Inflammation and its effect on how we think

There is increasing evidence confirming a relationship between inflammation and depression. That is, higher inflammation (as measured by inflammatory biomarkers) is associated with a lower mood. Even if someone is not currently depressed, having higher levels of C-reactive protein (a blood measure of inflammation) increases the risk of developing depression in the next 5 years [1].

Even though research demonstrates a relationship between depression and inflammation, this does not necessarily mean that inflammation causes depression. The reverse could also be true. That is, having depression can cause depression. For example, people with depression are more likely to be inactive, eat an unhealthier diet, and have poor sleep which can all increase inflammation.

However, there are several interesting studies that show inflammation can change the way we think. In these studies, a non-depressed, healthy volunteer is injected with an inflammatory endotoxin and its effects on mood, thoughts, and decision making is investigated.

In one study, exposure to an inflammatory endotoxin was associated with a reduction in mood and lower body-related self-esteem compared to a placebo. This indicates that inflammation can have a negative effect on mood and the way we think about ourselves [2].

In another study, two hours after receiving a placebo or endotoxin, participants completed a task involving a monetary reward. In women (but not men) exposed to the endotoxin, there was reduced activity in the ventral striatum (a part of the brain associated with reward/pleasure). This suggests that inflammation can have a negative impact on brain regions associated with reward/pleasure [3]. 

In another study, electroencephalography (EEG) was used to assess how inflammation affected three different distinct attentional processes: alerting, orienting, and executive control. In a double-blinded placebo-controlled study, healthy males were exposed to an endotoxin or a placebo injection. Inflammation caused significant alterations in task-related brain activity. In particular, volunteers had to exert greater cognitive effort when preparing for a task in order to maintain adequate behavioural performance [4].

Studies have also shown that endotoxin exposure can have an adverse effect on decision-making characterised by greater impulsivity and an inability to delay gratification. The authors of a paper concluded that these results suggest that inflammation may contribute to changes in decision-making patterns that can lead to undesirable personal and societal outcomes [5].

Finally, based on a systematic review of 12 studies, it was concluded that results from endotoxin-induced inflammatory trials suggest inflammation can have a negative effect on the perception of emotions, increases avoidance of punishment/loss, and increases levels of social disconnectedness [6].

The results from these studies suggest that inflammation can affect our mood and the way we think. This potentially means that one way to improve the way we think is to reduce levels of inflammation. This includes eating an anti-inflammatory diet, engaging in moderate exercise, ensuring good sleep patterns, and possibly taking anti-inflammatory herbal ingredients and nutraceuticals such as saffron, curcumin, ashwagandha, magnesium, and fish oils.

In a paper I published in the Australian and New Zealand Journal of Psychiatry, I demonstrated that there are studies showing inflammation reduces the effectiveness of cognitive-behaviour therapy (CBT) [7]. CBT works by targeting the way we think, but if someone presents with increased inflammation, there may be neurological/neurochemical alterations that change the way the brain works. CBT may, therefore, be ineffective because there are biological forces that modify thinking patterns. By including interventions that lower inflammation, biological changes in the brain may be altered that can then make CBT more effective. However, more research is needed to confirm this.

References

1. Valkanova V, et al. CRP, IL-6 and depression: a systematic review and meta-analysis of longitudinal studies. J Affect Disord. 2013 Sep 25;150(3):736-44.
2. Kotulla S, et al. Does Human Experimental Endotoxemia Impact Negative Cognitions Related to the Self? Front Behav Neurosci. 2018 Aug 21;12:183.
3. Moieni M, et al. Sex Differences in the Relationship Between Inflammation and Reward Sensitivity: A Randomized Controlled Trial of Endotoxin. Biol Psychiatry Cogn Neurosci Neuroimaging. 2019 Jul;4(7):619-626.
4. Balter LJ, et al. Selective effects of acute low-grade inflammation on human visual attention. Neuroimage. 2019 Nov 15;202:116098.
5. Gassen J, et al. Inflammation Predicts Decision-Making Characterized by Impulsivity, Present Focus, and an Inability to Delay Gratification. Sci Rep. 2019 Mar 20;9(1):4928.
6. Bollen J, et al. The effects of acute inflammation on cognitive functioning and emotional processing in humans: A systematic review of experimental studies. J Psychosom Res. 2017 Mar;94:47-55.
7. Lopresti AL. Cognitive behaviour therapy and inflammation: A systematic review of its relationship and the potential implications for the treatment of depression. Aust N Z J Psychiatry. 2017 Jun;51(6):565-582.