Depression and the Brain as a Thermostat
Definition: Depression is more than just a mental health problem. Alongside symptoms like low mood and loss of motivation, it’s also common to experience physical problems. For example, if you have depression you are twice as likely to develop conditions like type 2 diabetes, high blood pressure, obesity and heart disease. This effect works both ways: if you have these metabolic conditions, you are also twice as likely to develop depression. In other words, the relationship is bidirectional. Neuroscientists and psychiatrists increasingly think that this link between depression and metabolic diseases isn’t just incidental. Instead, it may be explained by a common underlying biological cause, which has to do with how the brain regulates our energy levels[1].
Metaphor: You can think of the brain as being similar to the thermostat which controls the heating in your home. The thermostat tries to keep the temperature in your house constant. If the temperature falls below a set value, it sends a signal to the boiler to turn on and produce more heat. Once the room reaches the desired temperature, the thermostat turns off the boiler. Similarly, the brain plays a significant role in keeping our own energy levels balanced. It continuously monitors how much energy we have available and regulates our appetite (how much energy we take in by eating) and our motivation (how much energy we are willing to expend on actions or thoughts) accordingly. Perhaps surprisingly, the brain’s role extends beyond just modifying our behaviour – it also influences important bodily processes like controlling our blood sugar levels and fat storage.
We can use this metaphor to shed light on the link between depression and metabolic disease. Imagine if your thermostat at home were damaged, and could no longer detect temperature accurately; it might then leave the heating running for longer, even when the house is already uncomfortably hot. Similarly, if the brain’s ability to monitor your energy levels were impaired, it might try to conserve energy unnecessarily: you would experience low motivation and difficulty with focus and concentration; you might spend more time asleep; and your appetite, and therefore weight, would increase as your brain seeks out the extra calories it thinks it needs. These are all symptoms of a type of depression called ‘atypical’, or energy-related, depression (which, despite the name, constitutes around 40% of depression diagnoses).
Crucially, as the body accumulates excess energy, this can cause damage to some of the metabolic systems, further impairing your ability to regulate your energy levels – the start of a vicious cycle. This may explain why depression and metabolic disease so often overlap – both may result from, and cause further changes to, the brain’s ability to keep our energy levels in balance.
[1] Specifically, 30%-40% of depression cases may be energy-related – during an episode of depression, patients in this subgroup experience reduced motivation, increased appetite and weight, longer time spent asleep and also tend to have higher levels of systemic inflammation