There is a lack of large-scale dietary or lifestyle interventions among depressed patients.
Based on the information of the impaired quality of diet and decreased dietary intakes of essential nutrients, it is evident that depressed individuals would benefit from dietary improvements that are possible to achieve in dietary or lifestyle specific interventions (171).
Positive effects have been shown in interventions (312), but the effects of improved lifestyle factors on depression course are still partly unclear.
The beneficial effect of physical exercise is suggested to be considerable in the treatment of depression. A recent Cochrane review concluded that exercise seems to improve depressive symptoms (27). The results indicated a moderate clinical effect in people with a diagnosis of depression when compared with no treatment or control intervention. Physical exercise is effective especially in the prevention of recurrent depression (101). However, 12 months of pure exercise intervention reported no statistically significant effect on depressive symptoms in elderly depressed individuals (305). In contrast, a psycho-educational group program aimed at healthier lifestyle in patients with depression showed benefits compared to usual care as a control (313).
A recent RCT clarified the effects of four lifestyle recommendations on the treatment of depression (314). The study consisted of 80 depressive out-patients on antidepressant treatment. The intervention aimed at four goals: improving sleep (detailed instructions, for example go to bed before 11 pm), walking at least one hour a day, being exposed to sunlight at least two hours per day and finally, eating a healthy and balanced diet and regular meals. Specific dietary aims were to eat fish at least three times per week, consume vegetables, fruits, cereals and nuts daily and avoid sugary drinks. The control group received only general advice, for example “try to eat a healthy and balanced diet”. After six months of intervention, all scales indicated better recovery in the intervention group (mean decrease of the BDI scores in intervention and control group was 9.3 and 5.3, respectively).
In addition, the number of psychopharmacological treatment prescriptions was reduced more in the intervention compared to the control group. The study suggested that lifestyle recommendations may be used as an effective antidepressant complementary treatment in clinical practice.
Recently, a study protocol was published for the “Supporting the Modification of Lifestyle In Lowered Emotional States” study, an RCT investigating the effects of dietary intervention for adults with MDD (315). This is the first diet-only based trial with 12 weeks of intervention focusing on advocating a healthy diet based on the Australian Dietary Guidelines and the Dietary Guidelines for Adults in Greece, started at the end of 2012.
After completion of this trial, there will be more evidence of the effects of dietary intervention in depressed individuals.
It has been assumed that current depression may affect adherence to intervention instructions, achieving goals and the possibility of dropping out during the intervention. In the elderly, intervention participants with high depressive symptoms did not benefit from physical exercise intervention as much as healthy controls (305). However, in that study adherence rates to the exercise sessions did not differ between depressed individuals and others. In a recent Finnish study, a diagnosis of MDD, chronic depression or specific symptoms of depression did not predict quitting a weight loss intervention (316). It was observed that only anhedonia predicted drop-outs.
7.3 POTENTIAL MECHANISMS
Lifestyle factors are interrelated, and when one of these habits is modified it usually facilitates change in others (16). Therefore, the mechanism that may explain why these lifestyle habits can prevent or improve depression is probably complex (312). The possible effects of dietary factors on depression have been presented in detail earlier in this thesis.
On biological level, it is likely that exercise improves the balance of serotonergic, dopaminergic and noradrenergic systems in contrast to stress, which impairs the systems (317). In addition, the magnitude of social support and regular meetings may affect depressive symptoms.
In summary, results from the lifestyle intervention studies aimed at improving diet and/or increasing physical activity in non-clinical high-risk populations, as well as in depressed individuals, are inconsistent. There is great heterogeneity between the studies. It seems that increasing physical exercise alone does not benefit participants as much as the combination of lifestyle factors. In addition, the effect of comprehensive lifestyle intervention aimed at healthy diet, increased physical activity and weight reduction has been less studied compared to intervention studies with only one aim, usually increasing the amount of exercise or losing weight.
In summary of the review of the literature section of this thesis, the evidence suggests that diet may have a role in the prevention and treatment of depression although heterogeneity between the studies is large. The inconsistency in the results may partly be explained by potential confounders, such as total energy intake, gender, age or smoking. Further prospective studies and intervention studies are especially needed.
8 Aims of the study
The general aim of this study was to examine the associations between diet and depression.
The specific aims for each sub-study were:
I To examine whether dietary folate and vitamin B12 are associated with the risk of depression in a prospective setting in middle-aged men (work I)
II To study if serum concentrations of total long-chain n-3 PUFAs (sum of EPA+DPA+DHA), individual PUFAs, or the ratio of n-6 to n-3 PUFAs are associated with the risk of depression in a prospective setting in middle-aged men (work II).
III To study if coffee or tea consumption or intake of caffeine are associated with the risk of depression in a prospective setting in middle-aged men (work III).
IV To examine if dietary patterns are associated with a prevalence of depressive symptoms or the risk of clinical depression in middle-aged men. Both cross-sectional and prospective settings where used to clarify the associations (work IV).
V To investigate how lifestyle intervention affects the depressive symptoms assessed by BDI score in middle-aged men and women, and to ascertain the determinants of this effect (work V).
9 Subjects and methods
The data used in this thesis were from two separate studies, the Kuopio Ischaemic Heart Disease Risk Factor (KIHD) Study and the Finnish Diabetes Prevention Study (DPS) (Table 11).
9.1 THE KUOPIO ISCHAEMIC HEART DISEASE RISK FACTOR (KIHD)