Summary: There is some sound evidence that metabolic inflammation is among the significant factors contributing to the rise of depression. Despite some sound evidence in favor of this theory, the role of chronic inflammation in depression development remains a relatively unexplored topic.
Depression is a complex mental disease caused by numerous factors. Depression used to be a rare disease till the middle of the 20th century, and now, after about 75 years, it is one of the most common mental health issues. So, why has this rise happened? Is increasing mental stress enough to explain this rise? Perhaps not.
It is quite likely that we are missing something, especially considering that, unlike metabolic disorders, depression is challenging to diagnose due to a lack of specific tests or biomarkers. Its diagnosis is mainly clinical. This also means that there are extremely high chances of missed diagnosis, and real numbers are much higher.
So, is it possible that metabolic inflammation might be a prime driving force behind the rise of depression and even other mental health issues and neurodegenerative conditions? Well, that is quite likely.
Figure 1 Depression, historical trend
If we look at the two graphs above, we see that the rise in depression rates coincides with the rise in the prevalence of metabolic disorders like diabetes.
It may be challenging to say how and to what degree depression is linked to metabolic disorders. Nevertheless, an increasing number of studies now suggest that metabolic inflammation might be one of the significant if not primary, contributing factors to depression risk.
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Of course, depression is a multidimensional ailment, which means that not all of those living with depression have metabolic disorders. Nonetheless, metabolic inflammation might be one of the significant factors behind the rise of depression.
Analysis of the 2009-2010 National Health and Nutrition Examination Survey (NHANES) survey data found that about 41% of those living with depression in the US had metabolic syndrome, and one-third of all those living with depression had elevated CRP.
Of course, we need to understand that CRP is not just a sole indicator of metabolic inflammation. If CRP is not elevated, this does not essentially mean that there is no inflammation. Researchers now think that metabolic inflammation is present in about half of all those diagnosed with depression.
Thus, new studies show that cytokines, which are inflammatory chemicals, are elevated in all those living with obesity, and they have a significant impact on mood and mental health. Moreover, in those living with obesity, cytokines are chronically elevated. It appears that metabolic inflammation disrupts the production of neurotransmitters and signal transduction.
Of course, to date, scientific understanding of how metabolic inflammation may cause depression is limited, and most evidence comes from animal studies. There is still a need to carry out more extensive studies on humans. Moreover, there is a need to find more reliable biomarkers that might suggest depression risk in those living with metabolic inflammation.
Additionally, it is also worth understanding that there is a dual-sided link between depression and metabolic inflammation. This means that not only does metabolic syndrome increase depression risk, but the opposite is also true. Those living with depression experience appetite and body weight changes, and they are more likely to experience metabolic disorders and, consequently, metabolic inflammation.
So, it is time to rethink and redefine depression. It is vital to start exploring the role of chronic inflammatory conditions in disease development. It is vital to understand the role of metabolic inflammation in depression. There is also a need to develop a better understanding of metabolic inflammation, as some of the commonly available blood tests are not reliable to diagnose the condition.
Understanding the role of metabolic inflammation in depression may even help enhance its diagnosis. It may help find some reliable biomarkers, which might help identify its risk and even monitor its progress.
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