The causal relationship between willpower and diet is reciprocal: We need self-control to eat healthy, and we need to eat healthy to have good self-control. The first relationship is obvious, but what’s the biological mechanism by which an unhealthy diet destroys willpower?
Inflammation
We typically think of inflammation as our immune system’s response to physical injury or infection that causes redness, heat, pain, and swelling. But inflammation has cognitive and motivational effects as well. Inflammation doesn’t only affect how we look and feel, but also how we think and act.
Think about the last time you were sick—with a hot, fatigued body, a sore throat, and swollen nasal passages. As you were coughing out your soul, you probably had no desire to eat, no strength to move, no energy to think. When you’re sick, the immune system demands all your body’s resources to fight off the virus, the infection, the sickness. This excludes being active, social, creative, and productive.
According to new research and the immunologic model of self-regulatory failure, “proinflammatory cytokine activity plays a key role in altering neural, cognitive, and behavioral dynamics that can contribute to self-regulatory failure” (Shields et al., 2017, p. 603). In simple English, inflammation undermines willpower. It weakens your will.
Here’s a brief overview of the biological mechanism:
- Cytokines are proteins that facilitate communication between cells in the immune system.
- Proinflammatory cytokines can stimulate neurons either directly via neural cytokine receptors or indirectly via the vagus nerve or neurotransmitter modulation.
- By stimulating neurons in the prefrontal cortex, proinflammatory cytokines undermine self-control, which stems from that brain region.
Now, what does an unhealthy diet do? It produces proinflammatory cytokines. It increases inflammation. Refined sugar (candy, soda, cookies, etc.), processed meat (burgers, sausages, etc.), and trans fats (fries, pastry, frozen pizza, etc.) inflame your body excessively, especially if you don’t eat enough anti-inflammatory foods like nuts, greens, fish, sprouts, and berries. This is how an unhealthy diet increases your odds of losing important self-control battles.
Naturally, an unhealthy diet is not the only cause of inflammation. Proinflammatory cytokine activity is also increased by bacterial and viral infections, physical injury, poor sleep, and psychological stress. All these things tax your immune system, inflame your body, and weaken your will.
For a more in-depth view, consider these five pathways between willpower and the immune system:
- Mindset. Positive beliefs about one’s self-efficacy, which promote self-control, are associated with reduced proinflammatory cytokine activity. Conversely, heightened proinflammatory cytokine levels contribute to a more negative and passive mindset. In short, inflammation weakens your will by weakening your mind. To develop a strong mindset, read my book Willpower Condensed.
- Motivation. Self-control only works if you’re motivated to achieve a goal. Proinflammatory cytokines drain your motivation. Consider again the extreme inflammation during severe sickness: there’s no motivation to do anything other than lie in bed and let the body recover. With junk food, the effects may be more subtle, but the mechanism is the same.
- Depletion. Willpower depletion, if it exists, may be accelerated by proinflammatory cytokines. After vaccination, for example, when cytokine levels are particularly high, activity in the prefrontal cortex is acutely exaggerated, but impaired afterwards. Although we have some evidence for this mechanism, nobody has yet tested it directly.
- Stress. Most stress is created by the mind, by its opinions and appraisals, by perceiving situations as stressful. This anti-stoic tendency of the mind may be intensified by proinflammatory cytokines, which amplify stress experiences. And if you’re stressed out, it’s hard to stay in control and resist temptation.
- Cognitive Control. Dozens of studies (correlational, longitudinal, experimental, and genetic) have linked inflammation to impairments in working memory, attentional control, and behavioral inhibition. All these executive functions are critical for succeeding at willpower challenges.
If you want to get into the scientific details, check out the review “Inflammation, Self-Regulation, and Health: An Immunologic Model of Self-Regulatory Failure” by Shields et al. (2017) in Perspectives on Psychological Science, Vol. 12(4), pp. 588-612.