A key strength of the study was that it was the first to show that major depression predicts increased risk for developing metabolic syndrome in middle-aged women. One of the key limitations of the study was that it only evaluated the role of depression in middle-aged women and not in men. This limits the external validity of the study. In addition, the use of cross-sectional data, self-reports, or the measurement of depressive symptoms as opposed to clinical depression only provided indirect support for the link between depression and the risk of developing metabolic syndrome.
Although a majority of the research agrees that a clear connection exists between depression and metabolic syndrome, several sources disagree. Hildrum, Mykletun, Midthjell and associates (2008) are a key example of research that does not support the connection between depression and anxiety with metabolic syndrome. This study used a cross sectional study of participants aged 20-89. The sample consisted of 9,571 participants, representing an exceptionally large sample population. The study used HADS self-report questionnaires to measure depression and anxiety, and metabolic syndrome assessed according to the 2005 International Diabetes Federation criteria. All tests were administered to patients in a hospital setting. The results of the study demonstrated only a weak association between depression and metabolic syndrome. The study adjusted for the presence of cardiovascular disease and antidepressants. This study, reviewed by Hildrum and associates, represents one of the largest studies on the connection between depression and metabolic syndrome. The study appears to conflict an overwhelming number of studies that suggest that a connection does exist between depression and metabolic syndrome. However, it does not say that no connection exists, only that the connection is weak. In light of the current research, it is important to remember that every individual patient is indivudal and results may vary. Even if the study proves true for a small number of patients, it will be helpful for those patients that fall within that category.
An examination of current literature on the connection between depression and metabolic syndrome reveals several trends and gaps in the literature. Another problem found throughout the research was that each study had to be taken on its own merit. Due to the lack of uniformity in the operational definition of the studies and their lack of uniformity in defining key terms, it is difficult to group them reliably. They can be grouped together in that a majority of the studies conclude that there is a connection between metabolic syndrome and major depression. Even those that were found to conclude that no connection existed still had a number of sample participants that exhibited co-morbidity between the two conditions. Conclusions that no connection exists, when the study results indicate a weak conclusion are flawed. The conclusion from these studies still supports the connection between depression and co-mobidity with metabolic syndrome. They just indicate that the connection is not as strong as other studies indicate. This was key flaw found in the research studies examined.
The entire field of research into this subject suffers from a lack of consistency. Many factors can influence both depression and metabolic syndrome, and unfortunately, many of the studies found failed to take these factors into consideration. Other gaps in the literature were that parameters of the conditions were not well defined. For example, patients with mild to severe depression or milder metabolic syndrome were grouped into the same category as those with severe symptoms. The duration of the condition was also found to vary within the same study category. The only criteria was that there be a presence of the symptoms, not the severity. If symptoms existed, then the parameters of the study had been met. These flaws in research support the need for research that takes these flaws into consideration. It also supports the original premise that depression is predictive of the development of metabolic syndrome in a select population of patients. Further research needs to be conducted on this group of patients so that their clinical needs can be met and the development of metabolic syndrome can be prevented in these patients.