It’s well known that conditions such as obesity may lead to physical chronic diseases such as type 2 diabetes or cardiovascular disease, but what about their impact on mental health? The world of psychiatry once considered the functioning of the body to operate separately from that of the mind, however it’s becoming increasingly evident that the body and mind work together in a bidirectional nature, as seen in systems such as the gut-brain axis and chronic diseases such as obesity.

While it was once thought that body fat was simply a means for the body to store excess energy, we now know that fat is actually biologically active endocrine tissue. This means that instead of simply taking up space, fat synthesises and secretes substances such as hormones and inflammatory mediators. These play a role not only in energy balance but can also affect other systems and conditions such as immunity and mental health.

Obesity is a common variable of interest in mental health research. At first glance it may seem like a no-brainer; depression can lead to reduced interest in physical health, and when your ability to be physically or mentally active is at an all-time low, healthy behaviours may not be an option anymore. With this accumulation of lifestyle changes body fat may increase and lead to obesity. However, it’s no longer so clear cut as there is good evidence for a bidirectional relationship between depression and obesity. Yes, depression can affect obesity, but obesity can also affectdepression. One possible mechanism is that those substances secreted by fat tissue can cause inflammation, playing a role in the development of depression.

Inflammation

Obesity places the body under chronic stress which activates aspects of both the innate and adaptive immune systems. This leads to increased levels of circulating pro-inflammatory substances called adipokines which have been found to relate to mental health. In fact, studies have also found anti-inflammatory medications to be effective for the treatment of depression, and conversely treatment of other conditions with pro-inflammatory medications have caused depressive side-effects. However, findings from various studies are not always in agreement, so there’s still more work to be done in this area before there is a consensus on whether anti-inflammatory medications may a viable therapeutic option for depression.

Structural changes

But how exactly do these circulating inflammatory substances impact the brain to promote depression? One explanation may be by causing structural changes in the brain. A reduction in the hippocampus and amygdala has been observed in people with type 2 diabetes (which can occur as a result of obesity) and these brain regions have both been associated with depression.

Our microbiome also appears to play a role in obesity and depression and it’s been hypothesised that it might be a mediator of the two, but that’s a whole other blog post topic!

So, what’s the take-away here? Well, like most emerging health research it’s not straight-forward and there’s still more to discover. But keeping body fat within a healthy range through lifestyle choices such as reducing alcohol, processed foods and sedentary behaviour, and consuming nutrient-rich wholefoods is still the recommended advice. Keeping body fat at healthy levels may help to reduce chronic inflammation, a risk factor for depression. This lifestyle advice isn’t new, or a quick-fix, but some things will always be consistent when it comes to optimising your health, both physical and mental.

Jessica Davis is a PhD candidate with the Food and Mood Centre researching the factors that predict muscle mass over time. She has qualifications in Food and Nutrition (Exercise Science) and Medical Science (Honours) and is also a champion body builder.

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