This study is one of the first studies that assessed depression in a healthy well-nourished over-15 years old population. Generally, this idea that a healthy population is at low risk of depression prevents researchers from assessing the depression within this population. However, the current study's findings showed that the risk and prevalence of depression even in a healthy population can be notable. This study hazards regarding the high risk of depression in healthy populations as well as assessment of risk factors.
In this healthy population, a total of 31% of them suffered from depression. Of this 31%, one hundred and thirty-two (17.2%), ninety-three (12.1%), and nineteen (2.5%) of the population had mild, moderate, or severe depression. The reported prevalence in different countries was estimated to be about 1.5% in China to 15.2% in India before the pandemic, with a mean of 7% globally regardless of age [16, 17]. However, this prevalence can be higher within the older adult population. As the result, one systematic review in 2019 estimated the prevalence of depression to be about 34.4% in the Indian 60 years old and above population . Another study in Brazil reported a prevalence of 11.1% and 25.6% for late-life depression and clinically significant depressive symptoms in individuals older than 70 years old .
High-quality population-based prevalence studies on the same population in Iran are not available. There is no recent data available to estimate the prevalence of depression in Mashhad either. However, one study in Yazd, Iran, reported a prevalence of 29% for depression in 2019 (11.1% mild, 12.2% moderate, and 6.9% severe depression) . Nevertheless, in a systematic review from 2001 to 2015 in Iranian people aged 50–90 years, the prevalence of severe depression and overall depression was estimated to be 8.2%, and 43%, indicating a higher prevalence of depression than the current study . The finding and comparisons have shown a high prevalence of depression in a healthy population. However, Iranian older adults can be at a higher risk of depression compared with other nations. This may be due to the special condition of Iranians during the last few years. The authors also believe the findings of the current study encourage researchers to concentrate on assessing healthy populations for depression and other mental health disorders.
previous literature has established that increased age is one of the main risk factors for depression [22–24]. The current study's findings showed a significant association between age and depression (OR = 1.027, P-value˂0.05). the weak correlation of age with the BDI-13 score (R = 0.110, P-value˂0.002) also confirms the effect of age on depression severity. One of the newest hypotheses that can explain this situation can be brain aging [24, 25]. In this thesis, when people stop routine activities, the brain may face some uncontrolled mental burdens including anxiety and worries. This pressure along with lowered annalistic power of brain generally leads to mental health disorders, especially depression [24, 25]. Based on this new thesis, the effect of age on depression can be through brain aging, and the age itself may have no independent effect on mental health disorders . In this new hypothesis, keeping the mind active lowers the process of brain aging, mental disorders, and depression as a result .
In the current study, males had a significantly higher prevalence of depression when compared to females. The finding in different studies are varied, some studies suggested males [22, 26], and some others suggest females are at a higher risk of depression [17, 23, 27]. The gender-related finding depends on the population, age, and occupation change. In the Iranian tradition, especially in the current population, men have more and heavier responsibilities than females (because of tradition and culture) which can explain their higher risk of depression. In the Iranian traditional culture, generally, males represent the family in the society. They are responsible to provide all the family needs as well as protecting the family . These burdens can put males at a higher depression risk. Nevertheless, there are several limitations for females but according to the current population’s age and tradition, they were less stressful [28–30]. However, this relation is not absolute and during the last few years, the responsibilities between families started to be shared more equally while limitations for females are being removed .
Previous or current smokers were at least four times more likely to be depressed in the current population. Smoking has always been a fixed risk factor in nearly all diseases, especially depression [31, 32]. The relationship between smoking and depression can be bidirectional . In the current population males who are not smoking had a significantly higher BDI-13 score. Nevertheless, in our population men and women who are smoking or quit smoking didn’t have any significant difference in BDI-13 scores. This higher BDI-score in males who are not smoking can be explained by the other risk factors of depression that are more frequent in the male group.
Education and economic level
Generally, education and economic level have a direct relationship with each other, and people with higher educational levels are more likely to occupy better-income occupations [33, 34]. In the current population, as was expected, people with higher income had a lower risk of depression. There was no significant difference in the risk of depression between people with low income or without income. However, people with moderate- or higher-income levels have significantly 55% and 58% less OR of depression. The importance of income on the risk of depression is demonstrated in figure one, as a trend emerged correlating income with the severity of depression. The same finding has been reported elsewhere [17, 35]. For example, Americans with less than 5000$ in savings are reported to be at a higher risk of depression . Regarding the differences between genders in our study, females in families with low and moderate incomes had a significantly lower BDI-13 score that required to be more investigation.
Despite the inverse relationship between educational level and the risk of depression [23, 35], people with higher educational levels had an increased risk of depression in the current study. Some reports suggest that the graduated population has a lower risk of depression than the under-graduated population [17, 35]. However, to confirm the effect of alliteration on depression in the current study, there was evidence that illiterate people are at the lowest risk of depression in other studies . Another study in Iran also confirmed the inverse relationship between depression and education . The best explanation for this difference in the findings can be related to the special condition of the Iranian population. During the last few years, the political and economic changes that were highly affected by the US sanction and political stands of the Islamic Republic government made a complicated condition [36, 37]. Generally, higher education people follow the news the most . By considering the impact of good and bad news on individuals, this following up put this population in the exposed to more disappointing news than people with lower education which is a suitable condition to develop disappointment and depression [38, 39]. The other explanation can be related to the relationship between income and education. In Iran, experimentally, the income level is nearly independent of the education which can be a mental pressure on educated people [33, 34]. Correspondingly, no relationship between income and education was identified in the present population that confirm this relation (p = 0.752).
Marriage and living with someone
The marital status is another indication of depression. As expected, in this population, currently married people have a significantly lower risk than those who are divorced or widowed. There was also a lower prevalence of depression between marriage groups in the single people who never married, but the odds ratio was insignificant. In addition, married people who live with their partner (with or without children) had 44 to 47% lower OR of depression than people living alone. Another study by Lotfaliany et al. also reported the same finding . Besides, other studies’ findings confirm that married people are at a lower risk of depression in comparison with people who had divorced, widowed, or never gotten married [17, 20, 22, 35]. These two simple findings show the importance of family and their presence in preventing depression.
Working and physical activity
In Iran, people continue working after their retirement to cover their living expenses. The findings in the current population indicated that people who engage in moderate physical activities are at 80% lower OR of depression. However, in comparison to moderate (80% decrease), high physical activity only decreased the OR of depression by 60 percent. Generally, having a highly active physical activity required a routine programmed sports activity that can provide a slight mental pressure to the individuals that can explain this relation . The finding of previous studies also reported that inactive people were significantly at higher risk of depression [17, 23]. In addition, in the current population, people who had any sports activity ranging from swimming and volleyball to yoga and aerobics had a significantly 54% lower OR of depression than the inactive population. There is also evidence that people who are employed or currently have working status are at lower risk of depression [20, 23]. The finding of the current study also showed that unemployed individuals are at 2.5 times higher OR of depression compared with those who are working. As a result, having an active life, including moderate physical activity, any sport, and working, was shown to be suitable methods to lower the risk of depression and BDI-13 score at this age.
The relationship between lifestyle factors including marital, economic, education, working, and physical activity status with depression are well established. To the authors' knowledge and online report of the World Health Organization (WHO), the SF-36 questionnaire is a good tool that considers physical function, general health, vitality, social, mental, and emotional function. In the current study, an 1% higher quality of life showed about 3.3% significantly lower OR of depression. In the current population better physical function, general health, vitality, social, mental, and emotional function scores had a significant 3.8 to 1.1% inverse relationship with the risk of depression that is completely reported in table-2. The R of the lifestyle indicators also shows a significant moderate inverse relation (more than − 0.300) for nearly all factors with the severity of depression (BDI-13 score). The finding of other studies also reported the same inverse relationship between depression and higher quality of life [17, 20, 23, 35, 41]. As evidence show, this is one of the first studies that used this tool to score the populations' lifestyle in depression. authors suggest SF-36 can be easy to use, reliable, and a good tool to assess the quality of life in the depression risked populations that can consider all aspects of lifestyle quality. However, a validation study in this population is still required.
In the current study, the nutrient intake was adjusted to the individual's weight to increase the efficacy and decrease the confounding effect. Weight is known as the main and the most effective component of BMR, and by this adjustment, a good comparison between groups was made that showed a more accurate effect of nutrient intake compared to the metabolic requirement of people without the effect of food value . After adjustment, no difference had been found between the main nutrients such as water (g/kg), energy (Kcal/kg), protein (g/kg), and total lipid (g/kg) intake which are the most important components to categorize foods in the food’s groups . To the authors' knowledge, this study is the first study that used this method to study the relationship between nutrition with depression. In the study, adjusted dietary starch, trans fatty acids, iron, copper, vitamin-B6 and magnesium intake are shown to be significantly associated with depression. However, the correlation with the BDI-13 score was significantly but weak. This difference between these nutrients' association with depression and BDI-13 shows the possibility that these nutrients are associated with the risk but have a small effect on the severity of depression that required research on more. The high OR in trans fatty acid and copper could be explained by both the high impact on depression reported previously  and the number of decimal places of factors.
This study did not find any difference between groups for dietary fiber, but according to other evidence, higher fruit and vegetable intake that has higher fiber can lower the risk of depression [6, 7, 17, 42, 44]. There was also a significant difference between groups for vitamin B6 in this study that confirms previous reports [45, 46], but neither the R nor OR were not significant for the risk of depression and BDI-13 score for this vitamin. In the present study, both trans fatty acid and starch had higher OR with depression. These two nutrients are the main components of cakes, cookies, crackers, and biscuits, known as depressive food groups that can explain this effect [6, 7, 42]. Nevertheless, there was no effect of sugar on depression in the current study. This situation can be explained by the dietary pattern of Iranians; but other studies confirm that higher sugar intake is associated with depression [6, 7, 42, 44].
It is well known that meats are a better source with more iron bioavailability than other food groups, especially plants . While both meat and oils have higher fat concentrations, especially trans fats and cholesterol . The current study's findings showed that the effect of meat and oil on depression can be related to their iron and trans-fat containing. However, in the case of copper, it must be considered that both vegetables and animals are the best sources but, in the present population, it presents higher organs like liver, brain, and head especially traditional foods named “kale-pache” (head, brain, buttocks, tongue and eyes) and “Jegaraki” (liver and tail) consumption that have the higher concentration of trans fatty acids either .
The effect of iron, copper and trans fatty acids on depression in the current study can explain the harmful effects of fast foods on depression described in previous studies [6, 7, 42, 44]. During food process, high oil temperature, and reheating, can be the main depressive effect of fast foods. During long-term or high-temperature heating and reheating that is common during the food process of fast foods, and even daily cooking in most of the Iranians cooking patterns, the trans fatty acids and elements like iron and copper increase and are more likely to be absorbed by the food from the oil . By considering both the food processor and the current findings, it seems that fast foods in nature are not harmful, but it is the process of preparing these foods that increase the risk of depression. However, more investigation is needed.
Regardless of the finding of the current study, different studies show a healthy diet rich in fruits and vegetables, replacing red meats with white meats, increasing the beans and lumen intake that is close to the Mediterranean diet can lower the risk of depression [6, 7, 17, 42, 44]. However, the food processor and changes that can affect the dietary value of foods should be considered as the other factor that may elevate the depression risk.
Suggestions, limitations, and strengths
The main strength of the current study is related to its population. As most the studies categorize healthy populations as a low-risk group and do not assess them, this study is one of the first studies that evaluate a healthy population. The main finding of this study shows depression can be prevalent more than expected in healthy people. Based on the current study's findings and the importance of prevention, the authors suggest researchers study healthy populations more. Studying a healthy population have two major advantages. First, by understanding the risk factors and diagnosing diseases in healthy populations, designing prevention strategies will be easier, and second, people with the disease will be diagnosed sooner, and experts can treat the disease before getting severe.
The strengths of the current study are its accurate methodology, comprehensive report of a vast range of depression-related factors, and the big sample size that presents the overall healthy population of Mashhad, Iran, with a very low risk of biases between groups. In addition, it is one of the first studies that has this comprehensive point of view on a vast range of depression factors in one single report. However, the limitation was using a one-day dietary recall instate of a 3-day dietary recall, but the main cause was decreasing the memory depending on such a detailed tool. However, this limitation was controlled by some specialized dietary questions about dietary habits and adjustments that were made by the RDN. Nevertheless, the present study's main limitation is its observational nature, which is uninformative on the temporal criterion for judging causality. However, this type of study provides a rationale for future research.