions remained significant for those in the highest three adherence categories compared to the lowest, although those with the three point adherence score no longer had reduced risk compared to those in the lowest category.
The researchers also found that people who ate more fruit, nuts and legumes (such as peas), and more monounsaturated compared to saturated fats were less likely to have depression.
What interpretations did the researchers draw from these results?
The researchers say that their results suggest that the Mediterranean diet may protect against depression, and that further studies are needed to confirm these results.
What does the NHS Knowledge Service make of this study?
This study has strengths in that it enrolled a large number of people, followed the majority up over a four-year period and made careful attempts to collect regularly a large quantity of information for them.
However, it also has the following limitations:
- All data was collected by mailed-out questionnaire and none was by assessment in person. As all answers were self-reported there may be some errors in the information on food intake, frequency and portion size, exercise and weight, and on diagnoses of depression or antidepressant use.
- Although the results were adjusted for various things that might affect a relationship between diet and depression, others, that could significantly affect both diet and depression risk, were not. For example, socioeconomic status, medical illness and stressful life events were not taken into account. It is also impossible to assess all of the various genetic, personality and mental behaviours of an individual that may predispose them both to poorer diet and depression.
- Although people with depression were excluded at the beginning of the study, others that were included may have had the beginning of a mood disorder that had not yet been diagnosed. If this was the case, it could have influenced the accuracy of the results.
- It was not possible to accurately determine when people began eating a Mediterranean-style diet, how long they continued to eat it, and how it related to the onset of depression.
- This was a healthy group of university-educated young people. Older people (who may have more medical illnesses or stressful experiences) or those of lower opportunity and educational status may not have had the same result. In addition, this group was only followed for four years, and the link between depression and diet may change over time.
Although this study does have these limitations, numerous previous studies have suggested that a diet high in fruit, vegetables, nuts, fish, moderate alcohol and low in meat and saturated fats is beneficial for health. This link with depression prevention is an interesting one and is worthy of further research.